A text-book of physiology, for medical students and physicians . nters. The tonus of the nerve centers is areflex tonus. In (lie plain muscle the condition of tonus is alsomarked. The blood-vessels, the bladder, the various viscera arerarely, if ever, entirely relaxed for any length of time. This tonusis also dependent, in many cases, upon a constant innervationthrough the motor nerves, but after these latter have been destroyed PROPERTIES OF THE HEART MUSCLE. 559 the plain muscle still shows this property of tonicity. So in theheart muscle the power to maintain a certain degree of contraction
A text-book of physiology, for medical students and physicians . nters. The tonus of the nerve centers is areflex tonus. In (lie plain muscle the condition of tonus is alsomarked. The blood-vessels, the bladder, the various viscera arerarely, if ever, entirely relaxed for any length of time. This tonusis also dependent, in many cases, upon a constant innervationthrough the motor nerves, but after these latter have been destroyed PROPERTIES OF THE HEART MUSCLE. 559 the plain muscle still shows this property of tonicity. So in theheart muscle the power to maintain a certain degree of contraction,a certain state of muscle tension quite independently of the sharpsystolic contractions, is very characteristic. At the end of a normaldiastole, for example, the ventricle is not entirely relaxed, it retainsa certain amount of tonicity as compared with its condition wheninhibited through the vagus nerve or when dead. The degree ofthis tonicity determines, of course, the size of the ventricularcavity and the extent of the charge it will take from the Fig. 230.—To show tone waves in heart muscle. The record shows contractions of a6trip of the sinus venosus (terrapins heart) suspended in a bath of blood-serum. In addi-tion to the sharp contractions marked by the lines there are longer, wave-like shorteningsand relaxations, irregular in character, which are due to variations in tone. As will be described in the next chapter the tone of the heartmuscle is dependent in part upon its extrinsic nerves, but it ismore dependent probably upon the composition of the the property of rhythmicity, that of tonicity is mostdeveloped at the venous end of the heart. At least this is thecase with the heart of the cold-blooded animals, upon whichthis property has been studied most carefully. The ventricleof the terrapin, or strips excised from the ventricle and sus-pended so that their movements can be recorded, often varygreatly in length with differences in condition. The
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