CHAPTER SEVET7TH. 



OF THE BLOOD AND CIRCULATION. 



350. THE nutritive portions of the food are poured into 

 the general mass of fluid pervading every part of the body, 

 out of which every tissue is originally constructed, and from 

 iiiue to time renewed. This fluid, in the general accepta- 

 tion of the term, is called blood ; but it differs greatly in its 

 essential constitution : in the different groups of the animal 

 kingdom, in polyps, and medusae, it is merely chyme; in most 

 mollusca and articulata it is chyle; but in vertebrata it is more 

 highly organised, and constitutes what is properly called blood. 

 351. 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 discs, or 

 globules. These Tary in number with the natural heat of the 

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

 numerous in birds than in mammals, and more abundant in 

 the latter than in fishes. In man and other mammals they 

 are very small, and nearly circular (figs. 208 and 209) ; they 

 are somewhat larger, and of an oval form, in birds and fishes 

 (figs. 210, 214, 215); and still larger in reptiles (figs. 211, 

 212, 213). [The blood-globules in man appear distinctly dis- 



Fig. 208. Globules 

 .a ~ f tne blood of man, 



J) 7 ^PLfc the blood havin s been 



jfCE?S!Sffl -JH drawn from a vein and 



S8 beaten, to separate the 

 C fibrin. A, blood glo- 

 bules, seen, a, on the 

 flat aspect ; 6, standing 

 on the edge ; *, three-quarter view. B, a congeries of blood-globules, 

 with their flat surfaces in opposition, and forming columns such as are 

 made by a number of coins laid one upon another. C, a blood globule in 

 process of alteration, such as simple exposure to the air will produce. D, 

 a lymph globule, mingled with the proper blood globules. 



N.B. The subjects of this and the succeeding figures of blood discs from 

 Wagner's Icones Physiologic^ are all magnified to the same extent, viz. 

 about nine hundred diameters. 



