972 THE INTERNAL SECRETIONS 



stricter mechanism. This is also true of adrenin, because inasmuch as 

 the normal glands discharge their product into the suprarenal veins, 

 it must first be carried through the heart into the arteries. With a 

 normally active circulation this requires from 12 to 14 seconds. At 

 the end of this period of time, the blood pressure rises rather abruptly, 

 but declines very soon until its normal value has again been estab- 

 lished. The amplitude of this reaction depends, of course, upon the 

 potency and quantity of the adrenalin. Upon the heart, this agent 

 acts in two ways, namely (a) by lessening the frequency of this organ 

 through vagus-inhibition, and (6) by augmenting its force of con- 

 traction by a direct influence upon the cardiac musculature. Con- 

 sequently, the division of the vagi nerves must augment the rise in 

 blood pressure, because it prevents henceforth the inhibitory dis- 

 charges of the center from reaching the heart. It should be emphasized, 

 however, that the adrenalin does not stimulate the cardio-inhibitor 

 center directly, but in an indirect way through its effect upon the blood 

 pressure. As has been pointed out in one of the preceding chapters, 

 a high blood pressure invariably elicits a reflex which slows the heart, 

 its cause being resident in the distention of the arteries, chiefly of 

 the root of the aorta. 



Regarding the nature of this reaction, it may be stated that the 

 adrenalin constricts the arteries, and especially the arterioles, thereby 

 preventing normal amounts of arterial blood from escaping into the 

 capillaries. Its action, therefore, is to increase the peripheral resistance 

 by lessening the size of the arterio-capillary outlet. At this point 

 of the vascular system two elements are present, namely the smooth 

 muscle cells and the terminals of the vasomotor nerves. Where 

 then is the point of attack of the adrenalin? Since this rise in blood- 

 pressure may also be produced after the destruction of the cord and 

 sympathetic ganglia and even after the completion of secondary 

 degeneration of the postganglionic fibers, it cannot justly be regarded 

 as a nervous reaction. Moreover, the evidence so far presented tends 

 to show that it does not affect the contractile elements of the smooth 

 muscle cells directly, but some substance interposed between the latter 

 and the terminals of the nerve. In accordance with Langley and 

 Elliott, it must be concluded that this structure is the myoneural 

 junction which is composed of receptor substance, i.e., of a type of 

 neuroplasm somewhat distinct from ordinary nerve tissue. Adrena- 

 lin, therefore, acts upon the myoneural connection between the sym- 

 pathetic nerve fibers and the muscle cells. 



At the hand of this fact, it will now be seen that the adrenin dis- 

 charged by the adrenal bodies, must exercise a similar function. It is 

 poured out as a rule in insignificant amounts and aids in keeping 

 the vascular system in a semi-constricted condition i.e., in a state of 

 tonus. Moreover, in consequence of definite stimuli, larger amounts 

 may be discharged at any time which actually constrict the blood- 

 vessels and give rise to a temporary increase in blood pressure. This 



