20 A COURSE ON ZOOLOGY. 



globules to the entire quantity of blood, we find fifteen 

 per cent, of corpuscles in the blood of the tortoise, nine 

 per cent, in that of the horse, and only six per cent, in 

 that of the frog. 



Besides the red corpuscles in blood, there are white 

 spherical bodies known as white corpuscles, or leucocytes ; 

 these are a little larger than the red corpuscles, but much 

 fewer in number, there being in the blood of a healthy 

 man only one white corpuscle for about three hundred 

 and fifty red ones. 



We have said in the chapter on Respiration that the 

 blood charged with oxygen is called arterial blood ; it is 

 scarlet in color. The blood which circulates in the tissues, 

 and which is charged with carbonic acid gas, presents, 

 on the contrary, a dark-red color, and is called venous 

 blood. 



Circulatory Apparatus. The organs that -compose the 

 circulatory apparatus may be divided into groups, a 



d 



a 



DIAGRAM OF THE STRUCTURE OF AN ARTERY (after Turner). 



1. External coat .......... 



2. Middle coat ............. c, muscular. 



3. Internal coat ........... 



central organ, the heart, and peripheral organs, the 

 vessels. 



We recognize three systems of vessels, the arterial 

 system, which receives the blood from the heart and 



