CHAP, iv.] THE VASCULAR MECHANISM. 



241 



up and down. The piston again bears on a lever e by means of which 

 its movements may be registered. When the ventricle contracts, and 

 by contracting diminishes in volume, there is a lessening of pressure in 



FIG. 67. PURELY DIAGRAMMATIC FIGURES OF 



I. PerfusioD canimla tied into frog's ventricle, a, entrance, b, exit-tube ; a, wall 

 of ventricle ; ft, ligature. 



II. Hoy's apparatus modified by Gaskell. a, chamber filled with saline solution 

 and oil, containing the ventricle a tied on to the profusion cannulay; b, tube leading 

 to cylinder c, in which moves piston d, working the lever e. 



the interior of the chamber ; this is transmitted to the cylinder, and 

 the piston correspondingly rises, carrying with it the lever. As the 

 ventricle subsequently becomes distended, the pressure in the chamber 

 is increased, and the piston and lever sink. In this way variations in 

 the volume of the ventricle may be recorded, without any great inter- 

 ference with the flow of blood or fluid through it. 



The heart of the frog, as we have just said, will continue to 

 beat for hours after removal from the body, even though the cavi- 

 ties have been cleared of blood, and, indeed, when they are almost 

 empty of all fluid. The beats thus carried out are in all import- 

 ant respects identical with the beats executed by the heart in its 

 normal condition within the living body. Hence we may infer 

 that the beat of the heart is an automatic action : the muscular 

 contractions which constitute the beat are due to causes which 

 arise spontaneously in the heart itself. 



In the frog's heart, as in that of the mammal, 108, there is a 

 distinct sequence of events which is the same whether the heart be 

 removed from, or be still in its normal condition within the body. 

 First comes the beat of the sinus venosus, preceded by a more or 

 less peristaltic contraction of the large veins leading into it ; next 

 follows the sharp beat of the two auricles together ; then comes the 

 longer beat of the ventricle ; and lastly the cycle is completed by the 



16 



