PROPERTIES OF THE HEART MUSCLE. 529 



bring it to rest also. In this animal, therefore, the amount of potassium in 

 the blood is so adapted that it holds the ventricular end entirely quiescent. 

 In the mammalian heart it may be assumed that the amount of potassium is 

 sufficient to keep the spontaneous rhythm of the ventricle slower than that 

 of the auricles or veins, and therefore subordinates the rhythm of the whole 

 heart to that of the venous end. In the terrapin's heart, at least, the re- 

 moval or reduction of the potassium or the increase of the calcium may lead 

 to an independent ventricular rhythm the beat of the heart becomes arhyth- 

 mical. 



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 and the extent of the charge it will take from the auricles. 

 Like the property of rhythmicity, that of tonicity is most developed 

 at the venous end of the heart. At least this is the case with the 

 heart of the cold-blooded animals upon which this property has 

 been studied most carefully. The ventricle of the terrapin, or 

 strips excised from the ventricle and suspended so that their move- 

 ments can be recorded, often vary greatly in length with differences 

 in condition. These variations are due to changes in tone. Not 

 infrequently these changes take on a rhythmical character; so 

 that if the ventricle is beating one sees upon the record regular 

 tone waves, an alternate slow shortening and slow relaxation quite 

 independent of the rhythmical beats. The tissue of the auricle and 



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