THE THYMUS
The thymus is an organ whose importance is greatest in the young animal. It begins to regress about the time of
Figure 7-61 Visceral surface of the spleens of horse (A), cattle (B), and dog (C) to show the distribution of the splenic arteries.
Branches to other structures are shown in blue.
Figure 7-62 Thymus of calf (HE) (70?). 1, Thymic lobules; 2, capsule.
puberty and may eventually almost disappear. Even when a more sizable vestige persists, this will be found to consist largely of fat and fibrous elements and the thymic tissue is suppressed.
The thymus has a paired origin from the third pharyngeal pouch (see Figure 6—5/6), although some uncertainty exists about the precise contribution made by the endoderm and subjacent mesoderm; an ectodermal contribution is even conjectured in some species. The buds grow down the neck beside the trachea and invade the mediastinum, in which they extend to the pericardium. The cervical part regresses prematurely in many species (including the dog), and the thymus then appears as a single, median organ whose bilateral nature is anything but obvious. At its apogee it is a lobulated structure (with some resemblance to a salivary gland) that fills the ventral part of the cranial mediastinum, fitting about the other contents of this space.
The thymus is divisible in microscopic preparations into a cortex and medulla. The cortex produces the immunocompetent T lymphocytes, which enter the bloodstream for distribution to the peripheral lymphoid organs (nodes and scattered lymph nodules) where they settle and multiply. The medulla is formed of epithelioid cells of more speculative significance (Figure 7-62). Because of its relevance to the postnatal development and maintenance of immunological competence, the thymus is of vital importance.