32 INTERNAL SECRETION 



or they may be simulated by lobules thrown out from the thyroid 

 itself. 



The parathyroids are extremely vascular and are supplied 

 with blood from the inferior thyroid artery. The veins open into 

 the lateral portion of the thyroid venous system. Up to now 

 there is no anatomical proof that nerves are present. 



HISTOLOGY OF THE PARATHYROID GLANDS. 



The parathyroid gland usually has the appearance of a closed 

 capsule composed of fibrous connective tissue. It is filled with 

 parenchyma, which is divided into more or less regular columns 

 of epithelial cells, by delicate septa which proceed from the 

 envelope. These septa convey the capillary vessels, and they 

 unite to form a kind of network, in the meshes of which the 

 parenchyma is enclosed. Occasionally, the parenchyma appears 

 to be a single coherent cell-mass traversed by a very close network 

 of capillaries. In other cases, owing to pronounced development 

 of the connective tissue, it is divided up into portions resembling 

 lobules or follicles. Sometimes all three structural forms appear 

 in one organ ; in other cases the structure is uniform throughout. 



The greater proportion of the parenchyma consists of rather 

 large polygonal cells, called principal cells (Hauptzellen), the 

 protoplasm of which does not stain readily. The nuclei, which 

 stain well, are about the size of erythrocytes ; they are furnished 

 with a regular, net-like, chromatin framework and have several 

 nucleoli. A second group of cells is oxyphile. These have a 

 delicate granulated protoplasm, which colours strongly with eosin 

 and other acid dyes, and a small central nucleus. The oxyphile 

 cells are, as a rule, smaller than the principal cells, but there 

 are numerous transition forms. Of these, one is believed by some 

 authors to constitute a separate cell-form. It is intimately 

 associated with the connective tissue of the capsule and the septa, 

 and appears as a cubic or cylindrical cell with an elliptical nucleus. 

 These cells unite to form palisade-like rows, which are arranged 

 either radially or perpendicularly to the direction in which the 

 septa run. 



The numerous capillary vessels unite to form a close net- 

 work ; their endothelial cells rest partly on the epithelial cells and 

 in part are covered with a delicate sheath of connective tissue. 

 Occasionally, and more particularly in old age, the arteries are 

 accompanied by large lymphatics. According to Sacerdotti and 

 Anderson, the vessels in the interstitial tissue are accompanied 

 by nerve-fibres by which they are enclosed as by a network, and 

 which may also penetrate between the epithelial cells. 



That the parathyroids possess a secretory function is shown 

 by the presence of fat-granules in the epithelial cells of the 

 parathyroids of both man and animals. These fat-bodies are 



