THE THYEOID GLAND AND ITS DISEASES 213 



Comparative Anatomy 



The thyroid gland is unknown in invertebrates, and becomes more 

 and more highly developed the higher we go up the vertebrate scale./ Its 

 primitive connection with the pharynx is maintained through the 

 ammocete stage of the development of the lamprey. Its homology with 

 the endostyle of tunicates is uncertain. In most fishes it consists of very 

 small groups of follicles scattered along the ventral aorta. In amphibia, 

 birds, and reptiles it is more condensed. The bilobed condition in man is 

 characteristic of mammals generally. Kummer states that "the iodin con- 

 tent depends also upon the species of animal. The herbivora show a 

 maximum, the carnivora a minimum, and the omnivora an average 

 amount of iodin." 



These chemical analyses, together with the difference in the sus- 

 ceptibility of different animals to thyroid and parathyroid extirpation, 

 lead one to believe that the glands are specifically adapted to meet the 

 particular needs of different groups of animals. An extension of the 

 statistical methods, which are bringing out such interesting points in 

 normal development, to the phylogenetic series in the investigation of 

 all the endocrin organs, is greatly needed. 



Follicles. The thyroid gland is made up of a mass of closed follicles 

 containing a viscid material called "colloid." The follicles vary greatly 

 in size and in shape. Sometimes they are very tiny and present little or 

 no lumen, while in other cases they may normally attain a diameter of 

 several hundred microns. Their shape is largely dependent upon intra- 

 glandular pressure. Saccular and tubular forms are of quite frequent 

 occurrence, in addition to the more or less spherical vesicles which pre- 

 dominate. The follicles are embedded in loose areolar and elastic con- 

 nective tissue, and the whole gland is surrounded by a fairly dense 

 capsule (Fig. 4). 



Interfollicular Epithelium. Embedded in this connective tissue be- 

 tween the follicles, we occasionally find small irregular clumps of dis- 

 tinctly epithelial cells. These intervesicular or interfollicular cells, as they 

 are called, are known to vary greatly in number at different ages, though 

 their history has not been carefully worked out in any form. They are 

 very numerous before birth and seem to decrease progressively as age ad- 

 vances. In the normal adult they are fairly abundant. According to 

 Wilson, they become rearranged into secreting follicles in cases of exoph- 

 thalmic goiter. Many investigators, therefore, regard them as persistent 

 embryonic cells, possessing the power of differentiation into secretory 

 epithelium as the demand arises. Bozzi, however, could detect no changes 

 in these cells after removal of large parts of the gland, suggesting perhaps, 

 that after all they may not possess secretory potencies (citation from 



