Skeletal system

Chapter 3 Skeletal system

KEY POINTS

• The skeletal system provides a supporting framework for the body, a firm base to which the muscles of locomotion are attached, and protects the softer tissues enclosed within the framework.
• The skeleton can be considered to be made up of three parts:
• Axial skeleton, forming the central axis of the animal and comprising the skull, vertebral column and the rib cage
• Appendicular skeleton, comprising the fore and hind limbs and the limb girdles which attach them to the body
• Splanchnic skeleton, which in the dog and cat consists only of the os penis found within the soft tissues of the penis.
• Each part of the skeleton consists of many bones, each of which plays an important part in the function of the skeletal system.
• Bones are covered in ‘lumps, bumps and holes’. Each has a specific descriptive name and a function, which contributes to movement, i.e. muscle attachment, or to maintaining the health of the tissue, i.e. blood or nerve supply.
• Bones are linked together by means of joints.
• Joints can be classified according to the type of tissue from which they are made, e.g. fibrous, cartilaginous or synovial joints, or according to the type of movement they allow, e.g. hinge or gliding joints.

The skeletal system is the ‘framework’ upon which the body is built – it provides support, protection and enables the animal to move (Fig. 3.1). The joints are considered to be an integral part of the skeleton. The skeletal system is made of the specialised connective tissues, bone and cartilage.

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Fig. 3.1 The skeleton of the dog showing the major bones.

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 102.)

The functions of the skeletal system are:

1 Support – it acts as an internal ‘scaffold’ upon which the body is built
2 Locomotion – it provides attachment for muscles, which operate a system of levers, i.e. the bones, to bring about movement
3 Protection – it protects the underlying soft parts of the body, e.g. the brain is encased in the protective bony cranium of the skull
4 Storage – it acts as a store for the essential minerals calcium and phosphate
5 Haemopoiesis – haemopoietic tissue forming the bone marrow manufactures the blood cells.

Bone structure and function

Bone shape

Bones can be categorised according to their shape:

• Long bones – these are typical of the limb bones, e.g. femur, humerus, and also include bones of the metacarpus/metatarsus and phalanges; long bones have a shaft containing a medullary cavity filled with bone marrow (Fig. 3.2)
• Flat bones – these have an outer layer of compact bone with a layer of cancellous or spongy bone inside; there is no medullary cavity, e.g. flat bones of the skull, scapula and ribs
• Short bones – these have an outer layer of compact bone with a core of cancellous bone and no medullary cavity, e.g. carpal and tarsal bones
• Irregular bones – these have a similar structure to short bones but a less uniform shape; they lie in the midline and are unpaired, e.g. vertebrae.
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Fig. 3.2 Structure of a typical long bone.

Some specialised types of bone are:

• Sesamoid bones – these are sesame-seed-shaped bones that develop within a tendon (and occasionally a ligament) that runs over an underlying bony prominence; they serve to change the angle at which the tendon passes over the bone and thus reduce ‘wear and tear’, e.g. the patella associated with the stifle joint (Fig. 3.1)
• Pneumatic bones – these contain air filled spaces known as sinuses that have the effect of reducing the weight of the bone, e.g. maxillary and frontal bones
• Splanchnic bone – this is bone that develops in a soft organ and is unattached to the rest of the skeleton, e.g. the os penis (the bone within the penis of the dog and cat).

Development of bone

The process by which bone is formed is called ossification and there are two types: intramembranous and endochondral ossification. The cells responsible for laying down new bone are called osteoblasts; the cells that destroy or remodel bone are called osteoclasts.

• Intramembranous ossification – this is the process by which the flat bones of the skull are formed. The osteoblasts lay down bone between two layers of fibrous connective tissue. There is no cartilage template.
• Endochondral ossification – this type of ossification involves the replacement of a hyaline cartilage model within the embryo by bone. The process starts in the developing embryo but is not completed fully until the animal has reached maturity and growth has ceased. The long bones of the limb develop by this method.

Endochondral ossification

The process of endochondral ossification (Fig. 3.3) is as follows:

1 A cartilage model develops within the embryo.
2 Primary centres of ossification appear in the diaphysis or shaft of the bone. The cartilage is replaced as the osteoblasts lay down bone, which gradually extends towards the ends of the bone.
3 Secondary centres of ossification appear in the epiphyses or ends of the bone, continuing the bone development.
4 Osteoclasts then start to remove bone from the centre of the diaphysis to form the marrow cavity, while the osteoblasts continue to lay down bone in the outer edges.
5 Between the diaphysis and epiphyses a narrow band of cartilage persists. This is the growth plate or epiphyseal plate, which allows the bone to lengthen while the animal is growing. Eventually, when the animal has reached its final size, this will be replaced by bone and growth will no longer be possible. The epiphyseal plate is then said to have ‘closed’ and the time at which it happens is different for each type of bone.
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Fig. 3.3 Stages of endochondral ossification. A A cartilage model of the bone exists in the embryo. B Ossification begins from the primary ossification centre in the shaft (diaphysis). C Secondary ossification centres appear in the ends of the bone (epiphyses). D Ossification continues in primary and secondary centres; osteoclasts start to break down bone in the shaft, creating the marrow cavity. E The first growth plate fuses and growth is now only possible at the proximal growth plate; the medullary cavity extends into the epiphysis. F The proximal growth plate fuses and bone growth ceases.

Rickets is a disease of young growing animals caused by a nutritional deficiency of vitamin D or phosphorus. The bones fail to calcify and become bowed, and the joints appear swollen because of enlargement of the epiphyses. Any animal kept permanently inside is at risk of developing rickets because vitamin D is formed by the action of ultraviolet light on the skin.

The skeleton

The skeleton (Fig. 3.1) can be divided into three parts.

1 Axial skeleton – runs from the skull to the tip of the tail and includes the skull, mandible, vertebrae and also the sternum
2 Appendicular skeleton – the pectoral (front) and pelvic (hind) limbs and the shoulder and pelvic girdles that attach (or append) them to the body
3 Splanchnic skeleton – in the dog and cat, this is represented by the os penis within the tissue of the penis.

When studying the anatomy of the skeletal system it is helpful to understand the terms that are used to describe the various projections, passages and depressions that are found on and within bones.

• Tuberosity/trochanter/tubercle – protuberances on bones, which are usually for the attachment of muscles
• Trochlea – bony structures through or over which tendons pass; they are usually grooves in the bone and allow tendons to act as pulleys
• Condyle – a rounded projection on a bone, usually for articulation with another bone
• Epicondyle – a projection of bone on the lateral edge above its condyle
• Foramen (pl. foramina) – an opening or passage into or through a bone, e.g. to allow the passage of blood vessels and nerves
• Fossa – a hollow or depressed area on a bone
• Head, neck and shaft are used to describe parts of a long bone (Fig. 3.2)
• Tendon – connects muscle to bone
• Ligament – connects one bone to another bone

The axial skeleton

The skull

The bones of the head include the skull, nasal chambers, mandible or lower jaw and hyoid apparatus. The functions of the skull are:

1 To house and protect the brain.
2 To house the special sense organs – eye, ear, nose and tongue.
3 To house and provide attachment for parts of the digestive system – teeth and tongue, etc.
4 To provide attachment for the hyoid apparatus and the numerous muscles of mastication and facial expression.
5 To provide a bony cavity through which air can enter the body.
6 To communicate – the muscles of facial expression are found on the head and are an important means of communication.

Cranium

The caudal part of the skull that provides the bony ‘case’ in which the brain sits is called the cranium (Figs 3.4, 3.5). The bones of the cranium include:

• Parietal – forms much of the dorsal and lateral walls of the cranium
• Temporal – lies below the parietal bone on the caudolateral surface of the skull. The most ventral part of the temporal bone forms a rounded prominence called the tympanic bulla, which houses the structures of the middle ear. There is an opening into the tympanic bulla, called the external acoustic meatus, which in life is closed by the tympanic membrane or eardrum. The cartilages of the external ear canal are attached to this region
• Frontal – forms the front aspect of the cranium or ‘forehead’. Contains an air-filled chamber called the frontal sinus, which connects to the nasal chamber
• Occipital – this lies at the base of the skull on the caudal aspect. In this region there is a large hole called the foramen magnum, through which the spinal cord passes. On either side is a pair of bony prominences, the occipital condyles. These articulate with the first cervical vertebra or atlas (Fig. 3.1). At the side of the occipital condyles are the jugular processes, which are sites for muscle attachment
• Sphenoid – this lies on the ventral aspect of the skull, forming the floor of the cranial cavity. It is penetrated by many small foramina through which nerves and blood vessels pass
• Sagittal crest – a ridge of bone on the dorsal midline surface of the skull, which can be prominent in muscular dogs
• Zygomatic – the zygomatic arch is an arch of bone that projects laterally from the skull, forming the ‘cheekbone’
• Lacrimal – lies at the base of the orbit, which houses the eye, and is the region through which the tears drain from the eye into the nose.
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Fig. 3.4 Lateral view of the dog skull showing main bony features.

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Fig. 3.5 Ventral view of the dog skull showing main bony features.

Nasal chambers

The most rostral part of the skull carries the nasal chamber, the sides of which are formed by the maxilla and the roof by the nasal bone. The nasal chamber is divided lengthways into two by a cartilaginous plate called the nasal septum. Each of the chambers is filled with delicate scrolls of bone called the nasal turbinates or conchae. These are covered in ciliated mucous epithelium (see Ch. 8). At the back of the nasal chamber, forming a boundary between the nasal and cranial cavities, is the ethmoid bone. In the centre of this bone is the cribriform plate – a sieve-like area perforated by numerous foramina through which the olfactory nerves pass from the nasal mucosa to the olfactory bulbs of the brain (see Ch. 5).

The roof of the mouth is called the hard palate and is formed from three bones on the ventral aspect of the skull: the incisive bone or premaxilla, part of the maxilla and the palatine. The incisive bone is the most rostral and carries the incisor teeth (Fig. 3.5).

Many of the bones of the skull are joined together by fibrous joints called sutures (p 41). Sutures are firm and immovable joints but allow for expansion of the skull in a growing animal.

Mandible

The mandible or lower jaw is comprised of two halves or dentaries, joined together at the chin by a cartilaginous joint called the mandibular symphysis. Each half is divided into a horizontal part, the body, and a vertical part, the ramus (Fig. 3.6). The body carries the sockets or alveoli for the teeth of the lower jaw. The ramus articulates with the rest of the skull at the temporomandibular joint via a projection called the condylar process. A rounded coronoid process, which projects from the ramus into the temporal fossa, is the point to which the temporalis muscle attaches (Fig. 3.6). There is a depression on the lateral surface of the ramus, the masseteric fossa, in which the masseter muscle lies.

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Fig. 3.6 Lateral (A) and medial (B) views of the dog mandible.

Hyoid apparatus

The hyoid apparatus lies in the intermandibular space and consists of a number of fine bones and cartilages joined together in an arrangement that resembles a trapeze (Fig. 3.7). The hyoid apparatus is the means by which the larynx and tongue are suspended from the skull. The apparatus articulates with the temporal region of the skull in a cartilaginous joint.

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Fig. 3.7 The trapeze-like hyoid apparatus suspended from the skull (dog).

Related posts:

  1. Common integument
  2. Muscular system
  3. Mammals
  4. The horse

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Skeletal system

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