THE PARATHYROID GLANDS 



1271 



Structure. The thyroid gland is invested by a thin capsule of connective tissue, which pro- 

 jects into its substance and imperfectly divides it into masses of irregular form and size. When 

 the organ is cut into, it is of a brownish-red color, and is seen to be made up of a number of 

 closed vesicles, containing a yellow glairy fluid, and separated from each other by intermediate 

 connective tissue (Fig. 1176). 



Colloid material - 



Colloid in 

 lymphatic vessel 



Cubical 

 epithelium 



FIG. 1176. Section of thyroid gland of sheep. X 160. 



The vesicles of the thyroid of the adult animal are generally closed spherical sacs; but in some 

 young animals, e. g., young dogs, the vesicles are more or less tubular and branched. This 

 appearance is supposed to be .due to the mode of growth of the gland, and merely indicates that 

 an increase in the number of vesicles is taking place. Each vesicle is lined by a single layer of 

 cubical epithelium. There does not appear to be a basement membrane, so that the epithelial 

 cells are in direct contact with the connective-tissue reticulum which supports the acini. The 

 vesicles are of various sizes and shapes, and contain as a normal product a viscid, homogeneous, 

 semifluid, slightly yellowish, colloid material; red corpuscles are found in it in various stages 

 of disintegration and decolorization, the yellow tinge being probably due to the hemoglobin, 

 which is thus set free from the colored corpuscles. The colloid material contains an iodine com- 

 pound, iodothyrin, and is readily stained by eosin. According to Bensley 1 the thyroid gland 

 prepares and secretes into the vascular channels a substance, formed under normal conditions in 

 the outer pole of the cell and excreted from it directly without passing by the indirect route 

 through the follicular cavity. In addition to this direct mode of secretion there is an indirect 

 mode which consists in the condensation of the secretion into the form of droplets, having high 

 content of solids, and the extension of these droplets into the follicular cavity. These droplets 

 are formed in the same zone of the cell as that in which the primary or direct secretion is formed. 



This internal secretion of the thyroid is supposed to contain a specific hormone which acts as a 

 chemical stimulus to other tissues, increasing their metabolism. 



Vessels and Nerves. The arteries supplying the thyroid gland are the superior and inferior 

 thyroids and sometimes an additional branch (thyroidea ima) from the innominate artery or the 

 arch of the aorta, which ascends upon the front of the trachea. The arteries are remarkable 

 for their large size and frequent anastomoses. The veins form a plexus on the surface of the 

 gland and on the front of the trachea; from this plexus the superior, middle, and inferior thyroid 

 veins arise; the superior and middle end in the internal jugular, the inferior in the innominate 

 vein. The capillary bloodvessels form a dense plexus in the connective tissue around the vesicles, 

 between the epithelium of the vesicles and the endothelium of the lymphatics, which surround 

 a greater or smaller part of the circumference of the vesicle. The lymphatic vessels run in the 

 interlobular connective tissue, not uncommonly surrounding the arteries which they accompany, 

 and communicate with a net-work in the capsule of the gland; they may contain colloid material. 

 They end in the thoracic and right lymphatic trunks. The nerves are derived from the middle 

 and inferior cervical ganglia of the sympathetic. 



THE PARATHYROID GLANDS (Fig. 1177). 



The parathyroid glands are small brownish-red bodies, situated as a rule between 

 the posterior borders of the lateral lobes of the thyroid gland and its capsule. 

 They differ from it in structure, being composed of masses of cells arranged in a 

 more or less columnar fashion with numerous intervening capillaries. They meas- 



1 American Journal of Anatomy, 1916. xix. 



