326 CIRCULATORY SYSTEM. 



time by injecting into the vessels methylene-blue, the color of 

 which shows through the blood-vessels. By this means he has 

 been able to study the circulation-time in the lungs, kidney, stom- 

 ach, and other organs of lower animals. He thinks that the 

 pulmonary circulation-time i. e., the time occupied by the blood in 

 passing through the pulmonary circulation is not usually much 

 less than 12 seconds nor more than 15 seconds ; the circulation-time 

 of the kidney, spleen, and liver is relatively long and much more 

 variable than that of the lungs, these organs being easily affected 

 by exposure and changes of temperature (increased by cold, 

 diminished by warmth), and that of the retina and heart is the 

 shortest of all. The total circulation -time in man he thinks is 

 not much less than a minute, nor more than a minute and a quarter. 



THE PULSE. 



As has been seen, the left ventricle expels at each beat about 

 70 c.c. of blood, pumping it into the elastic arteries ; these being 

 already filled, are still further distended by this additional amount, 

 and the elastic coat of the artery recoils upon the blood within the 

 vessel, driving it still further along. If a finger is placed upon 

 an artery, this distention can be recognized, and constitutes the 

 pulse. When the ventricle ceases its systole and begins its diastole, 

 although blood ceases to be expelled from it, still the current in 

 the arteries does not cease, for the elastic force of the vessels is 

 sufficient to keep up a continuous movement of the fluid, which 

 while it is in the arterial system is affected by the pulsation of the 

 heart, but which in the capillary and venous systems, is uniform in 

 rate. If it was possible to place a finger upon the carotid, another 

 on the radial, and still another on the dorsal artery of the foot, it 

 would be found that the pulse would first be felt in the artery 

 nearest the heart, then in that at the wrist, and finally in the most 

 distant one. Thus the pulse-wave starting at the left ventricle is 

 felt in 0.159 second at the wrist, and in 0.193 second at the foot. 

 It travels at the rate of about 9 meters per second. 



The number of pulsations varies in different conditions, increas- 

 ing in activity and diminishing during rest ; it also varies at dif- 

 ferent ages : At birth it is 140 to the minute ; atone year, 120; 

 two years, 110; three years, 90; seven years, 85; puberty, 80; 

 adult age, 70 ; old age, 60. These figures are approximate only. 



Any artery which is accessible may be used to obtain the pulse, 

 but physicians have selected the radial as the most convenient be- 

 cause of its accessibility and also because it lies upon an unyield- 

 ing bony bed, and can be readily compressed by the finger and its 

 character studied. The heart is one of the vital organs of the 

 body, and a knowledge as to how it is performing its functions is 

 very important for the physician to possess. Situated as it is 

 within the thorax, he cannot examine it directly, and is therefore 



