684 PRJXC/PLES OF GENERAL PHYSIOLOGY 



<m the rate of the beat, while the effect on the auriculo- ventricular junction and 

 the ventricle remain. In these conditions the vagus decreases the rate of trans- 

 mission from auricle to ventricle, and diminishes the duration of the state of 

 excitation in the muscle fibres of the ventricle. This is associated with weakening 

 of contraction in all parts of the ventricle, and not to failure of some of them to get 

 excited at all. This diminution of duration of excited state by the vagus explains 

 why it changes the sign of the final T-wave in the electro-cardiogram, if we admit 

 that the negativity of the base indicated by this wave is due to the greater 

 duration of the excitatory state at the base than at the apex. The action of the 

 vagus being more powerful at the base than at the apex, it has the effect of 

 reducing the duration of the excited state at the two to approximate equality, 

 and thus tends to produce a simple diphasic effect. The application of atropiue 

 to different parts of the ventricle, by which the vagus endings are paralysed 

 locally, confirms the conclusions arrived at. 



Augmentor Serves. Like smooth muscle, the heart is supplied with two kinds 

 of nerve fibres, inhibitory and excitatory. The latter were discovered by Von 

 Bezold (1863), and their existence proved in a convincing manner by Von Bezold 

 and Bever (1867). A tracing of their effect on the heart of the toad is given in 

 Fig. 114 (page 406). In general, it may be said that the effect of the accelerator 

 nerves is exercised on the rate and the strength of the beat, and on the conducting 

 power of the muscle and, in all cases, in an opposite direction to that of the 

 vagus. Fig. 233 shows the improvement of conduction. Increase of excitability 

 has not, so far as I am aware, been demonstrated directly. 



These nerves arise from the sympathetic system. Their antagonistic relation 

 to the vagi has been discussed above (page 407). 



Reflexes to Heart Serves. Both the vagus and accelerator nerves can be 

 excited reflexly. It is not yet definitely known whether there is reciprocal 

 innervation, such as that of the vasomotor reflexes, in the reflexes to the heart. 

 Whether, for instance, when reflex slowing of the heart is produced, there is, 

 along with excitation of the vagi, inhibition of tone of the accelerator centre. 

 Bainbridge (1914) finds that reflex acceleration of the heart is produced by 

 inhibition of vagus tone, together with excitation of accelerators, but could find 

 no evidence of inhibition of accelerator tone in reflex slowing. Possibly there 

 was no tone in the accelerator centre, although the heart was apparently beating 

 at a maximal rate. This may, however, have been from its own pace-maker, 

 when relieved from vagus control. 



The heart reflexes are closely interconnected with those to the blood vessels, 

 and further facts with regard to them will be referred to along with the latter. 



THE BLOOD VESSELS 



Exact investigation of the phenomena of the circulation was impossible until 

 Ludwig (1847) invented the graphic method of recording blood pressure. His 

 portrait will be found in Fig. 234. 



THE PULSE WAVE 



Owing to the necessity of the pump being rhythmically active, the fact that 

 the blood vessels possess elastic walls has considerable importance. Since the 

 blood cannot pass through the arterioles as rapidly as it is driven into the aorta, 

 if the arteries were rigid and unable to accommodate, temporarily, the excess of 

 blood driven in by the contraction of the heart, it is plain that the efficient 

 action of the heart would be put to great strain, owing to the incompressibility 

 of the blood. Moreover, much less blood could be expelled at each beat. The 

 result of the stretching of the arterial wall by the beat of the heart, owing to the 

 elastic reaction when the aortic valves are closed, is to convert the rhythmic flow 

 from the heart into a continuous one through the capillaries. 



A further necessary incidental result of the elasticity of the arterial wall is 

 the pulse wave. As the fijjst portion of the system is distended by the blood, it 



