THE THYROID AND PARATHYROIDS 261 



tissue is large in amount, and in certain regions and in certain 

 specimens forms large areas of solid tissue, which is indistin- 

 guishable from parathyroid. This condition of affairs in the 

 avian thyroid has been emphasized by Forsyth, and it has been 

 pointed out that from the very wide variations in the amount 

 of the intervesicular tissue found in different specimens of the 

 same species, and from feeding experiments, we must conclude 

 that the proportion between vesicular and intervesicular 

 tissue varies under different physiological conditions. 

 The parathyroid of birds (see Fig. 57) contains an abundance 



fe^Ts 



&>& 



FIG. 57. Pigeon. Portions of post-branchial body (on the left) and para- 

 thyroid (on the right), separated by a connective-tissue septum. Note 

 the presence in both of structures identical with Hassan" s corpuscles of 

 the thymus. These are numerous in the post-branchial body, and few 

 in the parathyroid. In both, the concentric bodies frequently show a 

 lumen. (F. D. Thompson.) 



c.c., concentric corpuscles ; c.t., connective tissue ; p. thyr., parathyroid. 



of fat. It does not contain vesicles like those of the reptilian 

 glandule. 



In close relationship, and in some regions anatomically 

 continuous, with the tissue of the parathyroid, we find a body 

 having an extraordinary structure. It is obviously of epithe- 

 lial nature, and is composed largely of structures which at 

 first sight resemble small arteries, but whose walls are com- 

 posed entirely of concentrically disposed spindle-shaped cells, 

 and projecting into the lumen are irregular cells lining the 

 tubules (Fig. 57). The rest of the body appears to be made 



