296 LIFE : OUTLINES OF GENERAL BIOLOGY 



but have as their chief function that of neutralising the poisonous 

 substance guanidine, which is formed from the creatine of muscle. 



THE SUPRARENAL OR ADRENAL CAPSULES.— These are 



present in most Xertcbrate animals, and occur in man as two 

 cocked-hat-shajx'd bodies, one on the upper border of each kidney. 

 They have a remarkable structure and a remarkable develop- 

 ment. In mammals they show two very distinct parts, a central 

 medulla and a peripheral, often yellowish, cortex, differing in minute 

 structure. The medullary substance arises in development from 

 the sympathetic nerve ganglia; while the cortical substance arises 

 between the two kidneys from a number of thickenings of the 

 epithelium that lines the body-cavity. Thus the suprarenal or 

 adrenal apparatus in higher Vertebrates is an intimate union of 

 a mcsodermic and an ectodcrmic primordium. 



In birds and reptiles the suprarenals are elongated and situated 

 near the reproductive organs; in Amphibians they lie as a yellowish 

 streak along the ventral surface of the kidney. But it is their con- 

 dition in Elasmobranch fishes that is most illuminating, for there 

 the two parts are separate from one another. In sharks and skates 

 the cortical substance is represented by yellow interrenals and the 

 medullary substance by independent chroma ffine organs. A remark- 

 able feature is that when the two fuse in Amphibians the chrom- 

 afhne component encloses the other — just the opposite of the final 

 state of affairs in mammals! There also the chromafFme tissue is 

 outside the cortical tissue to begin with. 



A pathological condition of the suprarenal apparatus in man 

 is associated with Addison's disease, marked by a bronzing of the 

 skin ; the removal of the organs from a mammal is always fatal, 

 but it is the mysterious corte.x which is essential for lite. The 

 specific hormone is adrenin or adrenalin, and it is produced by 

 the medulla only, and it may be noted that it is also produced 

 by isolated patches of chromafTme tissue that sometimes occur 

 on the peritoneum or along with the sympathetic ganglia. The 

 everyday physiological importance of adrenalin is to keep up the 

 tone of the muscles, including those of the blood-vessels, and it 

 does this through the endings of the sympathetic nerves. The 

 amount that passes into the blood-stream is affected by the 

 splanchnic nerves, and these again are excited by emotions such 

 as rage and fear. In the case of the angry man, the emotion excites 

 the splanchnic nerves, which stimulate the outflow of adrenalin. 

 When the hormone passes into the blood, it improves the vigour 

 of the heart and the tone of the muscles; it increases the amount 

 of sugar in the blood, and increases metabolism; in short, it pre- 

 pares the lx)dy for a fight. It also appears to increase the coagula- 

 bility of the blood, which will be of advantage if there is a wound. 



