THYREOID GLAXD. 173 



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

 recently been described as active constituents of the thymus they are gener- 

 ally 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 those within the medulla. There are many 

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

 and accompanying the blood vessels. There is nothing in the thymus to 

 correspond with a lymph sinus. The nerves, chiefly sympathetic fibers, 

 with some from the vagus, terminate on the vessels; a very few have free 

 endings in the medulla. 



THYREOID GLAND. 



The thyreoid gland is a median, entodermal downgrowth 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 con- 

 sists of two lateral lobes, each about two inches long and an inch wide, 

 connected by an isthmus, about half an inch w r ide, which crosses the median 

 line ventral to the second and third tracheal rings. An unpaired pyram- 

 idal lobe extends from the isthmus or adjacent part of the lateral lobe 

 toward the tongue (Fig. 189). Irregular detached portions of the gland 

 such as occur especially along the course of the thyreoglossal 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 

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

 age, low columnar or cuboidal cells being usually found. The mature 

 thyreoid gland consists, therefore, of rounded, closed spaces, or follicles, 

 bounded by a simple entodermal epithelium (Fig. 195). The follicles 

 vary greatly in diameter. Generally they are rounded, but sometimes 

 they are elongated, and occasionally they branch or communicate with one 

 another. Among them are cords or clumps of cells which have not 

 acquired a lumen. 



\Yithin the follicles, and forming the most conspicuous feature of the 

 thyreoid gland in ordinary sections, is a hyaline material which stains 

 deeply with cosine and is named 'colloid.' Its chemical nature is unde- 



