CHAPTER VII. 

 THE BLOOD. 



ALL the active cells and tissues are in intimate relationship with 

 capillary vessels, through which blood is continually flowing. Both 

 tissue elements and capillary are bathed with a fluid called lymph, and 

 a constant interchange of material takes place through the lymph 

 between the tissues and the blood. On the one hand, oxygen and other 

 nutritive substances pass from blood to tissue to repair loss of substance, 

 and to furnish a source of energy, and on the other hand, carbonic acid 

 and other waste materials pass from the tissues to the blood. An 

 exchange of water and salts also takes place through the capillary wall 

 by diffusion. In some organs certain substances, called hormones, are 

 supplied to the blood, and are carried in it to the glands or muscle 

 fibres which they are destined to excite. Further, as the blood 

 circulates, now through glands and muscles in which heat is produced, 

 now through other structures in which heat loss occurs, it serves to 

 equalise the temperature of the different parts of the body. 



Freshly shed blood is a red, viscid, opaque fluid with a specific gravity 

 of 1055. The specific gravity may be ascertained with a single drop 

 of blood by making a mixture of chloroform and benzole, and finding 

 the proportions of the two fluids in which a drop of blood remains 

 suspended without tending either to sink or rise. The specific gravity 

 of the mixture, ascertained by means of a hydrometer, is that of the 

 blood itself. 



When human blood is examined under the microscope, it is seen to 

 consist of two kinds of corpuscles floating in a pale yellow fluid, the 

 blood plasma. The corpuscles which are most numerous are the red 

 blood corpuscles, or erythrocytes. The other variety of corpuscle, the 

 white blood corpuscle, or leucocyte, is in the proportion of 1 to 500 red. 



The red corpuscles, when seen singly, are yellow in colour, but when 

 massed together they give blood its red appearance. They are circular, 

 biconcave, non-nucleated discs, each having a diameter of 7 '5 thousandths 

 of a millimetre (7 '5 /A). In all mammals except the camel tribe, the 



