THE CAROTID BODY 1327 



the mesothelium on both sides of the root of the mesentery into the mesoderm ventral to the 

 aorta. A httle later these become definite organs completely separated from the ccelomic 

 epitheUum and are soon vascularised, but the central vein does not become visible until con- 

 siderably later. The suprarenal glands after their separation from the peritoneum form a 

 ridge on either side of the posterior wall of the coelom medial to the mesonephros. Some little 

 time after the origin of the cortical portion of the gland has midergone cellular differentiation 

 and has become surrounded by a delicate capsule, the medullary portion is formed by the 

 migration of masses of sympatho-chromafiin cells from the medial side toward the centre of the 

 organ so that they surround the central vein as the anlage of the medullary nucleus. They 

 penetrate the cortical portion of the gland as development proceeds and become completely 

 surrounded by it. These migrating masses are entirely or for the most part of chromaffin 

 formative cells derived from the ectoderm. They are clearly differentiated from the cortical 

 cells by their small size and darker colour, in stained sections. Migration of these cell masses 

 into the gland seems to be continued even after birth. The differentiation of the cortex into 

 three layers occurs late in development. The suprarenal glands are relatively large in fcetal 

 life; and their relation to the kidneys is secondarily acquired. 



THE GLOMUS CAROTICUM 



The carotid bodies [glomera carotica] are small ovoid or spherical bodies 

 found at or near the point where the common carotid arteries divide into the inter- 

 nal and external carotids (fig. 1081). They are usually on the dorsal and medial 

 side of the angle of bifurcation of the arteries. There is ordinarily one body on 



Fig. 1081. — The Glomus Caroticum (Carotid Body). (From Testut, after Prince- 

 teau.) 1, Carotid body; 2,3, 4, common, external and internal carotids; 5, int. jugular; 7, inf. 

 cervical sympathetic ganglion; 8, vagus. 



each side, 5 or 6 mm. in length and 2 or 3 mm, in thickness. It is reddish-yellow 

 in colour and is attached to the carotid by fibrous tissue and by the vessels and 

 nerves which enter it. A small special fibrous band may sometimes be recog- 

 nised binding it to the common, external or internal carotid artery. 



The carotid body or gland is composed of two essential parts: (1) roimd, oval, or polyhedral 

 epithehal cells which contain chromaffin granules, and are bound together by a mass of fibrous 

 connective tissue; and (2) a rich plexus of capillaries and sinusoids forming a mesh. Large 

 lymph-vessels surround the outside of the gland. The carotid gland has a very abundant 

 nerve supply, mostly from the sympathetic system, and ganghon cells are found in it. It may 

 receive twigs from the superior larjTigeal, hypoglossal, or glossopharyngeal nerves, as recorded 

 by some observers. 



The size of the carotid body varies considerably. At times the carotid bodies are absent; in 

 other cases they are so small that they can be detected only in microscopic sections; occasionally 

 they are 8 mm. in length by 4 or 5 mm. in thickness. Rarely the carotid bodies may be broken 

 up into two or more smaller masses bound together by connective tissue. The carotid body 

 may be larger in old individuals due to an increase in the connective tissue or vascular elements 

 with a corresponding decrease in the epithelial cells. The origin is probably from sympatho- 

 chromaffin cells but some investigators believe that they are derived from the endothelium of the 

 blood-vessels and others that they arise from the endoderm of a branchial pouch. 



i 



