TEE THYBOID AND THYMUS GLANDS. i73 



the epithelium is less evident and uniform, the contents have become brown 

 and hold granules and nuclei in suspension, and, finally, often assume the 

 character of colloid matter in becoming viscous and of a yellow tint. 



Vessels and nerves. — The thyroid body is remarkable for the relatively 

 enormous volume of its blood-vessels ; the arteries chiefly come from the 

 thyro-laryngeal branch — a collateral of the primitive (or common) carotid 

 (they form plexuses on the vesicle walls) ; the veins pass to the jugular. 

 Its nervous filaments are from the first and second cervical pairs, with twigs 

 from the sympathetic. It has an abundance of lymphatics. 



Functions. — The thyroid is one of the organs classed, in a somewhat 

 arbitrary manner, in the ill-defined category of ductless or blood-vascular 

 glands. Our knowledge of its use is as uncertain at present as in the 

 infancy of anatomical science. So that we can say nothing more on this 

 subject, except that the successive or simultaneous excision of the two 

 lobes in the Horse do not appear to cause any derangement in the animal's 

 health. 



Neither does the study of its development throw any light on its func- 

 tions. It is certainly relatively larger in the foetus and young animals than 

 in adults ; but the difference is not sufficiently marked to authorize us in 

 drawing any physiological inductions therefrom. 



2. Thymus Gland. 



The thymus gland is a transitory organ, only present in the foetus and 

 very young animals, and in its nature closely resembling the thyroid gland. 

 Like it, it is divided into two lateral lobes placed close together in the 

 middle line, under the lower face of the trachea, partly without and partly 

 within the chest, between the two layers of the anterior mediastinum. It is 

 elongated from before to behind, of a whitish colour, and uneven or 

 lobulated on its surface like a salivary gland. 



Stkuctuee. — It owes its uneven aspect to its lobular structure, for it is 

 effectively reduced by dissection into a multitude of granular lobules, in the 

 centre of which are found vesicular cavities containing a lactescent fluid. 

 The vesicles are larger than those of the thyroid gland, and have for their 

 walls a very thin layer of delicate connective tissue ; they are filled by a 

 mass of nuclei. A wide, irregular cavity has been described as existing in the 

 middle of each lobe (reservoir of the thymus), and evidently communicating 

 with the vesicles of the lobules, as it contains a notable quantity of the same 

 milky fluid. This cavity is certainly not present at all periods ; for I have 

 not met with it in two young foetuses now lying before me as I write. But 

 without dwelling on this particular point, we may notice enormous blood- 

 vessels, lymphatics, and nerves, as complementary elements in the organisa- 

 tion of the gland, whose structure is very similar to that of the proper 

 glands^ though differing from them in an important feature — the absence of 

 an excretory duct. 



Nothing positive is known as to the functions of the thymus gland ; 

 it is only certain that they are exclusively related to the development of the 

 young animal, as it generally disappears some months after birth, though it 

 is sometimes found in adult, and even in very aged animals. 



(Its functions are supposed to be the same as, or analogous to, those of the 

 thyroid. Structurally, the organ may be said to consist of an assemblage of 

 hoUow glandular lobules joined together by connective tissue, each having 

 a cavity which opens into a central canal that has no duct, and being lined 



