320 ORGANS OF RESPIRATION. 



by a nearly cubic epithelium; the lobules are separated from each 

 other by a highly vascularized connective tissue, continuous with 

 the firm connective-tissue sheath surrounding the whole gland. 

 The connective-tissue framework of the thyroid has been studied by 

 Flint by means of the destructive digestion method. Relatively greater 

 amounts of connective tissue are found in connection with the blood- 

 vessels, while the follicular membranes are delicate. The follicles 

 are either round, polyhedral, or tubular, and are occasionally 

 branched (Streiff). At an early stage the acini are found to con- 

 tain a substance known as "colloid" material. 



Laneendorff has shown that two varieties of cells exist in 



& 



the acini of the thyroid body the chief cells and colloid 

 cells. Those of the first variety apparently change into colloid 

 cells, while the latter secrete the colloid substance. During the 

 formation of this material the colloid cells become lower, and their 

 entire contents, including the nuclei, change into the colloid mass. 

 Hiirthle distinguished two processes of colloid secretion ; in the one 

 the cells remain intact, in the other they are destroyed. He claims 

 that the colloid cells of Langendorff participate in the former pro- 

 cess, while in the latter they are first modified (flattened) and then 

 changed into the colloid substance. The secretion is formed in the 

 cells in the form of secretory granules. The colloid material may 

 enter the lymph-channels, either directly by a rupture of- the acini, 

 or indirectly by a percolation of the substance into the intercellular 

 clefts, whence it is carried into the larger lymphatics. 



The thyroid gland has a very rich blood supply. The vessels, 

 which enter through the capsule, break up into smaller branches 

 which form a very rich capillary network surrounding the follicles. 

 The veins, which are thin-walled, arise from this capillary network. 

 The gland is provided with a rich network of lymphatic vessels. 



Anderson (91) and Berkley (94) have studied the distribution 

 of the nerve-fibers of the thyroid gland with the chrome-silver 

 method ; the account given by the latter is the more complete and 

 will be followed here. The nonmedullated nerves entering the gland 

 form plexuses about the larger arteries, which are less dense around 

 the smaller arterial branches. Some of these nerve-fibers are vascular 

 nerves and end on the vessels ; others form perifollicular meshes 

 surrounding the follicles of the gland. From the network of nerve- 

 fibers about the follicles, Berkley was able to trace fine nerve fila- 

 ments which seemed to terminate in end-knobs on or between the 

 epithelial cells lining the follicles. Even in the, best stained prepa- 

 rations, however, not nearly all the follicular cells possess such a 

 nerve termination. In methylene-blue preparations of the thyroid 

 gland (Dr. De Witt) some few medullated fibers were found in the 

 nerve plexus surrounding the vessels. In a number of preparations 

 these were traced to telodendria situated in the adventitia of the 

 vessels, showing that at least a portion of these medullated nerves 

 are sensory nerves ending in the walls of the vessels. 



