COURSE OF CIRCULATION IN MAMMALS 143 



are much thinner than that of the left ventricle. This is so because 

 the energy required of the left ventricle must exceed that of the right 

 ventricle, inasmuch as the resistance in the systemic system exceeds 

 that in the pulmonary circuit. 



The Cardiac Cycle. The changes in form of the heart can be studied 

 in an animal the heart of which has been exposed by opening the 

 thorax under an anaesthetic, artificial respiration being meanwhile 

 maintained, the movements being recorded by levers writing on the 

 kymograph ; or the mammalian heart may be removed and fed with 

 warm oxygenated nutritive fluid (see p. 159); or the circulation may 

 be short-circuited by what is known as the heart-lung preparation, 

 the blood still being sent through the lungs of the animal to keep it 

 oxygenated (see p. 163). 



When the heart is watched beating in full vigour and rapidity, 

 it is an impossible task to unravel by the eye alone the sequence 

 of events. 



Harvey, the discoverer of the circulation of the blood, felt and 

 described this difficulty in his writings: 



" When first I gave my attention to vivisections, as a means of 

 discovering the movements and uses of the heart, and sought to 

 discover these from actual inspection, and not from the writings of 

 others, I found the task so truly arduous, so full of difficulties, that 

 I was almost tempted to think (with Fracastorius) that the movement 

 of the heart was only to be comprehended by God ; for I could neither 

 rightly perceive at first when the systole and when the diastole took 

 place, nor when and where dilatation occurred, by reason of the 

 rapidity of the movements, which in many animals is accomplished 

 in the twinkling of the eye, coming and going like a flash of lightning ; 

 .so that the systole presented itself to me, now from this point, now 

 from that, the diastole the same; and then everything was reversed, 

 the movements occurring, as it seemed, variously and confusedly 

 together. 



" When the heart begins to flag, to move more slowly, and, as it 

 were, to die, the movements then become slower and rarer, the pauses 

 longer, by which it is made much more easy to perceive and unravel 

 what the movements really are, and how they are performed. 



" In the pause," Harvey says, "as in death, the heart is soft, 

 flaccid, exhausted, lying, as it were, at rest. In the movement and 

 interval in which this is accomplished, three principal circumstances 

 are to be noted: 



" 1. That the heart is excited, and rises upward, so that at this 

 time it strikes against the breast, and the pulse is felt externally. 



"2. That it is everywhere contracted, but more especially towards 

 the sides, so that it looks narrower, relatively longer, and more drawn 

 together. 



" 3. The heart, being grasped in the hand, is felt to become harder 

 during its action. Now this hardness proceeds from tension, precisely 

 as. when the forearm is grasped, its tendons are perceived to become 

 tense, and resilient when the fingers are moved. 



