THE VASCULAR MECHANISM. 2OQ 



arranged to write one above the other. Fig. 76 represents a tracing 

 of the venous pulse taken by this method. 



The venous pulse shows three waves ; the first rise, a, corresponds 

 with the auricular systole, the second, c, is simultaneous with the 

 beginning of the ventricular systole, and the third more rounded wave, 

 v, is due to the gradual filling of the auricle towards the end of 

 ventricular systole. The waves correspond with the changes of 

 pressure in the auricle, being transmitted along the column of blood 

 in the vein. 



The venous pulse is confined to the large veins, and as the arterial 

 pulse is extinguished in the arterioles, there is normally no pulsation in 

 the medium-sized and smaller veins. The arterial pulse wave may, 

 however, extend into the smaller veins under certain conditions. When 

 the chorda tympani nerve is stimulated, for example, the arterioles of 

 the submaxillary gland are dilated, and not only is the amount of blood 



\fenous Pulse 



FIG. 76. Time-marker records i sees. 



flowing through the gland increased, but the pulse wave extends into 

 the veins coming from the gland. The transmission of the pulse into 

 the veins is due, in this and similar cases, to the diminution of the 

 resistance in the arterioles, which is a consequence of their dilatation. 



SECTION IV. 

 THE CAUSATION OF THE HEART BEAT. 



We have now to consider how the beat of the heart originates, and 

 the conditions upon which its normal rhythm depends and by which 

 that rhythm can be modified. For this purpose the slowly beating and 

 relatively simple heart of the frog has proved to be of great value. 



Anatomy of the Frog's Heart. The frog's heart consists of a sinus 

 venosus, two auricles (atria), a ventricle, and a bulbus arteriosus. The 

 blood from the venae cavse enters the sinus, passes into the right auricle, 

 and thence into the ventricle. The blood coming from the lungs enters 

 the left auricle and passes into the ventricle. The ventricle opens into 

 the bulbus arteriosus, from which the aortic arches arise and distribute 

 the blood to the entire body, including the lungs. ,The cardiac muscle 



14 



