274 THE DUCTLESS GLANDS 



D. The Intervesicular Cellular Tissue of the Thyroid 



It has been stated above that " in many lobules one finds in 

 addition to the vesicles solid cords and nests of epithelium cells, 

 and that these are more numerous in the young and developing 

 thyroid." It will be desirable to call particular attention to 

 this intervesicular material. This tissue very closely resembles 

 that forming the parathyroids. 



The amount of the intervesicular cellular material varies 

 within very wide limits in the thyroid of different species of 

 animals, in different individuals of the same species, and, to 

 some extent also, in different regions of the same gland. It is 

 not rare to find a pair of vesicles in close juxtaposition to each 

 other, so that the colloid of one is separated from the colloid of 

 the other by nothing more than two rows of vesicular cells. 

 In other cases there is a certain amount of connective tissue 

 separating the vesicles ; but, speaking for mammals generally, 

 it is more usual to find, separating the colloid vesicles from one 

 another, a variable mass composed of cells which are almost 

 identical in size, nature of cytoplasm, size and form of nucleus, 

 with those lining the colloid vesicles. This intervesicular 

 cellular tissue is proportionately much greater where the 

 vesicles are small. This is notably the case in the rabbit, and 

 is also strikingly true in young animals generally (see above, 

 p. 271). In the monkey the amount of intervesicular tissue is 

 proportionately great, and it is not possible to determine any 

 fundamental differences between its cells and those of the 

 vesicles. In the human thyroid there is often as much inter- 

 vesicular as vesicular material. 



This intervesicular cellular substance is shown in Fig. 

 77. 



E. Structure of the Parathyroids 



The parathyroids in man are built up of closely packed 

 polygonal cells, which are divided up by connective-tissue 

 septa into masses and cords of varying sizes and shapes. The 

 glandules are usually surrounded by their own capsule ; but 

 in the case of those placed on the surface of the thyroid, the 

 connective-tissue sheath is seen to be derived from, and 

 continuous with, that of the thyroid lobe, The capsule 



