STRUCTURE OF THE THYROID GLAND. 371 



they originally represent only the product of a physiological 

 process. 



The several gland vesicles present great variation in size, 

 and even in adults some may be found which are of much 

 smaller diameter than the largest of those discoverable in the 

 infant. It appears that in extra-uterine life the progressive 

 increase of the several gland vesicles, if any, is usually very 

 small. On the other hand, in a human embryo of the fifth 

 or sixth month, I have found their diameter to be O0252 

 0'0336 millimeter, whilst their diameter in the newly born 

 already amounts to O'l O16 millimeter, and may in adults 

 exceed 0*2 millimeter. The gland vesicles of the tortoise are 

 particularly well adapted for investigation, since they measure 

 from O'l 4 0'27 millimeter. Mammals possess in general very 

 small vesicles, which sometimes, by their further growth, so 

 press upon one another that the space required for the capil- 

 laries is only obtained by an inflexion of their opposite walls. 

 Such conditions I found to occur frequently in the dog, where 

 the walls of the vesicles form projections internally, in which 

 the epithelial cells are seated like the voussoirs of an arch. 



It is deserving of notice that the larger vesicles occupy the 

 centre of the several lobules, or, where these are not present, 

 the centre of the entire gland, whilst at the periphery they 

 appear much smaller and are compressed and flattened in 

 form. 



The epithelial cells, as already mentioned, are always some- 

 what higher than broad, and do not vary remarkably either 

 with age or with the species of animal. Thus, for example, in 

 an embryo of the fifth or the sixth month they were from 

 0-0060-0095 millimeter long, and from 0*004 0'005 milli- 

 meter broad; in adults they attain the length of O'Ol 0"16 

 millimeter; in the dog, from 0*008 0*0126 millimeter; in 

 the calf, of about 0'0105 ; in the tortoise, from 0'0168 milli- 

 meter, etc. 



The framework of the thyroid gland is a direct continuation 

 of the external investing membrane, and, like this, consists of 

 fasciculi of connective tissue, with numerous elastic fibres and 

 connective tissue corpuscles, which for the most part appear 

 fusiform or branched. The organ is partially traversed by 



