286 



THE CIRCULATION 



one of the functions of the elaborate vasomotor apparatus always to 

 adapt the pressure in a given part to the needs of that region at that time. 

 The blood-pressure therefore is continually changing in all parts of the 

 body as the needs of different areas require. Blood-pressure in general 

 is one index of the quantity and activity of the blood-supply. Under 

 certain conditions, however, the arteries and arterioles may be relaxed 

 and ample blood be coming to the part, although under a low pressure. 

 The average arterial pressure in young men appears to lie between 

 90 and 150 mm. of mercury. Mental excitement or fever, for example, 

 promptly raises the blood-pressure, while pain lowers it. The actual 

 blood-pressure in a vessel at any time is the result of an hydraulic balance 



FIQ. 152 



FIG. 153 



P.Car 



Tracing to show the rise of pressure in the 

 carotid from excitation of an afferent nerve other 

 than the depressor. The stimulus was applied 

 at E and continued fourteen seconds. (Meyer.) 



Frog-cardiograms to show the relations of 

 pulse-rate to the heart-muscle's temperature 

 The top curve records the beat of the heart 

 at 35 C., the middle line at 20, and the 

 bottom line at 5. By the suspension- 

 method. To be read from left to right. The 

 time-line is in seconds. Reduced. 



between the caliber of the arteries and the work done by the heart. If 

 the arteries are dilated and the heart is beating fast and vigorously the 

 pressure in a given artery may be the same as it would be if the artery 

 were constricted and the heart beating slowly and with less vigor. In 

 the capillaries the blood-pressure is from 20 to perhaps 70 mm. of mercury. 

 In the veins it is from 15 mm. or so to a negative quantity of from 3 

 to 7 mm. (See Fig. 149.) 



The Pulse-rate of the Heart is the number of times per minute that it 

 beats; sometimes the term heart-rate is used. In the average man, the 

 pulse-rate is about 72 per minute, and in the average woman not far 

 from 78 or 80. As an average rate (like many other averages, seldom 

 met with in fact), we may use 75, especially because then by chance the 



