CHAPTER SEVENTH. 



OF THE BLOOD AND CIRCULATION. 



227. THE nutritive portions of the food are poured into 

 the general mass of fluid which pervades every part of the 

 body, out of which every tissue is originally constructed, 

 and from time to time renewed. This fluid, in the general 

 acceptation of the term, is called blood ; but it differs greatly 

 in its essential constitution in the different groups of the 

 Animal Kingdom. In polypi and medusse, it is merely 

 chyme, (208 ;) in most mollusks and articulates it is chyle, 

 (209 ;) but in vertebrates it is more highly organized, and 

 constitutes what is properly called BLOOD. 



228. The BLOOD, when examined by the microscope, is 

 found to consist of a transparent fluid, the serum, consisting 

 chiefly of albumen, fibrin, and water, in which float many 

 rounded, somewhat compressed bodies, called blood disks. 



Fig. 78. 



These vary in number with the natural heat of the animal 

 from which the blood is taken. Thus, they are more nu- 



