THE SUPEAKENAL GLANDS. 1343 



(iii.) The glandula carotica (B.N.A. glomus caroticum; O.T. intercarotid body) 



, is a bilateral paired organ situated in close but slightly variable relation to the bifurcation 

 of the common carotid artery. Frequently it lies deep to the bifurcation ; sometimes it 

 is wedged in between the internal and external carotids at their commencement ; some- 

 times it is placed between them at a slightly higher level. Its shape varies with its 

 position. When free from pressure it is oval ; when compressed by the internal and 

 external carotids it is wedge-shaped. On the average, its height is 7 mm., its breadth 

 1-5-5 mm. Not infrequently it is split into two or more nodules. Its colour is grayish, 

 yellowish, or brownish red. 



Structure. The glandula carotica is built up of nodules of chromaphil tissue surrounded 

 and supported by fibrous tissue. The nodules are penetrated by a mass of sinus-like blood 

 capillaries and surrounded by large lymph vessels. Scattered nerve-cells are present, and the 

 whole organ is permeated by non-medullated nerve-fibres, which establish intimate connexion 

 with the chromaphil cells. 



(iv.) The meduMary portions of the suprarenal glands, although belonging to 

 the chromaphil system, are described below (p. 1346). 



Development of the Chromaphil System. All chromaphil tissue develops in intimate 

 relation with the sympathetic nervous system. It is not derived from the sympathetic tissue 

 nor is the sympathetic tissue derived from it. Both are the descendants of a primitive, 

 indifferent, sympatho- chromaphil blastema, which in a 16-mm. embryo occupies the regions 

 corresponding to those occupied by the sympathetic system of the adult. It is composed of 

 tightly packed deeply staining cells about 5 ^ in diameter. The ancestry of these cells can be 

 traced back with strong probability to the cells of the neural crest (see Development of the 

 Sympathetic Nervous System). 



The differentiation of chromaphiloblasts from sympathoblasts begins when the embryo is 

 about 18 mm. in length, but is not completed until late in gestation, if then. The process is 

 marked by an increase in size of the chromaphil formative cells and by a diminution in the 

 intensity of their reaction to ordinary stains. Later, the specific chrome reaction develops, but 

 the exact stage at which this occurs is unknown. It is important to note that if any cells in an 

 area differentiate, all do. The result is that, in spite of their intimate relations and common 

 origin, an intermixture of chromaphil and sympathetic cells is extremely rare. 



The first of the chromaphil masses to differentiate are the aortic bodies. They are prominent 

 structures in a 20-mm. embryo. Later, the paraganglia of the sympathetic plexuses develop, 

 and last the paraganglia of the gangliated trunk. 



The development of the glandula carotica requires special description. It takes origin from 

 a strand of sympatho -chromaphil blastema, which extends ventrally from the region of the 

 superior cervical sympathetic ganglion, deep to the internal carotid artery or between the 

 internal and external carotids. Differentiation begins when the embryo is about 20 mm. in 

 length, and is completed by the time it is 30 mm. long. In connexion with the development 

 of the glandula, there is a peculiar thickening of the wall of the internal carotid artery near the 

 developing gland. In the dog a similar thickening of the wall of the internal carotid artery 

 takes place, although in that animal the glandula lies beside the external carotid. Further, in 

 a 45-mm. foetus in which the glandula is fully differentiated the thickening is still present. 

 These facts show that in the higher animals the thickening has no connexion with the develop- 

 ment of the glandula, though the association of chromaphil bodies with blood-vessels in 

 cyclostomata and elasmobranchs (see Comparative Anatomy of the Chromaphil and Cortical 

 Systems) compels caution in excluding the possibility of there being at least some phylogenetic 

 relation between the two. It has frequently been stated, and is widely held, that the glandula 

 carotica is developed from or receives some contribution from the entoderm of the third 

 pliaryngeal pouch. This is not the case (see Parathyreoid Glands, Development). 



B. THE CORTICAL SYSTEM. 



The cortical system is composed of several masses of glandular tissue peculiarly 

 rich in lipoids. Its function is undetermined. The distribution of the masses is 

 shown in Fig. 1053. There are (i.) the cortical portions of the suprarenal glands, 

 (ii.) accessory cortical bodies. These are described below (see Accessory Suprarenal 

 G-lands and Cortical System, Development). 



C. GLANDULE SUPRARENALES. 



The suprarenal glands (O.T. suprarenal bodies or capsules, adrenal glands) 



are compound organs consisting of a capsule of cortical substance enclosing a 

 medulla of chromaphil tissue. Typically, there are two suprarenal glands, a 

 right and left, placed in the epigastric region, one on each side of the vertebral 



