174 HISTOLOGY. 



termined. The hyaline material in the thymic corpuscles, the hypophysis, 

 and in the coagulum in the cervical blood and lymphatic vessels, has also 

 been designated colloid. In sections of the thyreoid gland it usually does 

 not fill the follicle but has contracted, producing a spiny border. Gran- 

 ules, vacuoles, detached cells, leucocytes, and crystalloid bodies may 

 be found in it. It is a product of the epithelial cells, in the protoplasm of 

 which similar material has been detected. It has been said that it is trans- 

 ferred to the blood and lymphatic vessels. 



As has been learned by experiment, the thyreoid gland produces an 

 internal secretion which is essential for the normal growth and develop- 

 ment of the body. It is, however, not known whether this secretion leaves 

 the basal or free surface of the thyreoid epithelium, and its relation to the 

 colloid material is not clear. The finding of two sorts of thyreoid cells. 



Tangential section of a 

 tubule; the epithelium 

 viewed from the surface. 



%\*\ 3 ^%I^^^^r^7'^'\Y^^ r ~^~- Tubule in transverse 



Epithelium. -^C- SBbiMr^^^ 1 ^^^ section. 



.Connective tissue. 



FIG. 195. SECTION OP A LOBULE OF THE THYREOID GLAND FROM AN ADULT MAN. X 220. 



one of which produces colloid, and the other does not, lacks confirmation. 

 The cells may exhibit refractive, secretory granules which are larger and 

 coarser toward the free surface. In certain animals other granules of 

 fatty nature have been found, especially near the basal surface. Since the 

 terminal bars are said to be deficient at the angles where the epithelial cells 

 meet, an opportunity is afforded for the contents of the follicles to pass out 

 between the epithelial cells to the vascular tunica propria. 



The thyreoid follicles are surrounded by loose elastic connective tissue, 

 said to be reticular near the follicles, which contains very many blood and 

 lymphatic vessels. Denser connective tissue forms a capsule and lobular 

 partitions. The nerves from the cervical_sympathetic ganglia form peri- 



