ORGANS OF RESPIRATION. 237 



lined by a simple cubical epithelium made up of two 

 kinds of cells, (i) a smaller number of colloid cells 

 engaged in the production of colloid, and (2) inter- 

 vening chief cells which replace the former in case 

 they are lost. It is affirmed by some that the two 

 kinds of cells represent merely different stages of 

 secretion. 



The thyroid, being an epithelial organ, may be the 

 seat of a cancer. Goiter is a more common thyroid 

 tumor, consisting of accumulations within its vesi- 

 cles of colloid substance ; or an increase of the con- 

 nective-tissue elements; or a multiplication of the 

 thyroid vesicles. Removal of the thyroid produces 

 myxedema, while a congenital absence of the gland 

 is the cause of cretinism. In the latter case, thyroid 

 extract, regularly administered, will establish a nor- 

 mal growth of the child. In exophthalmic goiter the 

 thyroid gland is enlarged, but in this case the en- 

 larged thyroid is a symptom of a more general disease 

 involving other organs and systems. The thyroid 

 gland may be removed and grafted almost anywhere 

 in the body. It will readily grow in its new position 

 and assume its normal function with good results. 



Vessels and Nerves. The arteries of the thyroid 

 gland are the superior and inferior thyroid arteries on 

 each side, to which is sometimes added a fifth vessel, 

 the thyroidea ima. The organ has, therefore, a very 

 rich supply of blood. The smaller vessels and capil- 

 laries ramify in the connective-tissue elements be- 

 tween the gland vesicles. The veins, which are also 

 large, form an extensive plexus near the surface of 

 the gland, from which a superior, middle, and inferior 



