CIIAPTEK lY. 



CIEOULATION, RESPIRATION, AND EXCRETION. 



§ 1. ClKCULATION. 



The Blood. — ^We have seen, in the preceding chapter, 

 that the digested and resorbed nutrients of the food are 

 carried more or less directly into the blood, and it is from 

 this fluid that all parts of the body derive those substances 

 necessary for their growth and the performance of their 

 functions. 



The blood of the higher animals is a thickish, somewhat 

 viscid fluid, having a faint but peculiar odor, a slightly salt 

 taste, and a color varying from bright to a dark red. It is 

 somewhat heavier than water (sp. gr. 1.045 — 1.075), and 

 contains about 21 per cent, of solid matters. 



Under the microscope it is seen to consist of a clear 

 fluid, the jplmniaj holding in suspension a vast number of 

 small, round disks, the em-jpuseles. 



The coTjpumlm are of two kinds. By far the most nu- 

 merous are the red corpuscles. In man these are round 

 like a coin but thicker at the edges than in the centre, 

 andiiave a diameter of .0060— .0085 millimetres. Their 

 number is enormous, being estimated at 4™6i- millions per 

 cubic millimetre. The color and opacity of the blood are 

 due to the corpuscles. 



The corpuscles of each kind of animal are peculiar, both 

 in shape and size, but their general characteristics are the 



