224: COMPARATIVE PHYSIOLOGY. 



maintenance of the circulation, the quantity of blood entering 

 and leaving the heart must be equal, in consequence of the sec- 

 tional area of the great veins that_ enter the heart being greater 

 than that of the aorta, it follows that the venous flow even at its 

 quickest is necessarily slower than the arterial. 



Comparative. — There must be great variations in velocity in 

 different animals, as such measurements as have been made 

 demonstrate. Thus, in the carotid of the horse, the speed of 

 the blood-current is calculated as about 306 mm., in the dog at 

 from 205 to 357 mm. These results can not be considered as 

 more than fair approximations. 



Highly important is it to note that the rate of flow ia the 

 capillaries of all animals is very slow indeed, not being as much 

 as 1 mm. in a second in the larger mammals. The time occupied 

 by the circulation is also, of course, variable, being as a rule 

 shorter the smaller the animals. As the result of a number of 

 calculations, though by methods that are more or less faulty, 

 the following law may be laid down as meeting approximately 

 the facts so far as warm-blooded animals are concerned. 



The circulation is effected by 27 heart-beats ; thus for a man 

 with a pulse of 81, the time occupied in the completion of the 

 course of the blood from and to the heart would be ff = 3 ; i. e., 

 the circulation is completed three times in one minute, or its ■ 

 period is twenty seconds ; and it is to -be well borne in mind 

 that by far the greater part of this time is occupied in traversing 

 the capillaries. 



THE CIRCULATION UNDER THE MICROSCOPE. 



There are few pictures more instructive and impressive than 

 a view of the circulation of the blood under the microscope. 

 It is well to have similar preparations, one under a low power 

 and another under a magniflcation of 300 to 500 diameters. 

 With the former a view of arterioles, veins, and capillaries may 

 be obtained at once. Many different parts of animals may be 

 used, as the web of the frog's foot, its tongue, lung, or mesen- 

 tery ; the gill or taU of a small fish, tadpole, etc. 



The relative size of the vessels ; the speed of the blood flow ; 

 the greater velocity of the central part of the stream ; the aggre- 

 gation of colorless corpuscles at the sides of the vessels, and the 

 occasional passage of one through a capillary wall, when the 

 exposure has lasted sotne time ; the crowding of the red cells ; 



