556 MESFIRATOtiJt APPARATUS IN MAMMALIA. 



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 

 {j-eservoir 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 com- 

 plementary elements in the organization of the gland, the structure of which is 

 very similar to that of true 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 resemble those of the lymphatic glands, and 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 the 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 

 hollow glandular lobules joined together by connective tissue, each having a 

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

 externally by an almost amorphous membrane which divides it into "acini," 

 or gland-granules. Separate acini are often observed on the main canal. Each 

 lobule is made up of its greyish-white, soft parenchyma composed of free nuclei 

 and small cells, and has a minutely distributed capillary plexus. The lymphatics 

 terminate in two large ducts that commence at the upper extremities of the lobes 

 of the gland — the thymic ducts — and pass downward to terminate at the junction 

 of the jugular and axillary veins at each side.) 



Differential Characters in the Glandiform Bodies annexed to the Kespiratort 

 Apparatus in the other Animals. 



The thyroid body, peculiar to Mammals, is more developed in Ruminants, Pacyderms, 

 and Carnivora, than in Solipeds. The two lobes are closer together, and are often united by 

 the thyroid isthmus. In the Pig this i^ very marked, and the gland well merits its name, 

 as it forms a veritable shield in front of the trachea, towards the lower part of the neck. 



The thymus gland in young Ruminants is more voluminous than in tlie Foal, and is situated 

 higher up in the cervical region.' (In the Carnivora, it is divided into two brandies ; but it is 

 small, and completely lodged between the layers of the anterior mediastinum. It persists for 

 some time after birth, and seldom disappears in less than a year.) 



(Comparison of the Glandiform Bodies annexed to the Respiratory Apparatus in 

 Man, with those of Animals. 



In Man, the two lobes of the thyroid body are connected by an istlimus, and the upper 

 extremity of the lobe is carried up to the side of the thyroid cartilage. Tiie istlimus often gives 

 origin to a process of variable length and size, called the pyramid, or third lobe, which is 

 generally situated to the left. A muscle is sometimes found connected with the isthmus or 

 pyramid, and is attached above to tlie body of theos hyoidesor to the thyroid cartilage ; it has 

 been named the levator glandulas thyroidx. 



The thymus gland is composed of two lobes, a right and left, only joined by connective tissue, 

 and having no structural communication. There is a cervical and a thoracic portion, the whole 

 extending from the fourth rib as higli as the thyroid gland. After birth it continues to enlarge 

 until the end of the second year, and begins to diminish between the eighth and twelfth 

 years.) 



