570 CIRCULATION OF BLOOD AND LYMPH. 



respects it differs from the typical heart muscle of the vertebrate, 

 but the difference is perhaps sufficiently explained by the discovery 

 fp. 563) that the crustacean heart, in one form at least, is not an 

 automatically rhythmical tissue. Its rhythmical contractions, like 

 those of the diaphragmatic muscle in the higher vertebrates, depend 

 upon rhythmical impulses received from nerve centers. 



The Compensatory Pause.— It has been observed that when an extra 

 systole is produced by sthmilating a ventricle it is followed by a pause longer 

 than usual; the pause, in fact, is of such a length as to compensate exactly 

 for the extra beat ; so that the total rate of beat remains the same. The pro- 

 longed pause under these conditions is therefore frequently designated as the 

 compensatory -pause. It has been shown,* however, that the exact compen- 

 sation in this case is not referable to a property of heart muscle, but is due to 

 the dependence of the ventricular upon the auricular beat. When the auricle 

 or ventricle is isolated and stimulated the phenomenon of exact compensation 

 is not observed. In an entire heart, on the contrary, the beat originates at 

 the venous end of the auricle and is propagated to the ventricle. If the latter 

 chamber is stimulated so as to give an extra beat out of sequence it will remain 

 in diastole until the next auricular beat stimulates it, and will thus pick up 

 the regular sequence of the heart beat. 



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 ten- 

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

 dition is dependent upon their connections with the nervous sys- 

 tem. 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 de- 

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

 in the heart muscle the power to maintain a certain state of 

 tension quite independently of the sharp systolic contractions is 

 very characteristic. At the end of a normal diastole, for example, 

 it is usually assumed, although positive proof is lacking, that 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 deter- 



* Cushny and Matthews, "Journal of Physiology," 21, 227, 1897. 



