CHAPTER SEVENTH. 



OF THE BLOOD AND CIRCULATION. 



227. THE nutritive portions of the food are poured into 

 the blood or the general mass of fluid which pervades every 

 part of the body, and out of which every tissue is origi- 

 nally constructed, and from time to time renewed. 



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

 found to consist of a transparent fluid, the serum, in which 

 float many rounded, somewhat compressed bodies, called 

 globules. These globules vary in number with the natural 

 heat of the animal from which the blood is taken. Thus, 

 they are more numerous in birds than in the mammals, and 



Fig. 79. 



Fig. 80. 



Fig. 81. 



Fig. 78. 



more abundant in the latter than in fishes. In man and 

 other mammals they are very small and nearly circular 

 (Fig. 78) ; they are somewhat larger and of an oval form 

 in birds and fishes (Figs. 79, 81) ; and still larger in rep- 

 tiles (Fig. 80). 



229. The color of the blood in the vertebrates is bright 

 red ; but in some invertebrates, as in the crabs and mol- 

 lusks, it is nearly or quite colorless ; while in the worms 



