194 



ESSENTIALS OF PHYSIOLOGY. 



obtaining a cardiogram is to use a polygraph (p. 208), and to place 

 over the region of the cardiac impulse the small metal receiver which 

 is generally used for recording the venous pulse. The general form 

 of the tracings is shown in fig. 75, but it varies considerably with the 

 pressure used and with the spot at which the instrument is applied 

 to the chest. 



(4) The Heart Sounds. If a stethoscope is applied to the front 

 of the chest, two sounds are heard with each beat of the heart. They 

 are often compared with the sounds lubb dup, the first being long and 

 low-pitched, the second short and sharp. The time relation between 



Heart Sounds 



dup 



Lubb dup 



FIG. 65. Diagram of events constituting a cardiac cycle. (Starling's 

 Principles of Physiology. ) 



the heart sounds and the other events occurring during the cardiac 

 cycle (fig. 65) has been determined in the following manner. A 

 stethoscope is connected with an apparatus similar to the receiver of 

 a telephone ; the vibrations of air in the stethoscope, set up by 

 the heart sounds, throw the membrane of the receiver into vibrations 

 and so alter the contact between the silver and carbon which form 

 part of the receiver, and through which a current is passing. The 

 current also passes through an electro-magnet, which pulls on a 

 disc connected with the membrane of a recording tambour. Each 

 vibration of the membrane of the receiver alters the strength of the 

 current passing through the electro-magnet and the pull which it 

 exerts on the iron disc and then on the membrane of the tambour. In 

 this way the vibrations of the receiver caused by the heart sounds can 

 be recorded. If such a record is obtained simultaneously with a 

 cardiogram, it is found that the first sound occupies nearly the whole 



