390 Thyroid and Parathyroid Glands throughout Vertebrates. 



7. In Birds, we frequently find large areas of the thyroid either devoid of 

 colloid vesicles, or having these in a compressed, crowded condition. But the 

 parathyroids are still separate and distinct organs. The post-branchial body 

 presents certain peculiar features, described in the text, among these being 

 an accumulation of concentric corpuscles, such as is found in the epithelial 

 part of the thymus. 



8. In Mammals, there is much more intimate relationship between the 

 parts of the thyroid apparatus than in lower animals. The cells lining the 

 vesicles are practically of the same character as those accumulated in varying 

 amount between the vesicles, and do not differ in any essential respect from 

 those forming the parathyroid glandules. 



Many of the masses of intervesicular cells are indistinguishable from 

 parathyroids. The internal parathyroid is frequently in direct tissue 

 continuity with the thyroid, and every kind of transition form exists. 

 Parathyroid has only, indeed, to have colloid spaces in its iuterior to constitute 

 itself thyroid, and this occurs in the human subject under certain pathological 

 conditions. 



Parathyroids left behind after removal of the thyroid develop colloid 

 vesicles and become practically converted into thyroid. Moreover, the 

 changes in the thyroid, after removal of the parathyroids, may be interpreted 

 as the reverse change of thyroid into parathyroid tissue. The experimental 

 evidence as to a separate function for parathyroids is inconclusive. 



9. Thyroid and parathyroids are to be looked upon as structures of some- 

 what different embryological origin, which are anatomically separate and 

 distinct in the lower Vertebrata, but which come into very intimate anatomical 

 and physiological relationships with each other in the Mammalia. In this 

 latter group they are, in fact, to be looked upon as constituting one 

 appariatus. 



10. Parathyroidectomy, like thyroidectomy, causes compensatory changes 

 in the pituitary body. These consist in a notable increase of the colloid 

 vesicles in the pars intermedia. 



This observation confirms the general view that thyroid and parathyroid 

 are very intimately related to each other. The colloid vesicles of the ^wrs 

 intermedia of the i)ituitary resemble in all respects those of the thyroid and 

 of the parathyroid (where such occur), and it is probable that the intermediate 

 portion of the pituitary is to be looked upon as an integral part of the thyroid 

 — paratliyroid apparatus. 



