THE TRANSPORTATION SYSTEM 



503 



It was brought out in an earlier chapter 

 that when a muscle contracts, a minute 

 electrical impulse is set up which can be 

 detected with delicate instruments. This 

 fact has been employed in diagnosing heart 

 difficulties. The instrument employed is the 

 electrocardiograph. It records slight electri- 

 cal changes that occur when the various 

 parts of the heart muscle contract. A nor- 



sure is much higher than the venous pres- 

 sure. The highest pressure is maintained in 

 the aorta. As the blood passes through the 

 thousands of miles of capillaries, the pres- 

 sure gradually falls, owing primarily to the 

 friction of the walls of these tiny tubes 

 (Fig. 19-11). By the time it reaches tlie 

 veins, much of the pressure has been spent 

 and, in fact, when it reaches the large veins 



Fig. 19-12. Blood pressure readings can be obtained in this manner. With such a device much 

 " information can be gained about the circulatory system. 



mal heart produces a characteristic series 

 of peaks and valleys. Any deviation in this 

 pattern indicates abnormality, the exact 

 nature of which can be discerned rather 

 accurately. 



BLOOD PRESSURE 



If a vein is severed the blood oozes out 

 in a gentle flow, whereas if an artery is cut 

 it shoots out in spurts which are synchro- 

 nized with ventricular systoles. It is obvi- 

 ous that the blood is being forced along 

 under pressure, and that the arterial pres- 



entering the right auricle it has a slight 

 negative pressure. In other words, the 

 blood is "sucked" into the heart. 



The pressure in the arteries rises and 

 falls with each heart beat. This can be 

 easily observed by "taking the pulse," 

 which can be felt at the wrist or any other 

 artery that comes near the surface. The 

 artery swells and collapses in a rhythmic 

 manner. This interested an eighteentli-cen- 

 tury English clergyman, Stephan Hales, to 

 the point of experimentation. He placed a 

 crude canula (a small brass tube), con- 

 nected to a long glass tube, into the carotid 



