PROPERTIES OF THE HEART MUSCLE. 583 



if this happens their rhythm is slower than that of the sinus and 

 independent of it. So in the terrapin's heart, in which the sequence 

 of beat is so beautifully exhibited, if one ties a ligature between 

 auricle and ventricle, or cuts off the ventricle entirely, the sinus 

 venosus and auricle continue beating at their normal rhythm, while 

 the ventricle remains usually entirely quiescent. It would seem from 

 these facts that in the mammalian heart the ventricle when dis- 

 connected from the auricle is capable of maintaining a fairly rapid 

 rhythm of its own. At the other extreme, the terrapin's ventricle 

 when similarly treated shows ,no spontaneous beats at all. These 

 and many other facts that might be quoted support well the gen- 

 eral view proposed by Gaskell that the musculature of the venous 

 end of the heart (sino-auricular node) possesses the greater rhyth- 

 mical power and starts the heart beat, and that the wave of ex- 

 citation is propagated to the auricles and ventricles through the 

 muscular tissue, or the modified muscular tissue composing the 

 so-called conducting system. The precise course of the wave of 

 contraction from auricle to ventricle and through the ventricle is 

 discussed on p. 544. 



The Tonicity of the Heart Muscle. In describing the phys- 

 iology of skeletal and plain muscle attention was called to their 

 property of tonicity, that property by means of which they remain 

 in a more or less permanent although variable condition of con- 

 traction. So far as the skeletal muscles are concerned, this con- 

 dition is dependent upon their connections with the nervous system. 

 Cut the motor nerve, or destroy the motor center, and the muscle 

 loses its tone, becomes completely relaxed. Tonicity or tonic 

 activity is therefore characteristic of the motor nerve centers, and 

 is due, no doubt, to a more or less continuous inflow of sensory 

 impulses into those centers. The tonus of the nerve centers is a 

 reflex tonus. In the plain muscle the condition of tonus is also 

 marked. The blood-vessels, the bladder, the various viscera are 

 rarely, if ever, entirely relaxed for any length of time. This tonus 

 is also dependent, in many cases, upon a constant innervation 

 through the motor nerves, but after these latter have been destroyed 

 the plain muscle still shows this property of tonicity. So in the 

 heart muscle the power to maintain a certain degree of contraction, 

 a certain state of muscle tension quite independently of the sharp 

 systolic contractions, is very characteristic. At the end of a normal 

 diastole, for example, the ventricle is not entirely relaxed, it retains 

 a certain amount of tonicity as compared with its condition when 

 inhibited through the vagus nerve or when dead. The degree of 

 this tonicity determines, of course, the size of the ventricular 

 cavity (the diastolic volume) and the extent of the charge it will 

 take from the auricles. As will be described in the next chapter 

 the tone of the heart muscle is dependent in part upon its extrinsic 



