PARATHYREOID GLANDS. 



175 



vascular plexuses, and pass to the follicles, 

 between the epithelial cells. 



A few have been found to end 



PARATHYREOID GLANDS. 



It is generally stated that there are four para.thyreoid glands in man, 

 the anterior or upper pair being derived from the fourth entodermal 

 pouches, and the posterior or lower pair from the nodulus ihymicus of the 

 third (Fig. 189). Although they have been repeatedly investigated, their 

 origin is not yet established. In the adult they are round or oval bodies, 

 said to measure from 3 to 13 mm., found on the dorsal or tracheal surface 

 of the thyreoid gland. They may be imbedded in its capsule or attached 

 to it by pedicles. Sometimes they (the lower pair?) are found in the thy- 



FIG. 196. SECTION OP A HUMAN PARATHYREOID GLAND. (Huber.) 



It is not known that two pairs always occur. The parathyreoid 

 glands may be lacking on one side, where in other cases as many as four 

 have been recorded. Both pairs possess a similar structure unlike that of 

 either the thyreoid gland or the thymus, but resembling the corresponding 

 epithelial bodies of the lower vertebrates. They consist of masses and 

 cords of polygonal, entodermal cells, containing round nuclei with networks 

 of chromatin. The protoplasm is pale, " almost homogeneous" or " slightly 

 granular," sometimes containing vacuoles. Cell membranes are not promi- 

 nent. Between these cells and the large thin-walled blood vessels which 

 pass among them (Fig. 196), there is only a very small amount of connec- 

 tive tissue. A capsule surrounds the entire structure. The blood vessels 

 are branches of those which supply the thyreoid gland. Little is known 

 of the lymphatics or nerves. 



