130 ANIMAL PHYSIOLOGY. 



course by the heart must not be confused with, its velocity. 

 The velocity is at its mminium in the capillaries ; the pressure 

 diminishes from quitting the heart till the return to it, being 

 dissipated by the friction of the tissues which resist it. From 

 experiments on the lower animals, it is calculated to be equal 

 in large arteries, such as the carotid, to the support of a 

 column of mercury more than six inches high, and in small 

 arteries, like those of the foot, to be about a fourth less: 

 while in the veins, after having experienced the resistance of 

 the tissues in the capillaries, it is only about a twelfth of 

 what it is in the arteries. These observations, together with 

 the fact that defTbrinated blood has been injected through the 

 body of a dog with less pressure than that exerted by the 

 heart (Sharpey), point out that the heart is the motive 

 power which causes the blood to flow through the whole 

 system. It must not, however, on that account be supposed 

 that the tissues have no influence whatever on the circula- 

 tion, for we have proof to the contrary in the fact that in 

 interference with respiration, the unaerated blood fails to 

 pass the capillaries, and that in inflammations, examined 

 microscopically in the web of the frog's foot, blood corpuscles 

 are seen arrested in their course without any obstruction 

 existing in the channel beyond. 



95. The time required for a portion of the blood to be carried 

 through the whole circulation, has been made the subject of 

 most interesting experiments. An easily detected substance, 

 such as ferrocyanide of potassium, is introduced into a vein 

 on one side of the neck of an animal, and the time noted which 

 elapses before it is present in the blood allowed to flow 

 from the corresponding vein on the other side. The substance 

 introduced has to pass through the heart and lungs, and some 

 . part of the head or neck, before it can reach the aperture 

 where it is sought for, and thus makes a circuit through both 

 pulmonary and systemic circulation. In the horse, such a 

 circulation is completed in little more than half a minute, 

 and in smaller animals in a much shorter time. Small 

 animals have the pulse rapid; and the rule may be laid 

 down that a complete circulation takes place in from 20 to 

 30 beats of the heart, This may well appear incredible at 

 first, when it is considered how slowly the blood moves in 



