64 PHYSIOLOGY OF FARM ANIMALS [CH. 



ineiitally, there is no such recoi], the downstroke of the sphygiiio- 

 graph tracing being then continuous. In the veins there is 

 normally no pulse. 



The normal rate of the pulse (or of the heart beat) varies in 

 different animrds ; it also varies mider different conditions. 

 Exercise increases the rate of the pulse, just as it increases the 

 respiratory movements. The heart rate responds also to nervous 

 excitement, and it quickens when there is a rise of temperature 

 as in febrile conditions. The increased rate is. apparently due 

 to reflex stimulation, and in the case of fevers, the rise of tempera- 

 ture in the blood is probably the exciting cause. In young 

 animals the rate is always faster than in adults, and with the 

 advance of old age the pulsations become still slower and at the 

 same time weaker. The following are the average rates for the 

 heart beat in man and fully grown animals : 



In the newly born foal or calf the average rate is abovit 110 

 beats per minute. 



The velocity of the blood varies in the different parts of the 

 body. In the carotid artery of the horse it has been shown to 

 flow at the rate of 300 mm. per second (Volkmann). In the 

 jugular vein of the dog it may flow at the rate of 260 mm. per 

 second (Vierodt). In the smaller veins and arteries the rate of 

 flow is much slower, and in the capillaries it is slowest of all, 

 having been estimated at about an inch per minute. In no part 

 of the body, however, does the blood traverse more than an 

 extremely short distance through capillaries before being returned 

 to a vein. Vierodt, by injecting potassium ferrocyanide into the 

 jugular of a dog and awaiting its appearance at the other jugular, 

 found that a complete circulation only took 16*32 seconds. 



In the arteries the blood is kept in motion by the direct action 

 of the heart. In the veins the flow is due to several causes. One 

 of these is the back pressure due to the heart, but there are other 

 contributing causes. Thus the expansion of the thorax in 



