404 ESSENTIALS OF PHYSIOLOGY. 



the force and (if the vagi are divided) the rate of the heart, and at 

 the same time dilates the coronary vessels, so that, in spite of the rise 

 in blood pressure, the efficiency of the heart is maintained and its 

 output may even become larger. It inhibits the movements of the 

 digestive tract and (in many animals) of the bladder, but causes 

 constriction of the ileo-colic sphincter ; it may also produce sweating, 

 erection of hairs, and dilatation of the pupil. 



Adrenalin is an extremely active substance, and even 0*0025 

 milligram per kilo of body weight, when injected into the circula- 

 tion, produces a definite rise of blood pressure. 



The exact point of action of adrenalin is not on the sympathetic 

 endings proper, but probably on some receptive substance (neuro- 

 nmscular junction), which is believed to lie between the actual nerve 

 ending and the muscular fibre which it supplies, and which is un- 

 affected by degeneration of the nerve fibre. This can be shown by the 

 following experiment. When the cervical sympathetic nerve, which 

 supplies the pupil, is stimulated, or when adrenalin is injected into a 

 vein, the pupil dilates. If the superior cervical ganglion on one side 

 is removed, the post-ganglionic fibres degenerate, and when time has 

 been allowed for their degeneration electrical stimulation of these fibres 

 produces no effect on the pupil, whereas on the injection of adrenalin 

 the pupil dilates even more fully than in the normal animal. 



Adrenalin is constantly being formed by the suprarenal glands, 

 from which it passes into the blood stream, and is thus a true internal 

 secretion.. This secretion can be increased in amount by stimulation 

 of the splanchnic nerves, which contain secretory fibres for the supra- 

 renal glands. The nerves may be stimulated either directly, e.g. by 

 stimulation of the peripheral end of a divided splanchnic nerve, or 

 reflexly, the centre for this reflex being either the vaso-motor centre 

 or an adrenalin centre lying very near the vaso-motor centre. The 

 occurrence of reflex secretion of adrenalin by the suprarenal glands can 

 be demonstrated in the following manner : one splanchnic nerve, e.g. 

 the left, is divided in the cat, so as to cut off the efferent path to one 

 gland ; and it is found that, as the result either (1) of stimulation of 

 sensory nerves, or (2) of violent emotion, such as fear, that the 

 adrenalin is discharged more or less completely from the right supra- 

 renal gland, while the left gland remains unaffected. Evidently 

 division of the splanchnic nerve, by breaking the efferent side of the 

 reflex arc, prevents any reflex secretion of adrenalin from that gland 

 into the blood stream. The effects of stimulation of the splanchnic 

 nerve, and the consequent setting free of adrenalin, on the arterial 

 blood pressure and on the constriction of arterioles outside the 



