554 



RESPIRATORY APPARATUS IN MAMMALIA. 



GROUP OF GLAND VESICLES FROM THE THYROID 

 BODY OF A YOUNG SUBJECT. 



a, Connective tissue ; 6, basement membrane of 

 the vesicles ; c, epithelial cells. 



trachea. This connecting portion is far from being constant in the Horse, 

 while it is nearly always present in the Ass. 



Each lobe of the thyroid body is related, inwardly, to the trachea ; outwardly, 

 it is covered by the subscapulo-hyoideus muscle. 



Structure. — The thyroid body is 

 Fig. 334. composed of a fibrous envelope, and a 



proper tissue or parenchyma. 



The fibrous envelope is composed of 

 slender, but strong connective tissue ; 

 it sends from its inner face a large 

 number of thin nucleated layers that 

 intersect each other, forming spaces in 

 which the proper tissue is contained. 



The parenchyma is divided into 

 lobules, the presence of whic^ is mani- 

 fested on the surface of the organ. 

 They are composed of vesicles communi- 

 cating with each other, the shape and 

 contents of which vary considerably 

 with age and situation. In the foetus, 

 or very young animal, they are round 

 or elliptical, and constituted by a thin 

 amorphous membrane, lined by poly- 

 gonal cells with a large nucleus, and 

 containing a granular fluid. In the 

 adult, these vesicles are misshapen, and, after being distended, several run to- 

 gether, while the epithelium is less evident, but uniform ; the contents have 

 become brown, and hold granules and nuclei in suspension ; and, finally, they 

 often assume the character of colloid matter, in becoming viscid and of a yellow 

 tint. 



Vessels mid 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 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, which form a system of somewhat large cavities around 

 the alveoli. 



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

 arbitrary manner, in the ill-defined category of ductless 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. 



In those animals in which it is developed, it seems to have much importance, 

 and is said to play a part with regard to the brain, similar to that attributed to 

 the spleen in the abdominal circulation. Htemato-poietic functions have also 

 been ascribed to it, from the influence it exerts by means of the follicles placed 

 between its vesicular lobes ; these are supposed to destroy the mucine formed in 

 the body. 



The studj of its development does not throw any Ught on its functions. 



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