HEART 399 



movement is to press the heart more forcibly against the 

 chest wall. This gives rise to the cardiac impulse with 

 each ventricular systole (fig. 168), but this is not easily felt 

 in the horse unless the action of the heart is exaggerated. 



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

 this elevation of the apex is readily seen. 



(2) Position. — The position of the impulse is determined 

 by the relationship of the heart to the anterior chest wall 

 and to the lungs. 



(3) Character. — It is felt as a forward impulse of the 

 tissues, 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 means 

 of any of the various forms of cardiograph (fig, 173). One of 

 the simplest consists of a receiving and a recording tambour 

 connected by means of a tube (fig. 171) {Practical 

 Physiology). 



6. Changes in the Intracardiac Pressure. — These 

 have been studied in the horse and dog. 



(1) Methods. — The most common way of determining 

 the pressure in a cavity is to connect it to a vertical tube 

 and to see to what height the fluid in the tube is raised. 

 If such a method be applied to the ventricles of the heart, 

 the blood in the tube undergoes such sudden and enormous 

 changes m level that it is impossible to get accurate results. 



The same objection applies to the method of connecting 

 the heart with a manometer, a U tube filled with mercury. 

 When this is done, the changes in pressure are so sudden 

 and so extensive that the mercury cannot respond to them 

 on account of its inertia. 



Various means of obviating these difficulties have been 

 devised. (1) One of the best is to allow the changes of pressure 

 to act upon a small elastic membrane tested against known 

 pressures. A tube is thrust through the wall of the heart 

 and connected with a tambour covered by a membrane to 

 which a lever is attached. (2) Probably the most delicate 

 method is by the use of Piper's stilette manometer (fig. 172), 



