1236 PHYSIOLOGY 



destruction of the cord and sympathetic plexuses but even after time has 

 allowed for the peripheral (post-ganglionic) fibres to degenerate as a result of 

 extirpation of the corresponding ganglia. Although the effect is not altered 

 under these circumstances, and it may still produce either relaxation or 

 contraction of muscles according to the original action of the sympathetic 

 on these fibres, we are not justified in regarding it as acting on the contractile 

 material of the cells themselves. Rather must we assume with Langley and 

 Elliott that the action of adrenaline is on some substance mediating between 

 the nerve and the responsive tissue. We may speak of this reactive material 

 as the receptor substance (Langley), or we may locate it in the situation 

 where the nerve joins the muscle or gland cell, and describe adrenaline as 

 acting on the myoneural junction. 



Each suprarenal receives a number of filaments from the splanchnic 

 nerve on its own side. These pass to the medulla where they end apparently 

 without the interposition of any ganglion cells on their course (Elliott), the 

 cells of the medulla having themselves been developed by a modification of 

 sympathetic ganglion cells. Stimulation of the peripheral end of the 

 splanchnic nerve causes, as we have already seen, a discharge of adrenaline 

 into the blood stream. This discharge accounts for the secondary rise, 

 often accompanied with quickening of the heart, observed on a blood- 

 ptessure tracing as the result of stimulating the splanchnic nerve. Through 

 the splanchnic nerves a discharge of adrenaline can be excited by many 

 general conditions, such as pressure on the brain, puncture of the fourth 

 ventricle, administration of anaesthetics, mental disturbances such as excite- 

 ment or fright. Such a discharge is an important element in the adaptation 

 to environmental stress and enables the animal to react for the preservation 

 of its life either by offence or flight. If one splanchnic nerve be cut before the 

 administration of anaesthetics or the maintenance of a condition of irritation 

 or fright, the suprarenal gland on the corresponding side will be found to 

 contain two or three times as much adrenaline as the gland which has been 

 left in connection with the central nervous system. It is interesting that 

 no such condition of exhaustion can be produced by electrical stimulation of 

 the peripheral end .of the divided splanchnic. It has been s invested 

 therefore that the splanchnic nerve contains two sets of fibres, anabolic 

 and catabolic, that only the latter are stimulated by central irritation, 

 whereas electrical stimulation, exciting both sets of fibres, causes an increased 

 production of adrenaline in the gland, which exactly keeps pace with the 

 increased output. 



When adrenaline is injected into the blood stream the ctTeet is only 

 temporary. It is not excreted in the urine, but rapidly disappears from t he 

 blood. Since it is easily oxidised and is extremely unstable in alkaline 

 solution, we may conclude that after performing its excitatory function it is 

 ' destroyed by oxidation in the fluids. Adrenaline is thus a typical hormone, 

 a body of comparatively low molecular weight, having a drug-like excitatory 

 action on specific tissues of the body, easily diffusible, and rapidly destroyed 

 after discharging its office. 



