420 THE ORGANS 



The MEDULLA (Fig. 295, C) consists of spherical and oval 

 groups and cords of polygonal cells. After alcohol or formalin fixa- 

 tion these cells take a paler stain than those of the cortex. After 

 fixation in solutions containing chromic acid or chrome salts the cells 

 of the medulla assume a peculiar characteristic deep brown color, 

 which cannot be removed by washing in water and which is due to 

 chromafhn granules which they contain (p. 411). The chromaffin 

 content varies in different animals and with age. Thus while in the 

 adult human the chromaffin reaction is strong, little or no re- 

 action is present in the foetal adrenal. The secretion of the med- 

 ullary part of the adrenal is known as adrenalin, apparently the ma- 

 ture condition of the intra-cellular chromaffin granules. As already 

 noted (p. 417) it is probably an active agent in the regulation of 

 arterial tension. 



Scattered in irregular groups among the chromaffin cells are many 

 sympathetic ganglion cells. 



Blood-vessels. — The arteries supplying the adrenal first form 

 a poorly defined plexus in the capsule. From this are given off three 

 sets of vessels — one to the capsule, one to the cortex, and one to the 

 medulla. The first set breaks up into a network of capillaries, which 

 supply the capsule. The vessels to the cortex break up into capillary 

 networks, the shape of the mesh corresponding to the arrangement of 

 the connective tissue in the different zones. The v^essels to the me- 

 dulla pass directly through the cortex without branching and form 

 dense capillary networks among the groups of medullary cells. The re- 

 lations of the capillaries to these glands cells are extremely intimate, 

 especially in the reticular zone and medulla, where the cells in many 

 cases immediately surround the capillaries in much the same manner 

 as the glandular cells of a tubular gland surround their lumina. From 

 the capillaries of both cortex and medulla small veins arise. These 

 unite to form larger veins which empty into one or two main veins 

 situated in the center of the medulla. 



Lymphatics. — These follow in general the course of the blood- 

 vessels. The exact distribution of the adrenal lymph system has 

 not been as yet satisfactorily determined. 



Nerves. — The nerve supply of the adrenal is so rich and the 

 nerve elements of the gland are so abundant as to have led to its 

 classification by some among the organs of the nervous system. 

 Both medullated and non-medullated fibres — but chiefly the latter 

 — form plexuses in the capsule, where they are associated with groups 



