Introduction

Osteology is the study of bones. The skeleton provides the basic scaffolding for the body. The skeletal system includes the bones, cartilage, ligaments, and connective tissues that hold everything together.

Classification of Bones

The human skeleton contains 206 major bones, whereas the number of bones in different animals varies. The bones can be classified into five categories, including long bones, short bones, flat bones, irregular bones, and sesamoid bones (Fig. 6.1), named due to their resemblance to sesame seeds.

• Long bones are bones that are longer than they are wide. Some of the bones of the limbs are long bones. Long bones are characterized by an elongated shaft and somewhat enlarged extremities with articular surfaces. Examples of long bones include the humerus, radius, femur, tibia, metacarpal (MC) bones, and metatarsal bones.
  
• Short bones are usually shaped somewhat like a cube. Some examples include the carpal bones of the wrist and the tarsal bones of the ankle.
  
• Flat bones, as the name implies, are thin and flattened. They include two plates of compact bone separated by cancellous or spongy bone. Examples include the sternum (breastbone), ribs, scapula (shoulder blade), and certain skull bones.
  
• Irregular bones. Are complex and irregularly shaped bones. Examples include the vertebrae and certain facial bones.
  
• Sesamoid bones. Are small bones embedded in a tendon and resemble the shape of a sesame seed. The most prominent example is the patella (kneecap).

Gross Anatomy

Each bone consists of compact bone and cancellous bone. Compact bone is also called dense or cortical bone. Compact bone is found on the surface of bones. The interior of bones is made of cancellous bone.

Cancellous bone or spongy bone, consists of a network of small pieces of bone called trabeculae or spicules, interspersed with spaces filled with red or yellow bone marrow. Spongy bone predominates in short, flat, and irregular bones, as well as in the epiphyses of long bones. It is also found as a narrow lining of the medullary cavity of the diaphysis of long bones.

Long Bones

In developing long bones, the shaft is called the diaphysis, and each extremity is called an epiphysis (plural = epiphyses) (Fig. 6.2). The epiphysis is mostly cancellous bone with a thin outer coat of compact bone. It is usually enlarged relative to the diaphysis. The metaphysis is the region at the boundary between the diaphysis and epiphysis. Between the diaphysis and epiphysis of growing bones, there is a flat plate of hyaline cartilage called the epiphyseal plate. After growth is complete, this cartilage is replaced by the epiphyseal line. The medullary cavity (from the medulla, “innermost part”) is the space within the diaphysis that contains bone marrow. The joint surface of the bone is covered with a smooth layer of hyaline cartilage where one bone forms a joint with another bone.

The fibrous sheath surrounding that part of the bone, not covered with articular cartilage, is called the periosteum. It consists of dense irregular connective tissue. The innermost periosteal layer includes an osteogenic layer containing osteoblasts responsible for generating new bone and osteoclasts tasked with resorbing bone. The periosteum contains nerve fibers, lymphatic vessels, and blood vessels that supply the bone. The periosteum is attached to the underlying bone by Sharpey’s fibers extending from the fibrous layer of the periosteum into the bone matrix. Sharpey’s fibers are particularly dense where tendons and ligaments attach to the periosteum.

The internal surfaces of the bone are covered with the endosteum. The endosteum lines the medullary cavity in long bones and covers the trabeculae of spongy bones.

Short, Irregular, and Flat Bones

Short, irregular, and flat bones vary in the proportion of compact and cancellous bone (Fig. 6.3). In addition, these bones do not have a shaft or epiphyses. They contain bone marrow between their trabeculae, but there are no bone marrow cavities. The internal spongy layer in flat bones is called the diploë (folded).

Microscopic Anatomy of Bone

Four major cell types are found in bone (Fig. 6.4). The mature osteocytes account for most bone cells. Each osteocyte lies within a lacuna (see next section, “Compact Bone”). Osteoblasts are cells that secrete the extracellular matrix of bone. They produce collagen and matrix materials that account for unmineralized bone or osteoid. Eventually, osteoblasts become surrounded by the matrix they secrete, at which point they mature and become osteocytes. Osteoclasts are important in the resorption of bone and the liberation of calcium stored in bone and are most abundant in areas where the bone is being removed. Osteoclasts are giant multinucleated cells. Bone also contains a small number of mesenchymal cells known as osteoprogenitor cells. These cells can produce osteoblasts and are therefore important in fracture repair. Osteoprogenitor cells reside in the inner, cellular layer of the periosteum, the endosteum lining the marrow cavity, and along vascular passageways in the matrix.

Compact Bone

Although compact bone appears solid to the unaided eye, it is microscopically complex (see Fig. 4.53, Fig. 4.54, and Fig. 4.55). The structural unit of compact bone is the osteon, or Haversian system (Fig. 6.5). Each osteon appears as a cylindrical unit consisting of 3–20 concentric lamellae of the bone matrix surrounding the central Haversian canal that runs parallel to the long axis of the bone. The lamellae can be imagined like paper towels wrapped around a cardboard roll (i.e., osteonal or Haversian canal). The osteonal canal contains the vascular and nerve supply of the osteon.

A second group of canals, called perforating or Volkmann’s, or lateral canals, runs at right angles to the long axis of the bone. These canals provide pathways for the blood vessel and nerve supply to the periosteum inside the osteonal canal. These canals are lined with endosteum.

During bone formation, osteoblasts secrete the bone matrix. Osteoblasts maintain contact with one another via gap junctions (see Fig. 4.33). As the matrix hardens, the osteoblasts become trapped within it, thus forming the lacunae and canaliculi. The osteoblasts become osteocytes, or mature bone cells.

Osteocytes, the spider‐shaped mature bone cells, are found in lacunae, the small cavities at the junctions of the lamellae. Only one osteocyte is found per lacuna, and these cells cannot divide. Numerous processes extend from each osteocyte into little tunnels running through the mineralized matrix called canaliculi, which connect adjacent lacunae. Therefore, a continuous network of canaliculi and lacunae contains osteocytes and their processes running throughout the mineralized bone. Canaliculi are important because they provide a route by which processes from one osteocyte can contact those of adjacent osteocytes. Therefore, via the canalicular system, all osteocytes are potentially in communication with one another. They pass information, nutrients, and/or waste from one place to another.

Osteocytes can synthesize or absorb bone matrix. If the osteocyte dies, bone matrix resorption occurs due to osteoclast activity, which is later followed by repairing or remodeling by osteoblast activity.

While mature compact bone has a lamellar structure in which the fibers run parallel, immature bone, also called woven bone, has a nonlamellar structure. Woven bone is deposited rapidly during growth or repair, and its fibers are aligned at random resulting in reduced strength. Woven bone typically is replaced by lamellar bone as growth continues.

Cancellous or Spongy Bone

Unlike compact bone, spongy bone does not contain osteons but instead consists of an irregular lattice network of bone spicules called trabeculae. In specific regions of certain bones, red bone marrow can be found in the space between the trabeculae. Osteocytes are found in lacunae within the trabeculae, and canaliculi radiate from the lacunae.

Chemical Composition of Bone

Bone consists of both organic and inorganic components. The major inorganic component is calcium phosphate, Ca₃(PO₄)₂. It accounts for two‐thirds of the weight of bone. Calcium phosphate interacts with calcium hydroxide, Ca(OH)₂, to form hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. As crystals of hydroxyapatite form, they also incorporate other inorganic materials including calcium carbonate, sodium, magnesium, and fluoride.

The remaining organic portion of the bone is made up of cells (osteoblasts, osteocytes, and osteoclasts) and osteoid, which includes collagen fibers and ground substance elements (proteoglycans and glycoproteins). Osteoid is secreted by osteoblasts.

Hematopoietic Tissue in Bones

Red bone marrow, which is hematopoietic (i.e., blood‐forming), is found in the spongy bone of long bones and the diploë of flat bones. Red bone marrow consists of mature and immature red blood cells, white blood cells, and associated stem cells. In newborn animals, the medullary cavities of spongy bones contain red bone marrow. In adult long bones, the medullary cavities of spongy bone become large hollow cavities extending into the epiphysis and containing yellow bone marrow. Yellow marrow functions in fat storage and contains mostly fat cells. Therefore, blood cell production in adult long bones is restricted to the head of the femur and humerus. However, if an animal becomes anemic (has very few red blood cells), the yellow marrow can revert to red marrow to supplement red blood cell production. In contrast, the spongy bone found in flat bones, such as those in the hips, remains hematopoietic throughout life and therefore a reliable source there is a need to sample bone marrow.

The osteonal and lateral canals are also the pathways by which blood cells formed in the marrow enter circulation. Because bone marrow sinuses connect with the venous vessels running through these channels, newly formed blood cells are released into osteonal, and lateral canals have a path to enter the general circulation.

Intramembranous Ossification

Early in embryonic development, mesenchymal cells migrate and aggregate in specific regions of the body. Remember, mesenchyme is tissue from which all connective tissue develops. As these cells condense, they form the membrane from which the bone will develop (Fig. 6.6). This presumptive bone site becomes more vascularized with time, and the mesenchymal cells enlarge and become rounder. As the mesenchymal cells change from eosinophilic (i.e., stained shades of red with eosin dyes) to basophilic (affinity for basic or blue dyes), they differentiate into osteoblasts. These cells secrete the collagen and proteoglycans (osteoid) of the bone matrix. As the osteoid is deposited, the osteoblasts become increasingly separated from one another, although they remain connected by thin cytoplasmic processes.

The site where the matrix first begins calcification is called the ossification center. Eventually, as the matrix becomes calcified, the osteoblasts become osteocytes. The osteocytes are contained in canaliculi. Some of the surrounding less differentiated cells proliferate to produce osteoprogenitor cells. These cells migrate to reside adjacent to the spicules and become osteoblasts, thus adding more matrix. This results in appositional growth in which the spicules (areas of calcification extending from the ossification center) enlarge and become joined into a trabecular network mimicking the shape of bone.

Endochondral Ossification

Endochondral ossification begins similarly to intramembranous ossification, with migration and aggregation of mesenchymal cells (Fig. 6.7). However, these cells now become chondroblasts, instead of osteoblasts, and begin making a cartilage matrix. Once made, the cartilage matrix grows by both interstitial and appositional growth. Interstitial growth is responsible for most of the increase in the length of the bone, whereas the increase in width is produced by new chondrocytes that differentiate from the chondrogenic layer of the perichondrium surrounding the cartilage mass.

Formation of mature bony tissue begins when perichondrial cells in the middle of the cartilage model begin to produce osteoblasts rather than chondrocytes. Subsequently, the connective tissue surrounding the middle of the cartilage changes from perichondrium to periosteum. A thin layer of bone begins forming around the cartilage model. This bone is called either periosteal bone because of its location or endochondral bone because of its mechanism of development. This periosteal bone may be called the bony collar.

As the chondrocytes in the mid‐region become hypertrophic, the matrix is compressed. These cells begin to synthesize alkaline phosphatase, and the surrounding matrix begins to calcify. As the chondrocytes die, the matrix breaks down and the neighboring lacunae become interconnected. At the same time, blood vessels begin to enter this diaphyseal area, vascularizing the developing cavity.

Cells from the periosteum migrate inward with the blood vessels and become osteoprogenitor cells. Other cells also enter to give rise to the marrow. The breakdown of the matrix leaves spicules that become lined with osteoprogenitor cells that then differentiate into osteoblasts. Osteoblasts then produce the osteoid on the spicule framework. The bone formed in this manner is called endochondral bone, and this region becomes the primary ossification center. As the cartilage is resorbed, the bone deposited on the calcified spicules becomes spongy bone.

Thereafter, a secondary ossification center develops in each epiphysis. The bone develops in these areas like the process occurring in the primary ossification center. As the secondary ossification develops, the only cartilage remaining is at the ends of the bones, and a transverse region known as the epiphyseal plate separates the diaphyseal and epiphyseal cavities.

As the cavity in the diaphyseal marrow enlarges, a distinct zonation develops in the cartilage at either end of the diaphysis (Fig. 6.8). The following five regions develop beginning most distal from the diaphysis:

1. Zone of reserve cartilage. This region contains no cellular proliferation or matrix production. Small and scattered chondrocytes are present.
   
2. Zone of proliferation. Cartilage cells are divided and organized in distinct columns in this area. The cells are larger than in the reserve zone and are producing a matrix.
   
3. Zone of hypertrophy. Cartilage cells in this region are large with a clear cytoplasm containing glycogen. The matrix is found in columns between the cells.
   
4. Zone of calcified cartilage. Enlarged degenerating cells form this region. The matrix is calcified.
   
5. Zone of resorption. Nearest to the diaphysis, the cartilage in this region is in direct contact with connective tissue in the marrow cavity.

Bone Growth

As bone grows, a continuous internal and external remodeling takes place in the epiphyseal plate. The epiphyseal plate remains constant in size because new cartilage is produced in the zone of proliferation, while a similar amount of cartilage is removed in the zone of resorption due to the action of osteocytes. The resorbed cartilage is replaced by spongy bone, produced by osteoblasts, which is found between the zone of resorption and the diaphysis. As the cells in the proliferative region divide, an increase in the length of the bone occurs as the epiphysis is moved away from the diaphysis.

The width of bone is increased by appositional growth of bone that occurs between the cortical lamellae and the periosteum as bone resorption occurs on the endosteal surface of the outermost region of the bone. As bones elongate, they are constantly remodeling, which involves both resorption of bone and concomitant deposition in other areas.

Eventually, new cartilage production ceases. The cartilage that is present in the epiphyseal plate is converted to bone until no more cartilage exists. This signifies epiphyseal closure and growth of the bone is complete. The only remaining cartilage is at the articular (i.e., regions where bones form joints) surface of the bone. The epiphyseal plate now becomes the epiphyseal line.

The major hormone controlling bone growth in young animals is growth hormone from the anterior pituitary gland. Excessive secretion of growth hormone can cause gigantism, or hyposecretion can cause dwarfism. Thyroid hormones also play an important role in bone development. The action of these hormones is discussed in Chapter 12.

Bone Remodeling and Repair

While bones may appear to be dormant after animals reach adulthood, it is not. In fact, bone remains very active and is constantly being broken down (resorbed) and replaced in response to various physical or hormonal changes. This constant breakdown by osteoclasts and formation by osteoblasts is called remodeling and occurs at both the periosteal and endosteal surfaces.

The breakdown of bone by osteoclasts is termed bone resorption. Resorption begins when osteoclasts bind tightly to the surface of the bone (either the endosteum or periosteum) to create a seal. This sealing effect is important because the osteoclasts release lysosomal enzymes, which digest the collagen and organic matrix while the acid digests the minerals. The tight seal of the osteoclast to the bone surface protects other areas from resorption. The digested components are engulfed by the osteoclasts via endocytosis, packaged into vesicles, translocated across the osteoclast by transcytosis, and released by exocytosis into the interstitial space where the material is absorbed into the capillaries. The canal that is formed establishes the future Haversian system. Eventually, the osteoclasts are replaced by osteoblasts that rebuild the bone.

Repair of Fractures

Fractures can be classified in several ways:

• Bone end alignment. If the bone ends remain aligned following a fracture, it is called a nondisplaced fracture. Displaced fractures occur when the bone ends are out of alignment.

  

  

  
  
• Degree of break. If the break is all the way through the bone, it is termed a complete fracture; if not all the way through, it is an incomplete break.
  
• Orientation of the break. If the break is parallel to the long axis, it is a linear fracture; if it is perpendicular to the long axis, it is a transverse fracture.
  
• Skin penetration. If bone protrudes through the skin, it is an open, or compound, fracture. A nonprotruding break is called a closed, or simple fracture.

The repair process for a fractured bone involves four steps (Fig. 6.10):

1. Hematoma formation. As a result of a fracture, the blood vessels tear causing the formation of a hematoma, a mass of clotted blood, at the fracture site. Bone cells begin to die, and the site shows the classic signs of inflammation, i.e., pain, swelling, redness, and loss of function.
   
2. Fibrocartilaginous callus formation. Capillaries grow into the hematoma, providing an entrance for phagocytic cells to invade and remove the debris. Fibroblasts and osteoblasts migrate into the fractured area from the periosteum and endosteum. The fibroblasts form collagen fibers, which serve to span the space in the break, thus connecting the two ends. Stimulated mesenchymal cells are induced to differentiate into chondroblasts which begin making cartilage matrix. Finally, osteoblasts close to the capillaries begin forming spongy bone; those found further away secrete a bulging cartilaginous matrix. This entire mass, called a fibrocartilaginous callus, spans the fractured area.
   
3. Bony callus formation. Bone trabeculae begin to appear because of the actions of the osteoblasts converting the fibrocartilaginous callus into a bony callus made of spongy (or woven) bone. Bony callus formation continues until the two ends of the bone are rejoined.
   
4. Bone remodeling. Remodeling begins during bony callus formation and continues until the bony callus is complete. Excess material is removed from both the periosteal and endosteal area, and compact bone is formed along the shaft. Normally, the final product has the same shape as the original bone.

Osteomalacia and Rickets

Osteomalacia is a condition in which the bones are insufficiently mineralized. Rickets is a name for the same condition when it is present in prepubertal animals. Although osteoid is produced, calcium salts are not sufficiently deposited, and the bones remain soft. Inadequate calcium or vitamin D in the diet are frequent causes of osteomalacia. Vitamin D is necessary for intestinal absorption of Ca²⁺. When blood calcium levels decrease, due to inadequate intestinal absorption, PTH maintains plasma Ca²⁺ by stimulating the release of Ca²⁺ from the bone.

Parturient Paresis (Milk Fever)

As dairy cows begin milk production, the first milk produced (colostrum) contains high concentrations of Ca²⁺. Colostrum requires approximately 3 g of calcium per hour to produce. When a cow cannot mobilize this amount of calcium, she can develop milk fever within 72 hours. Following parturition. Symptoms include loss of appetite, followed by muscle weakness, decreased body temperature, labored breathing, and paralysis of the hind legs. If left untreated, the cow can collapse into a coma and die.

To prevent milk fever, cows should be given sufficient dietary vitamin D prior to parturition. If milk fever develops, cows are given oral or intravenous calcium, according to individual severity.

Egg‐Laying Fatigue in Birds

Like milk fever in cows, high‐producing egg‐laying hens can develop weak and brittle bones. A hen must deposit as much as 8–10% of her total calcium into the eggshell each day. Because the eggshell is deposited during the night, the hen must draw upon the calcium reserves located in a specialized type of bone called medullary bone. Under the influence of estrogens and androgens secreted from the developing ovarian follicles, medullary bone is produced in hens 2 weeks prior to the commencement of egg laying. As blood calcium levels decrease during eggshell formation, the hen releases PTH, which mobilizes bone calcium. If insufficient stores of calcium are present in the bones, the bones become weaker as they become demineralized.

Markings on Bones

The surface of the bone is seldom smooth. Instead, various depressions, bumps, and ridges serve as sites where muscles and tendons originate or attach, and blood vessels and nerves travel. These various markings are outlined in Table 6.1 and illustrated in Figure 6.12. Learning these terms is helpful when studying the origins and insertions of muscles.

Skeleton

The skeleton includes all the bones of the body. These bones, and their articulations, have been altered during evolution to accommodate various functions. Consequently, the skeleton is an excellent example of the complementary nature of form and function. The theme of structure reflecting function appears yet again. The skeletons of various species are shown in Figure 6.13. Most of the remainder of the chapter is concerned with mammals, however, a brief discussion of features unique to avian species is included at the end of the section on the skeleton.

Term	Description	Example
Projections, depressions, and openings where muscles and ligaments attach
Crest	Narrow ridge of bone; usually prominent	Iliac crest
Epicondyle	Raised area on or above a condyle	Lateral epicondyle of the humerus
Fossa	Shallow depression, often serving as an articular surface	Olecranon and radial fossae of the humerus
Line	Narrow ridge of bone; less prominent than a crest	Gluteal line on wing of ilium
Process	Generally, any bony prominence; sometimes used to name specific prominences	Crest, spine, trochanter, tubercle, tuberosity, etc.; olecranon process
Ramus	Arm‐like bar of bone	Ramus of the mandible
Spine	Sharp, slender, often pointed projection	Spine of the scapula
Tuberosity	Large, rounded projection	Deltoid tuberosity of the humerus
Trochanter	Very large, blunt, irregular‐shaped process; found only on the femur	Trochanter of the femur
Tubercle	Small‐rounded projection or process	Greater tubercle of the humerus
Projections that help form joints
Condyle	Rounded articular projection	Occipital condyle of the skull
Cotyloid	A deep articular depression	Acetabulum of the hip joint
Facet	Smooth, nearly flat articular surface	Superior costal facet of the vertebrae
Head	Bony expansion carried on a narrow neck	Head of the femur
Trochlea	A pulley‐shaped, articular structure	Trochlea of the femur
Depressions and openings, allowing blood vessels and nerves to pass
Fissure	Narrow, slit‐like opening	Palatine fissure
Foramen	Round or oval opening through a bone	Foramen magnum
Fovea	A shallow, nonarticular depression	Fovea capitis on the head of the femur
Incisure	A notch‐shaped depression at the edge of a bone	Semilunar notch of the ulna
Meatus	Canal‐like passageway	External auditory meatus
Sinus	Cavity within a bone, filled with air and lined with mucous membrane	Nasal sinuses
Sulcus	Furrow‐like groove	Brachial groove of the humerus

Functions of the Skeletal System

The skeleton has five primary functions:

1. Support. The skeletal system provides the structure to which the bones attach, as well as the structural support for the entire body.
   
2. Storage of minerals and lipids. The bones provide a major storage for various minerals, particularly calcium. In addition, the bones contain a substantial amount of lipids.
   
3. Blood cell production. The bone marrow is a site of formation for all types of blood cells.
   
4. Protection. The vital organs of the body are protected by the skeletal system. The ribs surround the visceral organs, whereas the central nervous system is encased within the skull and spinal cord.
   
5. Leverage. Many of the joints of the body act as levers therefore assisting with movement.

Skeletal Cartilage

Growth of Cartilage

Cartilage can continue to grow through two processes. Appositional growth occurs when new cartilage forms on the surface of preexisting cartilage. Interstitial growth occurs from inside of the cartilage mass in which lacunae‐bound chondrocytes inside the cartilage divide and secrete new matrix, thereby expanding the cartilage from within.

Skeleton Classification

The skeleton is divided into the appendicular skeleton and the axial skeleton (Fig. 6.14). The axial skeleton includes the skull, hyoid apparatus, vertebral column, ribs, and sternum. The appendicular skeleton includes the bones of the limbs and limb girdles. The thoracic limb or pectoral limb includes the scapula, humerus, radius, ulna, carpal bones, MC bones, phalanges, and sesamoid bones. The thoracic girdle or shoulder girdle includes the two scapulae, and the clavicle in man, which holds the shoulder laterally, but which is only vestigial in domestic animals.

The Skull

The skull is a very complex structure made mostly of flat bones. Except for the mandible that is attached via a movable joint, the bones of the skull are connected by interlocking joints called sutures. The suture joints are characterized by a saw‐toothed or serrated appearance that keeps the bones attached but allows the cranium to expand and contract while remaining intact.

Suture lines are visible between the bones of the skull (Fig. 6.15). The internasal suture is between the two nasal bones while the frontonasal suture separates the frontal bones from the nasal bones. The frontoparietal suture separates the frontal bones from the parietal bones. The nasomaxillary suture separates the nasal bones from the maxillary bones.

The skull contains both cranial and facial bones (Table 6.2). The cranium includes those bones that surround the brain. The cranium consists of the cranial vault, also called the calvaria, forming the superior, lateral, and posterior aspects of the skull, and the cranial base or floor that forms the inferior aspect of the cranium. The cranial base is divided by bony ridges into three distinct fossae: the anterior, middle, and posterior cranial fossa. The cranial bones form the cranial cavity that houses the brain and provide the site for attachment of head and neck muscles.

The skull contains approximately 85 named openings, including foramina, canals, fissures, and orbits. These provide passageways for the spinal cord, blood vessels, and the 12 cranial nerves to enter and leave the brain.

Cranium

The roof of the cranium is formed by the paired frontal and parietal bones (Fig. 6.16). The caudal aspect of the skull is formed by the unpaired occipital bone. The floor of the cranium is formed by the unpaired sphenoid bone. Finally, the rostral wall of the cranium is formed by the ethmoid bone.

The facial bones include those bones enclosing the nasal and oral cavities. These bones form the structure of the face; contain cavities for special senses, including sight, taste, and smell; provide openings for air and food; secure teeth; and provide attachment sites for facial muscles. The facial region is divided into the oral, nasal, and orbital regions.

Term	Description
Cranial bones (number)
Frontal (2)	The rostral portion of the roof of the cranial cavity in most domestic species; in the ox, it forms the entire roof of the cranial cavity
Parietal (2)	Along with the frontal, forms the roof of the cranial cavity in most domestic animals except ox
Occipital (1)	Forms caudal aspect of the cranial cavity, as well as the skull
Temporal (2)	Forms caudolateral wall of the cranial cavity
Sphenoid (1)	Unpaired bone forms the floor of the cranial cavity; it has several parts, including the body, greater wings, lesser wings, and pterygoid processes
Ethmoid (1)	Unpaired bone forms the rostral wall of the cranial cavity; forms part of the nasal septum, caudal wall of the nasal cavity, and part of the medial wall of the orbit
Facial bones (number)
Mandible (1)	The lower jaw
Maxilla (2)	Form the upper jaw, and parts of the hard palate, orbits, and nasal cavity
Zygomatic (2)	Cranial portion of zygomatic arch; forms part of the cheek and orbit
Nasal (2)	Along with the cranial portion of the frontal bone, forms the osseous roof of the nasal cavity
Lacrimal (2)	Forms medial surface of orbit
Palatine (2)	Forms part of the hard palate along with maxillary and incisive bones
Vomer (1)	Unpaired bone forming part of osseous nasal septum
Ventral nasal concha (2)	A fragile scroll of bone that increases the nasal surface area
Pterygoid (2)	Small bones in the caudal part of the nasopharynx
Incisive or premaxillary (1)	Holds upper incisors

The oral region is formed by parts of the incisive, maxillary, and palatine bones, as well as the mandible surrounding the oral cavity. The nasal region is formed by portions of the nasal, maxillary, palatine, and incisive bones that surround the nasal cavity. The orbital region consists of the bony socket holding the eye and is formed by portions of the frontal, lacrimal, palatine, sphenoid, and zygomatic bones. The zygomatic arch, which forms the ventral wall of the orbit, consists of the zygomatic bone and the zygomatic process of the temporal bones. An exploded view of the equine skull is shown in Figure 6.17.

Species Differences

Unique to the horse and cat, the interparietal bone is found between the two parietal bones. In other species, this bone is present in the fetus but fuses with surrounding bones during gestation. In the ox, the frontal bone forms the entire roof of the cranium, whereas the parietal bones help form the roof in other species.

In the dog, three types of skulls are described based on the variations in the facial bones and cranial cavity (Fig. 6.18).

1. Mesaticephalic. Average conformation. For example, Siberian husky.
   
2. Dolichephalic. Has an elongated facial component. For example, collie.
   
3. Brachiocephalic. Has a shorter facial component. Example, bulldog.

The Vertebral Column

The vertebral or spinal column, also colloquially (though incorrectly) called the spine (a spine is a pointed projection from any bone), protects the spinal cord, supports the head, and serves as an attachment site for muscles affecting body movements. The bony column consists of irregular bones connected by joints that are characterized by different degrees of mobility in different regions of the column.

The vertebrae (sing. = vertebra) are the irregularly shaped bones making up the vertebral column. Five general regions are described: cervical (neck), thoracic (back), lumbar (loin), sacral (croup), and caudal (tail) vertebrae. Each is named by the first letter of the group followed by the number within the group, for example, C1, T3, L5, S3, and Ca20. The number of vertebrae typically present in various species is shown in Table 6.3.

Typical vertebrae are shown in Figure 6.19. Features common to all vertebrae include the body, vertebral arch, vertebral foramen, and processes. The body is the thick, spool‐shaped ventral portion of the vertebra. The vertebral body is convex at the cranial end and concave at the caudal end, allowing articulation with the adjacent vertebrae. The vertebral arch is the dorsal portion of the vertebra consisting of two upright pedicles that form the wall of the vertebral foramen. Paired (right and left) half‐ (or hemi‐) laminae project from the pedicles toward the back, and meet in the midline to complete the lamina, form the roof of the vertebral foramen. In the articulated vertebral column, consecutive vertebral foramen completes the vertebral canal. In the intact state, the spinal cord passes through the vertebral canal.

Within each region, the common vertebral features are modified to optimize the ability of each vertebral region to accomplish its function. Regional vertebrae are characterized by features that are more like each other than adjacent regions. However, the transition from one region to the next is gradual, with the last vertebra of a given region bearing a strong resemblance to the first vertebra of the following region.

Potentially, seven processes arise from each vertebra. The dorsal spinous process projects vertically toward the back, and the paired transverse processes extend laterally from each side of the vertebral body. Two pairs (right and left) of cranial and right and left caudal articular processes project from a region‐specific area of the lamina to allow adjacent vertebrae to articulate with each other at a second site (the first is at the cranial and caudal articular surfaces of each vertebral body, thus simultaneously allowing greater flexibility and stability than could otherwise be achieved).

The first and second cervical vertebrae are very highly specialized and are unique in form from each other and from any other vertebra. Only these two vertebrae are given specific individual names (Fig. 6.20). The atlas (C1) supports the head, hence, its name (like the mythological Atlas, who carried the Earth upon his shoulders). Cranially, the atlas articulates with the occipital condyles of the skull (see Fig 6.17) forming the atlanto‐occipital joint. Motion at this joint is a back‐and‐forth rocking motion, which allows the neck to flex and extend as in nodding “yes.” The atlas is unique in that its spinous process is reduced to a bump (the dorsal tubercle), and the body is modified into the ventral arch. The axis (C2) possesses a large ridge‐like spinous process. The dens a heavy, peg‐like cranially directed process that forms a pivoting articulation with the atlas and allows a twisting motion of the head on the neck, as when shaking the head “no.” The dens are developmentally derived from a part of the atlas.

Thoracic vertebrae are generally characterized by robust spinous processes. The anticlinal vertebra is the one with the most vertically oriented dorsal process. Cranial to this vertebra, the dorsal processes are inclined cranially while those caudal to this vertebra are inclined caudally. The anticlinal vertebra is an important landmark when reading radiographs. Their bodies possess an articular facet at the cranial and caudal end for articulation with the ribs.

Lumbar vertebrae are characterized by their massive bodies, shorter spinous processes, and long, flat transverse processes. These vertebrae also lack costal facets because ribs do not articulate with them.

The sacral vertebrae fuse in adult individuals to form the sacrum. Each sacral vertebra has dorsal and ventral foramina allowing the passage of spinal nerves. The wings of the sacrum (Fig. 6.19) articulate with the ilium of the pelvis forming the sacroiliac joint. This is the only site of connection between the axial skeleton and pelvic limb.

Thorax

The thorax is the bony cavity formed by the sternum, ribs, costal cartilage, and bodies of the thoracic vertebrae (Fig. 6.21). The sternum, or breastbone, is the composite of the unpaired bones (sternebrae) forming the floor of the thorax. Species‐specific numbers of sternebrae are as follows: six in pigs, horses, and humans; seven in ruminants; and eight in carnivores. The manubrium is the enlarged first sternebra, while the xiphoid process is the last sternebra capped by the xiphoid cartilage. The thoracic inlet is the entrance into the bony thoracic cavity as delineated by the last cervical vertebra, the first pair of ribs, and the sternum.

The ribs consist of long, curved bones that form the lateral wall of the thorax. The ribs can be grouped as follows:

1. Sternal (sometimes called “true”) ribs. Articulate directly to the sternum via their costal cartilage. Costal cartilage consists of hyaline cartilage.
   
2. Asternal (sometimes called “false”) ribs. Costal cartilages merge to form the costal arch, which indirectly joins them to the sternum in all domestic species.
   
3. Floating ribs. End in short costal cartilages that join neither the sternum nor the costal arch, but end “blindly” in the flank musculature. One pair of floating ribs is present in dogs and cats, two pairs in humans and cattle, but none in horses.

As shown in Figure 6.22, the proximal part of each rib consists of a head and a tubercle. The head articulates with the caudal and cranial costal fovea of adjacent thoracic vertebrae and the intervertebral disc found in between. The tubercle of the rib articulates with the transverse process of the same numbered vertebra. Between each rib is the intercostal space.

Thoracic Limb

In humans, the clavicle braces the shoulder against the sternum, which facilitates climbing and other similar behaviors. In domestic mammals, the thoracic limb bears weight and has a much more restricted range of motion, and the clavicle is vestigial. The proximal end of the thoracic limb begins at the scapula. This “shoulder blade” is a flat, triangular bone in the shoulder region (Fig. 6.23). Together the two scapulae constitute the thoracic girdle.

The lateral surface of the scapula presents the spine of the scapula that ends in the acromion, the expanded distal end of the spine of the scapula. The acromion is absent in the horse. The area cranial to the spine is the supraspinous fossa; the area caudal to it is the infraspinous fossa. Most of the medial surface of the scapula is called the subscapular fossa. On the dorsal border of the scapula is the scapular cartilage. On the opposite end of the bone, the cavity in which the humerus articulates is the glenoid cavity. The supraglenoid tubercle, located near the cranial aspect of the glenoid cavity, is the site of proximal attachment (origin) of the biceps brachii muscle. The coracoid process (Greek for “crow like”) is a small process on the medial side of the supraglenoid tubercle where the coracobrachialis muscle arises. Found only in cats, the suprahamate process is a caudal projection from the acromion.

The humerus, sometimes called the brachial bone, is the largest bone in the thoracic limb (Fig. 6.24). Proximately, the humerus articulates with the scapula at the glenoid cavity thereby forming the shoulder joint. Distally the humerus articulates with both the radius and ulna contributing to the elbow joint.

The head of the humerus is a rounded process articulating with the glenoid cavity. The greater (lateral, major) tubercle is the large process craniolateral to the head and can be palpated as the point of the shoulder. The lesser (medial, minor) tubercle is located on the medial side of the head. The bicipital, or intertubercular, groove is a sulcus between the greater and lesser tubercles through which the tendon of the biceps brachii muscle passes. The body of the humerus connects the two epiphyses of the bone. The laterally positioned deltoid tuberosity, to which the deltoid muscles attach, is the largest tuberosity on the bone. The distal end of the bone is called the humeral condyle and includes the humeral capitulum and humeral trochlea, which are the two articulating surfaces, two fossae, and the medial and lateral epicondyles. The olecranon fossa is a groove on the caudal surface of the distal end of the humerus in which the olecranon process of the ulna rests. The radial fossa, opposite the olecranon fossa, receives the proximal end of the radius while the elbow is flexed. In the dog, and sometimes the pig, the supratrochlear foramen, is a hole that connects the olecranon and radial fossa. Nothing passes through the supratrochlear foramen. In cats only, the supracondylar foramen lies on the lateral surface just proximal to the condyle; the median nerve and the brachial vessels pass through this hole.

The radius is the weight‐bearing bone of the forearm (Fig. 6.25). Proximally, the radius articulates with the humerus and ulna at the elbow (the cubital joint); distally the radial articulations are with the carpal bones and ulna forming the antebrachiocarpal joint. The head of the radius articulates with the capitulum of the humerus and the ulna. The (medial) styloid process lies on the distal end of the radius.

The ulna functions mainly as a site for muscle attachments and the formation of the elbow. Proximately, it articulates with the humerus and radius, and distally with the radius and carpal bones. The proximal end of the ulna is called the olecranon process, the point of the elbow, where the extensor muscles of the elbow attach. The trochlear notch is the crescent‐shaped articulation site with the humerus. The distal end of the ulna also ends in the styloid process, referred to as the lateral styloid process to distinguish it from the radial styloid process.

The radius and ulna fuse in the horse and ruminants. Because they are fused, these animals cannot supinate or pronate (rotate the wrist), and the mannus (hand of humans) is committed to permanent pronation (palm‐down position). In contrast, these bones are not fused in carnivores; therefore, these animals can at least partially supinate (turn upward) their paws.

Premature closing of the GPs in the radius or ulna can cause deviations in these bones, resulting in valgus or vargus deviations. Valgus is a lateral deviation distal to a joint and vargus is a medial deviation distal to a joint. For example, carpus valgus or carpus vargus are lateral and medial deviations distal to the carpus. Carpal valgus, a lateral deviation of the joints distal to the carpus, is also called “knock‐knee”; carpal vargus, a medial deviation of the bones distal to the carpus, is called “bowlegged.”

The distal portion of the thoracic limb is technically the manus (hand), commonly called the forepaw in carnivores (Fig. 6.26). The manus consists of the carpus, metacarpus, and digits. Each digit is a complete toe, including two or three separate bones called sesamoid bones that are associated with the joints between each MC bone and digit, as well as some of the joints between phalanges (interphalangeal joints).

The carpus, the wrist of man, consists of two transverse rows of carpal bones. The number of carpal bones varies among species. Although the pig and horse have eight carpal bones, the first carpal bone in the distal row is often absent in the horse. Dogs and cats have seven carpal bones due to the fusion of two of the carpal bones in the proximal row. Ruminants have six carpal bones because the first carpal bone is absent, the second and third are fused.

The MC bones are located between the carpus and digits (toes). Potentially, five are present and numbered IV from medial to lateral. Species differ in the number and relative size of MC bones due to the absence or fusion of these bones as related to which are weight‐bearing. Generally, paired proximal sesamoid bones are present on the palmar (ground‐facing) surface at the metacarpophalangeal joint (junction of MC bone and its associated digit). In dogs, only a single sesamoid bone is present for the dewclaw.

Carnivores have five MC bones because all five toes are present. The first MC bone is much shorter than the others because digit I (the “dewclaw”) is not weight bearing.

The pig has four MC bones because MC I is absent. The third and fourth MC bones are weight‐bearing and therefore the longest, while the second and fifth are somewhat shorter and nonweight‐bearing.

The horse has three MC bones, with MC I and V being absent. The second and fourth MC bones are commonly called splint bones because they are greatly reduced in size and end proximal to the digit. The rounded distal ends of each splint bone are called the buttons of the splints. The large, heavy third MC is called the cannon bone.

The digits of the forelimb correspond with the fingers of man. Like the MC bones (which support the digits), potentially five digits are present and again are numbered from medial to lateral. Their numbers correspond to their supporting MC bones. However, the number of digits present varies by species.

Each digit contains multiple bones called phalanges. The first digit (when present) possesses two phalanges, while the remaining digits possess three. The phalanges are named proximal phalanx, middle phalanx, and distal phalanx.

Carnivores possess weight‐bearing digits. As with MC I the first (nonweight‐bearing) digit is reduced in size. Pigs possess digits II–V but bear weight only on III and IV. Superficially, ruminants seem to have four digits because they have four hooves, but only have two digits (III and IV) because no bony elements are associated with the lateral‐and‐medial hooves (which are referred to as “claws”). Horses possess only digit III corresponding with the MC III.

Pelvic Limb

The pelvic girdle, or bony pelvis, consists of the two hip bones (ossa coxarum), the sacrum, and the first few caudal vertebrae (Fig. 6.27). The pelvic cavity is the internal space defined by the bony pelvis. Each hip bone (os coxae) consists of the fused ilium, ischium, pubic, and acetabular bones. The acetabulum is the site where the head of the femur articulates. It is formed by the fusion of the ilium, ischium, pubic, and acetabular bones. The small acetabular bone lies in the center of the acetabulum, and in the adult is fused with the other bones. The two hip bones are fused on the ventral midline at the pelvic symphysis. This fusion includes the two pubic and two ischial bones.

The ilium is the largest and most cranial of the os coxae, consisting of a wing and body. The ilium forms the cranial part of the acetabulum and articulates with the sacrum at the sacroiliac joint. In carnivores, the wing of the ilium is relatively unspecialized, and its broad plane is oriented parallel to the longitudinal axis of the body. In large animals, the iliac wing is expanded and twisted so that the broad plane is oriented almost transverse to the longitudinal axis of the body. The modifications relate to increasing surface area for attachment of the powerful muscles of the hindlimb in these larger heavier animals. The tuber coxae are the more lateral prominences of the ileac wings. Because the tuber coxae are particularly prominent in cattle, it is commonly referred to as the “hook.” The tuber sacrale is the medial process of the wing that articulates with the sacrum. The ischium is the caudal‐most portion of the os coxae and forms the lateral portions of the obturator foramen, the large opening in the floor of each ox coxae. The ischiatic tuberosity is the caudal most part of the ischium and is quite prominent. The ischiatic tuber is referred to as the “sit bones” in humans and carnivores and as the “pin bone” in cattle. The pubis forms the cranioventral part of the os coxae. The pubis consists of a central body and two branches. The more medial branch contributes to the obturator for humans, while the more lateral branch contributes to the acetabulum.

The femur, or thigh bone, articulates proximally with the os coxae at the acetabulum, forming the hip joint, distally with the tibia forming the stifle (true knee) joint (Fig. 6.28). A small depression on the medial surface of the head of the femur, the fovea capitis, provides attachment for the round ligament of the femur. This ligament inserts in the acetabular fossa, anchoring the head of the femur into the acetabulum. The head of the femur continues with the body of the femoral by the neck. The greater trochanter is the large prominence found lateral to the head of the femur and the lesser trochanter is the smaller prominence found distal to the head on the medial side. In horses, a prominent third trochanter also lies on the lateral side, distal to the greater trochanter. Note that trochanters are unique to the femur. The medial and lateral condyles are the two large, rounded prominences at the distal end of the femur, and articulate with the tibia. Also on the distal end of the femur is the patellar surface, a groove bordered by two ridges that articulates with the patella. The patella, or kneecap, is the largest sesamoid bone of the body.

The tibia and fibula, or bones of the “lower leg” (crus), are located between the femur and metatarsal bones (Fig. 6.29). The tibia, or shin bone, is located medially and is the weight‐bearing bone of the crus. Located at the proximal end of the tibia are the medial and lateral condyles, separated by the intercondylar eminence. The tibia condyles articulate with the corresponding condyles of the femur. The fibula is located more laterally and is not weight‐bearing. Distally, the fibula articulates with the tibia and the fibular tarsal bone. The distal fibula in cattle is represented by the separate malleolar bone.

The tarsus, or “hock”, consists of the three rows of bones between the crus and metatarsal region (Fig. 6.30). Like the carpus, this region is characterized by multiple bones arranged in multiple rows. However, the hock has a more complex arrangement than the carpus, with a proximal row, a sort of intermediate bone, and then a distal row. Again, marked species variation occurs in the number of individual bones present. In all species, the proximal row consists of two bones. Beyond that, generality, marked variations occur among species in fusions among different bones and the resulting number of separate bones. To simplify, carnivores and pigs have seven tarsal bones, horses have six, and ruminants have five separate tarsal bones.

In all species, one of the two largest bones of the tarsus is the talus or tibial tarsal bone. The talus is located dorsomedially and articulates with the tibia and fibula or its equivalent via its trochlea. The calcaneus or fibular tarsal bone is the other bone in the proximal row, just lateral to the talus. The calcanean tuberosity is a large process of the calcaneus acting as a lever for the common calcanean (Achilles) tendon and is commonly called the point of the hock.

Metatarsal bones and digits are located at the distal end of the tarsus. In the horse and pig, they follow the same pattern as the thoracic limb. In carnivores, the first metatarsal bone is greatly reduced, and the digit is usually absent. Exceptions exist among certain breeds of dog, in which the dewclaw may be present or even doubled. Certain breed standards require the presence of hind dewclaws. Usually, these rear dewclaws are like the dewclaws of cattle, in that they contain only cutaneous [skin‐related] and no bony elements. In ruminants, the first and fifth metatarsal bones are absent, and the second is reduced to a tiny element.

The digits of the pelvic limb follow the same pattern as in the thoracic limb: each weight‐bearing digit is composed of three phalanges, with paired sesamoid bones on the plantar aspect of the metacarpophalangeal joint and single distal sesamoid on the plantar aspect of the distal interphalangeal joint.

Anatomical Types of Joints

Arthrology is the study of joints. Joints are necessary to allow for the movement of the skeleton. By their structure, joints can be classified in several ways according to (1) the number of articulating bones, (2) structural classification, and (3) functional classification:

1. Number of articulating bones. A simple joint has only two articulating bones forming the joint, but a compound joint has three or more articulating bones.
   
2. Structural classification. Joints can be classified by the medium holding the joint together (Table 6.4):

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Structural Class	Characteristics	Type	Mobility	Example
   Fibrous	End of bones united by fibrous tissue	1) Sutures	Immobile (synarthrosis)	Bones of the cranium
   		2) Syndesmosis	Slightly mobile (amphiarthrosis) and immobile	Distal tibiofibular joint
   		3) Gomphosis	Immobile	Articulation of a tooth with its socket
   Cartilaginous	End of bones united by cartilage	1) Synchondrosis (hyaline cartilage)	Immobile	Epiphyseal plates
   		2) Symphysis (fibrocartilage)	Slightly movable	Pubic symphysis
   Synovial	Ends of bones covered with articular cartilage, and a joint cavity enclosed with a joint capsule	1) Ball‐and‐socket	Freely movable	Coxofemoral (hip) joint and glenohumeral (shoulder) joint
   		2) Pivot	Rounded end of one bone projected into sleeve or ring on another bone; freely movable but allows only uniaxial rotation	Between atlas and dens of axis; proximal radioulnar joint in animals where pronation and supination possible
   		3) Ellipsoidal	Both articulating surfaces are oval; freely movable allowing flexion, extension, abduction, adduction, and circumduction	Radiocarpal joints
   		4) Saddle	Each articulating surface has both concave and convex areas, resembling a saddle; freely movable	Carpometacarpal joint of the thumb in human
   		5) Plane (or gliding)	Articulating surfaces ∼flat; freely movable, but only slipping or gliding motions	Intercarpal and intertarsal joints; vertebral processes
   		6) Hinge	Cylindrical projection of one bone into trough‐like depression of another	Knee, elbow, and interphalangeal joints

   
   
   
   
   1. Fibrous joints—fibrous tissue between bones holds the bones together but allows little or no movement; consequently, there is no joint capsule (Fig. 6.32). These joints usually ossify later. According to details of formation, fibrous joints are classified as sutures, syndesmoses, or gomphoses.
      
   2. Cartilaginous joints—fibrocartilage, hyaline cartilage, or both hold the joint together (Fig. 6.33). These joints only allow slight movement, so like fibrous joints, they lack a joint capsule. Two types of cartilaginous joints are synchondroses and symphyses. The best examples of synchondroses are the epiphyseal plates in long bones, which eventually close, and the joint between the first rib and the manubrium. An example of a symphysis joint is the pubic symphysis.
      
   3. Synovial joints—the articular surfaces of two hyaline cartilage‐covered bones are joined by a synovial joint capsule and are freely movable. The structure and types of synovial joints are discussed below.
   
   
3. Functional classification. This indicates the degree of mobility in the joint:
   
   
   
   1. Synarthrotic. Movements in these joints are absent or extremely limited. Examples of these joints include the sutures in the cranium.
      
   2. Amphiarthrotic. There is slight movement in these joints. Examples include the intervertebral joints of sternoclavicular joints.
      
   3. Diarthrotic. Also called synovial joints, these joints have considerable movement. They allow for one‐, two‐, or three‐dimensional movement, and contain articular cartilage and synovial membranes. Many such joints also contain bursae sacs. Examples include shoulder, knee, wrist, and elbow.

Synovial Joints

Intervertebral Articulations

The intervertebral articulations consist of cartilaginous and synovial joints. The cartilaginous joints are formed by the intervertebral discs joining the bodies of the vertebrae. The synovial joints are formed by the caudal and cranial articular processes of the adjacent vertebrae.

The first two joints within the vertebral column are atypical. The first, the atlanto‐occipital joint, is a modified hinge type of synovial joint between the occipital condyles and the cranial articular surfaces of the atlas (i.e., the first vertebral vertebra). This joint has a spacious joint capsule and is specialized to allow a “yes” motion. The atlantoaxial joint is a pivot type of synovial joint. It is between the dens of the axis and the cranial articulation surfaces on the atlas.