454 COMPARATIVE ANATOMY 



Cannon's well-known experiments upon animals led him to his emer- 

 gency theory of adrenal action. He found that in a quiet undisturbed 

 animal adrenin is absent from the blood. When, however, the animal 

 is excited by pain, fear, or anger, adrenin increases. As a result, the 

 heart beats more strongly, breathing becomes deeper and more rapid, 

 intestinal action ceases, the liver releases sugar more rapidly, the muscles 

 respond more quickly to stimulation, the tonus of the blood vessels is 

 raised, the coagulability of the blood increases, and so the animal is pre- 

 pared either to run or fight. Thus the medullary secretion reinforces the 

 action of the sympathetic nerves. 



Cannon has also shown that a substance which resembles adrenin in 

 its effects is given off by organs stimulated by the sympathetic nervous 

 system. This substance, the chemical nature of which is unknown, has 

 been called sjrmpathin. 



Development. Corresponding with its double function and its 

 division into cortex and medulla, each adrenal has a double origin. The 

 cortex develops from mesoderm, while the medulla is derived from sympa- 

 thetic ganglia and is therefore ectodermal in origin. In the human 

 embryo of 6 mm., the cortical substance makes its first appearance as a 

 proliferation of the coelomic epithelium near the root of the mesentery. 

 The cells of the anlage lose their connexion with the epitheHum and take 

 positions at the sides of the abdominal aorta. There is no evidence of a 

 segmental origin. 



The cells of the medulla, on the other hand, come from the chain 

 of sympathetic ganglia. In a 6 mm. human embryo, the cells which later 

 form the adrenal medulla may be distinguished in the ganglia by their 

 affinity for chromic acid which stains them brown while other cells of the 

 ganglia remain unstained. The migration of chromafl&n cells into the 

 adrenal cortex begins in embryos of seven weeks (17 mm.) and may 

 continue until after birth. In embryos of the third month, the adrenals 

 are more conspicuous organs than the kidneys, but as development pro- 

 ceeds, they fail to keep pace with the enlargement of the body. 



Evolution. Adrenal glands are limited to vertebrates. Homologous 

 organs are lacking in invertebrates and protochordates. In Cyclostomes, 

 both the cortical layer and the chromaffin are present, but are spatially 

 separated. The cortical substance is represented by clusters of cells, 

 the interrenal bodies, which lie throughout the length of the body cavity 

 near the post-cardinal veins. The chromaffin cells, on the other hand, 

 are arranged as strands along the dorsal aorta. In the Elasmobranchs, 

 the interrenal bodies tend to aggregate in the posterior part of the body 

 cavity, while the chromaffin cells are arranged in metameric groups near 

 the sympathetic ganglia. 



