The primary function of the thymus is to produce T-lymphocytes. The thymus is derived from the third pharyngeal pouches as buds that grow down the neck and merge at the level of the mediastinum and cranial pericardium. In dogs and cats, the cervical parts of the gland regresses early, leaving a single, midline thoracic lobe. In calves, the cervical thymus often persists as elongate lobes on each side of the trachea. In addition to the thoracic and cervical lobes, sheep and goats have two, ovoid cranial lobes, located medial to the mandibular salivary gland. In most animals, the thymus begins to regress with the onset of sexual maturity and may disappear or be largely replaced by adipose and fibrous tissue. The thymus in rodents and rabbits is usually maintained throughout life (except in athymic strains like nude mice). In fresh carcasses, the thymus has a pink tinge that rapidly fades to gray with autolysis (Figure 14.1). The thymus has a lobulated structure, and the lobules are separated by delicate fibrous trabeculae that allow them to move freely on each other.

The lymph nodes filter antigen and infectious agents from the afferent lymph and produce efferent lymph rich in antibodies and lymphocytes. Lymph nodes are situated where they are protected by the surrounding tissue but produce minimum interference with the function of the musculoskeletal and vascular systems. Thus, they are usually located in fat stores at the flexor angles of joints, in the mediastinum and mesentery, and at the angle formed by the origin of many of the larger blood vessels.

Each lymph node drains a reasonably well-defined area of regional anatomy, but extensive anastomosis is present, so drainage areas overlap. Lymphocenters consist of a single lymph node or a group of lymph nodes that occur consistently in the same region of the body and receive afferent lymphatics from approximately the same regions in all species: (a) Head (mandibular, parotid, and retropharyngeal lymph nodes), (b) Cervical (superficial and deep cervical lymph nodes), (c) Thoracic limb (axillary lymph node), (d) Thoracic cavity (dorsal and ventral thoracic lymph nodes, mediastinal lymph nodes, and tracheobronchial lymph nodes), (e) Abdominal and pelvic wall (lumbar, iliosacral, inguinofemoral, and ischiatic lymph nodes), (e) Abdominal viscera (celiac, cranial mesenteric, caudal mesenteric lymph nodes), and (f) Distal pelvic limb (popliteal lymph node).

In most mammals, lymphocyte replication in response to antigen stimulation occurs in the peripheral part of the lymph node (cortex), and then mature lymphocytes and antibodies flow through the medullary sinuses to exit the node through the efferent lymphatics at the hilar region. In pigs, the structure of the node is inverted, and afferent lymph is channeled to the central part of the node, where antigen stimulation and lymphocyte replication occurs. Cells and antibodies then flow to the periphery of the node, where the efferent lymphatics emerge (Figure 14.2). Efferent lymphatics eventually return tissue fluid and leukocytes to the venous blood supply via the thoracic duct terminates in the left external jugular vein or the cranial vena cava.

The spleen is a lymphoid organ that lies in the left hypogastric region and roughly parallels the greater curvature of the stomach. The shape of the spleen varies by species (see Table 14.1 and Figure 14.3). The spleen is loosely attached to the greater curvature of the stomach by the gastrosplenic ligament, which is part of the greater omentum. The dorsal end or head of the spleen lies between the gastric fundus and the cranial pole of the left kidney (Figure 14.3). Species differences in the amount of smooth muscle in the splenic capsule and trabeculae determine the degree to which the size of the spleen varies. Dog and horse spleens contain thicker bands of smooth muscle, and the size of the spleen can vary significantly depending on physiologic level of contraction. When contracted, the capsular surface is gray to brown with a purple cast but when congested is dark red to purple. The splenic artery arises as a branch of the celiac artery and divides into a dorsal and ventral branch. These branches give rise to up to splenic branches that penetrate the capsule at the longitudinal hilus along the visceral surface. The venous outflow parallels the arterial inflow and empties into the portal vein.

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