204 THE VASCULAK MECHANISM. 



with the handle of a scalpel, the heart will stand still in diastole with all 

 the phenomena of vagus inhibition. If the nervi mesenteriei, or the connec- 

 tions of these nerves with the spinal cord, be stimulated with the interrupted 

 current, cardiac inhibition is similarly produced. If in these two experiments 

 both vagi are divided, or the medulla oblongata is destroyed, inhibition is 

 not produced, however much either the intestine or the mesenteric nerves be 

 stimulated. This shows that the phenomena are caused by impulses ascend- 

 ing along the mesenteric nerves to the medulla, and so affecting a portion of 

 that organ as to give rise by reflex action to impulses which descend the 

 vagi as inhibitory impulses. The portion of the medulla thus mediating 

 between the afferent and efferent impulses may be spoken of as the cardio- 

 inhibitory centre. 



Reflex inhibition through one vagus may be brought about by stimula- 

 tion of the central end of the other. In general the alimentary tract seems 

 in closer connection with the cardio-inhibitory centre than other parts of the 

 body ; and if the peritoneal surface of the intestine be inflamed, very gentle 

 stimulation of the inflamed surface will produce marked inhibition. But 

 apparently stimuli, if sufficiently powerful, will through reflex action pro- 

 duce inhibition, whatever be the part of the body to which they are applied. 

 Thus, crushing a frog's foot will stop the heart, and adequate stimulation of 

 most afferent nerves will produce some amount of inhibition. 



The details of the reflex chain and the portion of the centre concerned in 

 the development of augmenting impulses have not been worked out so fully 

 as in the case of inhibitory impulses, but there can be little doubt that the 

 former, like the latter, are governed by the central nervous system. 



146. So far we have been dealing with the heart of the frog, but the 

 main facts which we have stated regarding inhibition and augmentation of 

 the heart beat apply also to other vertebrate animals, including mammals ; 

 and, indeed, we meet similar phenomena in the hearts of invertebrate 

 animals. 



If in a mammal the heart be exposed to view by opening the thorax, and 

 the vagus nerve be stimulated in the neck, the heart may be seen to stand 

 still in diastole, with all the parts flaccid and at rest. If the current em- 

 ployed be too weak, the result as in the frog is not an actual arrest, but a 

 slowing or weakening of the beats. If a light lever be placed on the heart 

 a graphic record of the standstill or of the slowing, of the complete or incom- 

 plete inhibition, may be obtained The result of stimulating the vagus is 

 also well shown on the blood-pressure curve, the effect of complete cardiac 

 inhibition on blood-pressure being most striking. If, while a tracing of 

 arterial pressure is being taken, the beat of the heart be suddenly arrested, 

 some such curve as that represented in Fig. 75 will be obtained. It will be 

 observed that two beats follow the application of the current marked by the 

 point a, which corresponds to the signal x on the line below. Then for a 

 space of time no beats at all are seen, the next beat b taking place almost 

 immediately after the shutting off the current at y. Immediately after the 

 last beat following a there is a sudden fall of the blood-pressure. At the 

 pulse due to the last systole the arterial system is at its maximum of disten- 

 tion ; forthwith the elastic reaction of the arterial walls propels the blood for- 

 ward into the veins, and, there being no fresh fluid injected from the heart, 

 the fall of the mercury is unbroken, being rapid at first, but slower afterward, 

 as the elastic force of the arterial walls is more and more used up. With the 

 returning beats the pressure correspondingly rises in successive leaps until 

 the normal mean pressure is regained. The size of these returning leaps of 

 the mercury may seem disproportionately large, but it must be remembered 

 that by far the greater part of the force of the first few strokes of the heart 



