THE CIRCULATION 223 



heart more forcibly against the chest wall. This gives rise 

 to the cardiac impulse which is felt with each ventricular 

 systole over the prsecordium (Fig. 109). 



If the chest is opened and the animal placed on its back 

 this elevation of the apex is readily seen. If the animal is 

 placed on its belly, so that the heart when relaxed hangs 

 forwards, the apex is tilted back during contraction. 



Since the apex is twisted to the left, the movement of the 

 ventricle is not simply directly forward, but also from left 

 to right. This tilting of the apex from left to right is 

 further favoured by the direction of the muscular fibres of 

 the ventricles which pass from the auriculo- ventricular rings 

 downwards and to the left. 



The increased thickness of the heart from before back- 

 wards also assists, to some extent, in the production of the 

 impulse. 



The study of the position and characters of the cardiac 

 impulse is of great importance in medicine. 



The position is determined by the relationship of the heart 

 to the anterior chest wall and to the lungs. The boundaries 

 of the part of the heart lying in relationship to the chest 

 wall have been already defined (p. 218), and it is at the outer 

 and lower part of this area, a region bounded above by the 

 fifth rib, below by the sixth rib, outside by a line drawn verti- 

 cally through the nipple, and inside by a line drawn vertically 

 midway between the nipple and the left edge of the sternum, 

 that the cardiac impulse is felt. Normally it does not extend 

 outwards beyond the nipple line, but frequently when the 

 left lung is voluminous the impulse only extends out to an 

 inch or so inside of the nipple line. In children, on account 

 of the size of the liver, it is often felt between the fourth and 

 fifth ribs. This impulse is often called the apex beat but it 

 is not the apex which presses on the chest wall but a part of 

 the front of the right ventricle. 



In character it is felt as a forward impulse of the tissue 

 occupying the fifth interspace, which develops suddenly, 

 persists for a short period, and then suddenly disappears. 

 In many forms of heart disease its character is markedly 

 altered. 



The cardiac impulse may be recorded graphically by 



