150 INTRODUCTION TO GENERAL PHYSIOLOGY 



blood pressure by vascular dilatation, also produces slowing of the 

 heart through the vagus nerves. 



The endings of both kinds of nerves in the heart are accessible 

 to chemical stimuli. Thus the vagus endings are stimulated by 

 acetyl-choline, the accelerator endings by adrenaline. The former 

 are paralysed by atropine (E., p. 225). 



Origin and Transmission of the Heart Beat. If the heart of 

 the frog or tortoise be observed carefully (E., p. 225), it will be seen 

 that the different cavities contract in a regular order, beginning at 

 the junction of the great veins with the sinus venosus, and ending at 

 the commencement of the aorta. Thus there is a place which has the 

 property of beating more rapidly than other parts. In fact, if the 

 various cavities are cut away from one another, it is found that the 

 natural rate of the activity of each part decreases in order of its 

 distance from the sinus. Since the contraction of one cavity is 

 transmitted to the next, it is clear that the most rapid one sets the 

 pace for the others. In the mammalian heart, although the sinus 

 no longer exists as a separate cavity, there is a mass of tissue 

 of similar structure to the sinus, and situated at the point where 

 the great vein of the head and neck joins the right auricle. From 

 this tissue each heart beat is initiated, and it is known as the 

 *' Keith-Flack or sinu-auricular node." In the frog, the contraction 

 progresses as a wave in the ordinary muscular tissue, which is 

 continuous throughout the series of chambers. There is, how- 

 ever, evidence that even here there is a certain degree of specialisa- 

 tion of a part of the connecting tube or funnel between auricles 

 anJ ventricle, such that this part conducts more rapidly than 

 other parts. In the mammal, this conducting tissue has become 

 a bundle of a peculiar kind of muscle cells which have developed 

 the capacity of rapid conduction. Consideration of the anatomical 

 arrangement by which the blood enters the ventricle at what is 

 called the base, where it is united to the auricle, and also leaves 

 it at the same end, leads us to realise that it must be an advantage, 

 especially in large hearts, if the ventricle contracts as simultaneously 

 as possible in all its parts, instead of in a rather slow wave 

 progressing from base to apex. This is provided for by the 

 auriculo-ventricular bundle above mentioned, which conducts about 

 ten times as fast as the ordinary ventricular muscle, sending out 

 branches to all the various regions of the ventricle. Thus con- 

 traction at the apex is almost coincident with that at the base, 

 and the contents of the cavity are expelled more effectively than 

 if different parts were not in the same phase of contractile stress 

 at the same time. 



The nerves which regulate the beat of the heart are in 

 especially intimate connection with the node from which the con- 

 tractions start. 



