THE VASCULAR MECHANISM. 



193 



FIG. 71. 



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

 its movements may be registered. When the ventricle con- 

 tracts, and by contracting diminishes in volume, there is a 

 lessening of pressure in 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 interference 

 with the flow of blood or fluid through it. 



The heart of the frog, as we have just said, will con- 

 tinue to beat for hours after removal from the body, even 

 after the cavities have been cleared of blood, and, indeed, 

 when they are almost empty of all fluid. The beats thus 

 carried out are in all important respects identical with 

 the beats executed by the heart in its normal condition 

 within the living body. Hence we may infer that the A p er fusion Camiia. 

 beat of the heart is an automatic action ; the muscular 

 contractions which constitute the beat are due to causes which arise spon- 

 taneously in the heart itself. 



In the frog's heart, as in that of the mammal, 115, there is a distinct 

 sequence of events which is the same whether the heart be removed from, or 



Purely Diagrammatic Figures of I. Perfusion canula tied into frog's ventricle : a, entrance : 

 b, exit-tube ; A, wall of ventricle ; B, ligature. 



II. Roy's apparatus modified by Gaskill , a, chamber filled with saline solution and oil, con- 

 taining the ventricle A tied on to perfusion canula /; 6, tube leading to cylinder c, in which 

 moves piston d, working the lever e. 



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 beat of the bulbus arteriosus, which does not, like the 

 mammalian aorta, simply recoil by elastic reaction after distention by the 

 ventricular stroke, but carries out a distinct muscular contraction passing in 

 a wave from the ventricle outward. 



When the heart in dying ceases to beat, the several movements cease, 

 as a rule, in an order the inverse of the above. Omitting the bulbus arte- 



