SECRETION. 313 



membrane lined by a single layer of short cylindrical or cubical cells. 

 These vesicles are filled with transparent nucleo-albuminous colloid 

 material. The colloid substance increases with age, and the cavities 

 appear to coalesce. In the interstitial connective tissue is a round 

 meshed capillary plexus, and a large number of lymphatics. The nerves 

 adhere closely to the vessels. 



Fig. 824 Part of a section of the human thyroid, a, Fibrous capsule ; &, thyroid vesicles filled 

 with, e, colloid substance; c, supporting fibrous tissue; d, short columnar cells lining vesicles; /, 

 arteries ; g, veins filled with blood ; h, lymphatic vessel filled with colloid substance, x (S. K. 

 Alcock.) 



In the vesicles there are in addition to the yellowish glassy colloid 

 material, epithelium cells, colorless blood -corpuscles, and also colored 

 corpuscles undergoing disintegration. 



Accessory Thyroids. These are small bodies possessing the structure 

 of the thyroid and apparently performing the same function. They are 

 found in the neck and in the mediastinum as far as the heart. The ac- 

 cessory thyroids undergo hypertrophy when the thyroid has been removed. 



Parathyroids. In addition to the accessory thyroids, parathyroids 

 are found in the neck, lying behind or to the side of the thyroid, or 

 even within its substance (in the rat). They are small bodies, differing 

 from the th} T roid in structure in that they consist of solid columns of 

 cells, not of acini; yet they seem capable of performing the function of 



