140 ORGANS OF RESPIRATION. 



It is covered in front by the sterno-hyoid and thyroid mus- 

 cles, and laterally by the omo-hyoid and the sterno-mastoid. 

 Embracing the trachea and the sides of the larynx, its lobes re- 

 pose upon the primitive carotids, and the internal jugular 

 veins. 



The thyroid gland has a capsule which is not very easily 

 raised up, but serves to give it a polish; it is also invested by 

 the condensed cellular membrane of the part called fascia pro- 

 funda colli. Its surface is smooth and uniform. It is of a dark 

 brown colour. When cut into or torn, it is seen to consist of 

 several lobules adhering to each other; but this arrangement is 

 not very distinct, except in an enlarged or diseased state; and 

 may be traced most easily by following the course of the large 

 blood vessels, which pass in the interstices between the lobules. 

 The latter contain many small vesicles, or cells filled with a 

 transparent or yellowish and somewhat unctuous fluid: the cells 

 are frequently in a collapsed state, which prevents them from 

 being manifest to the naked eye. 



There are four considerable arteries which supply this body, 

 two on each side, one coming from the external carotid and 

 the other from the subclavian. The veins follow the course of 

 the arteries for the most part. Bichat has made a very inte- 

 resting remark on the subject of its blood vessels ; that, notwith- 

 standing their size and number, and minute ramifications in it, 

 much less blood remains in its capillary system than in that of 

 the liver or kidney, as is proved by the quantity of water it 

 tinges in maceration; he, therefore, infers that the capillary 

 system is less abundant. 



Anatomists have sought in vain for one or more excretory 

 ducts to this body, and some have imagined that they had found 

 them terminating in the trachea, or in the larynx. Santorini 

 considered the pyramidal process from the isthmus as the de- 

 sired duct. The emphysema with which the gland has some- 

 times been affected, was supposed to be a proof of its commu- 

 nicating with the trachea by excretory tubes : but it is more 

 probable that the air was forced into the cellular substance, 

 nuiting its lobules, and not into the structure itself of the gland. 



The settled opinion now seems to be, that whatever fluid it 

 secretes is conveyed away by the lymphatic vessels. Meckel 

 has suggested, that as this gland is more voluminous propor- 



