458 RESPIRATORY SYSTEM OF THE HORSE 



tissue which extends forward from the isthmus for a variable distance; this is termed the pyra- 

 midal lobe. The position of the lateral lobe is indicated approximately by the junction of the 

 external maxillary and jugular veins. 



Structure. — The gland is enveloped l^y a thin, fibrous capsule, from which 

 trabeculae pass into the gland, dividing it into lobules. The lobules consist of 

 non-communicating alveoli or follicles of varying form and size. The alveoli are 

 lined by cubical epithelium, and contain a viscid colloid substance. 



Blood-supply. — The arteries are relatively very large; there are usually two 

 thyroid arteries, which arise from the carotid and enter each extremity of the gland. 



Nerve-supply. — The nerves are derived from the sympathetic system. 



Nodules of thyroid tissue of variable size, sometimes as large as a pea, may be 

 found near the anterior extremity of the lateral lobes, on the course of the anterior 

 thyroid artery or on the trachea, even at a considerable distance from the thyroid 

 region. They are termed accessory thyroids (Glandula? thyreoidse accessorise). 



The parathyroids are small glandular bodies found in the thyroid region. 

 They are paler and not so dense as the thyroid tissue, but often cannot be differen- 

 tiated by their naked-eye appearance. In the horse there is usually only one on 

 each side in the connective tissue over the anterior extremity of the lateral lobe of 

 the thyroid. They resemble in structure embryonic thyroid tissue. 



THE THYMUS GLAND 



The thymus is a ductless gland which has a close resemblance to the lymphoid 

 tissues. It is well developed only in late foetal life and for a few months after birth. 

 After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, 

 so that in the adult it is represented by a thin remnant in the lower part of the 

 anterior mediastinum or has entirely disappeared. In the new-born foal it is of a 

 grayish-pink color, and consists of two lateral lobes (Fig. 358). The greater part 

 of the gland is situated in the anterior mediastinum, but the two lobes are con- 

 tinued into the neck by a chain of lobules which lie along the course of the carotid 

 artery, extending sometimes as far as the thyroid gland. ^ 



Structure. — The thymus is inclosed in a thin, connective-tissue capsule, and 

 consists of secondary lobules (Lobuli thj^mi), of varying shape and size, held to- 

 gether by areolar tissue and fat, and all attached to a cord of adenoid tissue termed 

 the tractus centralis. These lobules are subdivided into polyhedral primary 

 lobules, which are composed of lymphoid tissue, and present a dark vascular cortex, 

 and a lighter medulla that contains Hassal's corpuscles. 



Blood-supply. — Internal thoracic and carotid arteries. 



Nerve-supply. — Sympathetic and vagus nerve. 



RESPIRATORY SYSTEM OF THE OX 



THE NASAL CAVITY 



The nostrils, situated on either side of the muzzle, are relatively small, and are 



much less dilatable than those of the horse. The alee are thick and firm. The 



superior commissure is narrow. There is no false nostril or diverticuhun nasi. 



The skin is smooth, bare, and moist, and there is no clear line of demarcation be- 



^ The cervical part of the gland is very variable. The thoracic lobe of one side inay have 

 no cervical continuation; the other lobe may then give off a single prolongation which bifurcates. 

 The thoracic part in the new-born foal occupies most of the space which is later taken up by the 

 apices of the lungs. Its two lobes are in contact, and its deep surface is molded on the anterior 

 part of the pericardium and the large vessels. 



