THE CIRCULATION. 185 



the nozzle of a hose at once increases the pressure of the 

 water in it. In fact, the explanation of such a common 

 thing as a cold is found in the fact that, due to a compres- 

 sion of the vessels of the skin, caused as a rule by an 

 exposure to suddenly lowered temperatures, an increased 

 arterial pressure results and the blood is driven in undue 

 amounts through the vessels of the interior organs which 

 are thereby congested and an inflammation results which we 

 designate as a cold. 



In the third place, there is a slight variation produced 

 by the rhythmic beat of the heart. It is easily understood 

 how the sudden injection of a ventricle full of blood into 

 the arteries will cause the pressure in them to rise a little, 

 while during the diastole of the heart as the blood is con- 

 tinually flowing out into the capillaries the pressure tends 

 to sink a little. This variation is, however, very slight, 

 amounting to little more than a few millimetres of mercury. 

 As the average pressure of the aorta is as much as 200 mil- 

 limetres of mercury, the periodic rise and fall of but a few 

 millimetres causes practically no effect. 



THE CHANGING OF THE INTERMITTENT FLOW FROM THE HEART 

 INTO THE CONSTANT FLOW OF THE CAPILLARIES. 



If capillaries like those in a frog's foot be examined and 

 the blood be seen circulating through them, it may be 

 noticed that the blood flows smoothly and regularly, show- 

 ing no pulsations. Even through the smaller arteries the 

 blood stream seems to all external appearances to be per- 

 fectly constant and regular. As the blood is, however, 

 poured into the arteries by the heart in periodic pulsations 

 the question arises, how such an intermittent flow is changed 

 into a regular and continuous one. This is readily ex- 

 plained if it be remembered that the real force which drives 

 the blood from the arteries into the capillaries is the force 

 exerted by the elastic walls of the arteries which are much 

 distended and which in trying to regain their original di- 

 mensions press upon the contained blood. Much as water 



