232 COMPARATIVE PHYSIOLOGY. 



they become more conical ; that the long diameter is not appre- 

 ciably altered ; that the antero-posterior diameter is lengthened ; 

 and that the left ventricle at least turns on its own axis from 

 left to right. This latter may be distinctly made out by the eye 

 in watching the heart in the opened chest. 



THE IMPULSE OF THE HEART. 



When one places his hand over the region of the heart in. 

 man and other mammals, he experiences a sense of pressure 

 varying with the part touched, and from moment to moment. 

 Instruments constructed to convey this movement to recording 



Fio. 199.— Marey's cardiac sonnd. which may be used to explore the chambers of the 

 heart (after Foster), a is made of rubber stretched over a wire f rameworli:, witli 

 metallic supports above and below; 6 is a long tube. 



levers also teach that certain movements of the chest wall cor- 

 respond with the propagation of the pulse, and therefore to the 

 systole of the heart. It can be recognized, whether the hand 

 or an instrument be used, that all parts of the chest wall over 

 the heart are not equally raised at the one instant. If the beat- 

 ing heart be held in the hand, it will be noticed that during 

 systole there is a sudden hardening. The relation of the apex 

 to the chest wall is variable for different mammals, and with 

 different positions of the body in man. 



As a result of the investigation which this subject has re- 

 ceived, it may be inferred that the sudden tension of the heart, 

 owing to the ventricle contracting over its fluid contents, 

 causes in those cases in which during diastole the ventricle 

 lies against the chest wall, a sense of pressure beneath the 

 hand, which is usually accompanied by a visible movement 

 upward in some part of the thoracic wall, and downward in 

 adjacent parts. 



It will not be forgotten that the heart lies in a pericardial 

 sac, moistened with a small quantity of albuminous fluid ; and 

 that by this sac the organ is tethered to the walls of the chest 

 by its mediastinal fastenings ; so that in receding from the 

 chest wall the latter may be drawn after it ; though this might 



