496 COMPARATIVE ANATOMY 



forms the cortical substance enclosing the suprarenal portion 

 or medulla, with its chromaffin tissue, 1 and the whole organ 

 is richly supplied with pigment, lymph-vessels, and blood- 

 vessels, although it possesses no portal system, as in Amphibia and 

 Sauropsida. As regards form, there is considerable variation in 

 the Mammalian adrenals ; they may be compact and smooth, or 

 more or less lobed, and are usually asymmetrical in position. 



Physiological experiments on the adrenals indicate that they 

 are of great functional importance, and the two chief theories with 

 regard to them are briefly as follows. The medullary portion is 

 said to be a gland with " internal secretion," and to give off into 

 the blood a substance (adrenalin) which reacts on the muscular 

 contraction of the heart and blood-vessels, and increases the blood- 

 pressure. On the other hand, there is some evidence that the 

 cortex takes part in the manufacture of a substance which is 

 passed directly or indirectly into the blood-stream, and which 

 destroys the useless or harmful products of destructive metabolism. 

 Future researches must show whether and to what extent the 

 theory of " internal secretion " or of " auto-intoxication " is 

 universally applicable. 



1 So-called " accessory suprarenals," consisting of nests of chromaffin cells, 

 are found in all Vertebrates in relation with the sympathetic system and blood- 

 vessels. 



