180 PHYSIOLOGY FOR DENTAL STUDENTS. 



frog's foot be examined by means of a microscope, several inter- 

 esting facts will be noted. The red blood corpuscles will be seen 

 flowing in the center of the blood vessels, while in the clear plas- 

 ma which surrounds them are the much less numerous white 

 cells. This arrangement is explained by the fact that the red 

 corpuscles are heavier than the white cells or the plasma and are 

 held in the center of the stream by a principle of hydraulics. The 

 white cells flow more slowly along the sides of the vessels than 

 the red corpuscles do in the center of the stream, which is sug- 

 gestive of the function of the white cells as phagocytes (see p. 

 152) ; thus, any injury to the vessel wall will necessarily slow the 

 flow of blood through the veins and allow a greater number of 

 leucocytes to collect at the point of injury. 



The Pulsatile Acceleration of Blood Flow. The flow of blood 

 in the arteries differs from that in the veins and the capillaries 

 in that it is swifter and pulsatile in character. This pulsatile 

 variation is due to the acceleration of the blood flow caused by 

 each heart beat, and the reason that this is not seen in the capil- 

 laries and veins is that the resistance which the walls of the 

 capillaries and arterioles offer to the blood is so great that the 

 cardiac factor, acting only for a brief time, is lost. The energy 

 represented in the increased rate of flow, is spent in stretching the 

 walls of the arteries, which contract after the pulsatile wave has 

 passed, and thus force the -blood onward. 



The Pulse. The pulsatile expansion of the arteries at each 

 heart beat has been mentioned in connection with the factors 

 which help to maintain the normal blood pressure. It is this 

 also which produces the phenomenon which is known as the pulse. 

 From time immemorial the physician has been accustomed to 

 come to an idea concerning the condition of the circulation by 

 feeling the pulse, for it represents changes in the arterial ten- 

 sion occurring during each cardiac cycle. In order to study 

 the pulse wave more carefully, instruments have been devised 

 which graphically record its wave on a piece of paper. Such an 

 instrument is known as a sphygmograph (Fig. 23), and some 

 of these have been cleverly arranged so as to enable us to record 

 simultaneously the pulse from different blood vessels. 



