282 FUNCTIONS OF NUTRITION. 



point of the heart is carried in the same direction. If an ivory or 

 porcelain rod be held horizontally just above the heart in this position, 

 the apex rises visibly toward the rod at each ventricular systole, and 

 recedes from it in the same degree at each diastole. 



Such an elongation can only be explained by the arrangement of the 

 fibres in the ventricular wall. Every muscular fibre, during contrac- 

 tion, increases in thickness while diminishing in length; so that its 

 volume remains the same. The superficial cardiac fibres which run 

 obliquely downward to the point of the heart, and then turn upward 

 along its internal surface to their insertion in the auriculo-ventricular 

 zones, would have the effect, if they acted alone, to draw the point and 

 base of the organ together and thus to shorten the heart. But between 

 their superficial and internal layers there are deep-seated fibres, running 

 in a nearly circular direction round the axis of the ventricular cavity. 

 These circular fibres, which are nearly wanting on the right side, are 

 very abundant in the left ventricle and form a 

 large part of its muscular walls. In the ventricu- 

 lar systole they contract upon the blood in the 

 ventricular cavity like the fingers of a closed hand. 

 By their contraction they tend to obliterate the 

 ventricular cavity, and by their lateral swelling 

 at the same time they exert a pressure from the 

 base of the heart toward its point, causing a pro- 

 trusion of the apex. 



The impulse of the heart is a stroke of the apex 

 against the walls of the chest, usually perceptible 

 both to sight and touch, at the time of ventricu- 

 LEFT VENTRICLE OF BUL- i ar systole. In man it is felt, as a rule, in the 



LOCK'S HEART, showing -.,.. . -, 



its deep fibres. fifth intercostal space, about midway between the 



left edge of the sternum and a vertical line drawn 

 through the left nipple. But its location varies somewhat with the 

 attitude and the respiration, owing to changes of position of the heart 

 within the chest. In the recumbent posture, when lying on the left 

 side, the situation of the heart's impulse is shifted from one and a half to 

 two centimetres farther outward from the median line. When lying on 

 the right side, it may be altogether imperceptible. In every posture 

 it disappears when the chest is fully expanded in inspiration, as the 

 cardiac surface is then completely covered by the lungs. But if inspira- 

 tion be performed by the diaphragm alone, the chest remaining fixed, the 

 descent of the heart, as it follows the diaphragm, is indicated by the 

 changed position of the impulse. This is most distinctly shown in the 

 recumbent posture on the left side ; when, in moderate expiration, the 

 heart's impulse is felt in the fifth intercostal space, but in full inspi- 

 ration, using the diaphragm alone, it disappears from that point and 

 is felt in the sixth intercostal space. In the erect posture the impulse 

 may also be felt in the sixth intercostal space after full inspiration by 

 the diaphragm alone. 



