226 HISTOLOGY 



transformation they may become a part of the corpuscle. The larger 

 corpuscles are due to a fusion of smaller ones, or to hyaline changes occur- 

 ring simultaneously in a group of cells. The central portion of a corpuscle 

 may become calcified. Sometimes it is vacuolated, containing fat. The 

 hyaline substance may respond to mucous stains, but generally it does not; 

 it has been considered similar to the 'colloid' of the thyreoid gland. 

 Leucocytes are said to become imbedded in the corpuscles, or to enter 

 them and assist in their disintegration. Thymic corpuscles have been 

 regarded not only as degenerative products of the entodermal epithelium 

 but also as concentric connective tissue masses, and as blood vessels 

 with thickened walls and obliterated cavities. Injections show that 

 they are not connected with the blood vessels. Although they have recently 

 been described as active constituents of the thymus, they are generally 

 regarded as degenerations. 



The arteries of the thymus enter it along the medullary strand, and 

 extend between the cortex and medulla, sending branches into both but 

 chiefly into the cortex. The cortical branches empty into veins between 

 the lobules; the others into veins within the medulla. There are many 

 interlobular lymphatic vessels, beginning close to the surface of the gland 

 substance, and accompanying the blood vessels. There is nothing in the 

 thymus to correspond with a lymph sinus. The nerves, chiefly sympa- 

 thetic fibers, with some from the vagus, terminate along the vessels; a 

 very few have free endings in the medulla. 



THYREOID GLAND. 



The thyreoid (i.e., shield-shaped) gland is a median, entodermal down- 

 growth from the tongue; its thyreoglossal duct becomes obliterated, 

 leaving the foramen caecum to mark its former outlet. The downgrowth 

 is joined by cells from the postbranchial bodies, which fuse with it. 

 This entire structure comes to lie beside and in front of the upper part of 

 the trachea. It consists of two lateral lobes, each about two inches long 

 and an inch wide, connected by an isthmus, about half an inch wide, which 

 crosses the median line ventral to the second and third tracheal rings. 

 An unpaired pyramidal lobe extends from the isthmus or adjacent part of 

 the lateral lobe toward the tongue (Fig. 208). Irregular detached por- 

 tions of the gland, such as occur especially along the course of the thyreo- 

 glossal duct, are called accessory thyreoid glands. 



The proliferating mass of entodermal cells forms at first a network of 

 solid cords. This becomes separated into small masses, within each of 

 which a lumen may appear. The lumen enlarges and becomes spheroidal; 

 the entodermal cells which surround it form a simple epithelium, either 

 coleumnar, cuboidal, or flat. Flat cells are said to occur especially in old 



