PARATHYREOID GLANDS 



229 



and disappear, or increase in number by subdivision. 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 vetebrates. 

 They consist of masses and cords of polygonal, entodermal cells contain- 

 ing round nuclei with networks of chromatin. The protoplasm is pale, 

 "almost homogeneous" or "slightly granular," sometimes containing 

 vacuoles. Cell membranes are not prominent. Between these cells 

 and the large thin- walled blood vessels which pass among them (Fig. 217), 

 there is only a very small amount of connective tissue. A capsule sur- 

 rounds the entire structure. The blood vessels are branches of those 

 which supply the thyreoid gland. Little is known of the lymphatics or 



nerves. 



GLOMUS CAROTICUM. 



c.t. 



The glomus caroticum (carotid gland) is largely a knot of blood vessels 

 at the bifurcation of the common carotid artery. It is a reddish body 

 "5-7 mm. long, 2.5-4 

 mm. broad, and 1.5 mm. 

 thick." Between its 

 thin-walled, dilated 

 capillaries there are 

 strands of polygonal 

 chromafime cells, which 

 are prone to disintegrate 

 (Fig. 218). Many nerve 

 fibers, both medullated 

 and non-medullated, 

 enter the glomus, and a 

 few multipolar ganglion 

 cells are associated with 

 them. Since the nature 

 of the glomus caroticum 

 is undetermined, the 

 three views regarding it 

 may be mentioned. 

 First, it has been consid- 

 ered as derived from the third pharyngeal pouch. Since it has recently 

 been asserted that the "carotid gland" of Echidna comes from the 

 second pouch, the non-entodermal origin of the human glomus is per- 

 haps not beyond question. Second, it has been considered ganglionic or 

 paraganglionic in nature, so that it is classed with nervhus structures, 

 and this opinion is probably correct. Third, it is considered essentially 

 a vascular formation, containing strands of modified mesenchymal cells. 



FIG. 218. SECTION OF A PART OF THE GLOMUS CAROTICUM OF 

 MAN. (After Schaper.) 



b.v., Blood vessels; e.v., efferent vein; tr., trabecula; c.t., connective 

 tissue septum. 



