Physiology of the Blood. 



[THE blood is aptly described by Claude Bernard as an internal medium, 

 which acts as a " go-between " for the outer world and the tissues. 

 Into it are poured those substances which have been subjected to the 

 action of the digestive fluids, and in the lungs or other respiratory 

 organs it receives oxygen. It thus contains new substances, but in its 

 passage through the tissues it gives up some of these new substances, 

 and receives in exchange certain effete and more or less useless sub- 

 stances which have to be got rid of. Its composition is thus highly 

 complex, containing, as it does, things both new and old. It is at 

 once a great pabulum-supplying medium, and a channel for getting rid 

 of useless materials. As the composition of the organs through which 

 the blood flows varies, it is evident that its composition must vary 

 in different parts of the circulatory system ; and it also varies in the 

 same individual under different conditions. Still, with slight varia- 

 tions, there are certain general physical, histological, and chemical 

 properties which characterise blood as a whole.] 



1. Physical Properties of the Blood. 



(1.) Colour. The colour of blood varies from a bright scarlet-red 

 in the arteries to a deep, dark, bluish-red in the veins. Oxygen (and, 

 therefore, the air) makes the blood bright-red ; want of oxygen makes 

 it dark. Blood free from oxygen (and also venous blood) is dichroic 

 i.e., by reflected light it appears dark-red, while by transmitted 

 light it is green (Briicke). 



In thin layers blood is opaque, as is easily shown by shaking blood 

 so as to form bubbles, or by allowing blood to fall upon a plate with 

 a pattern on it, and pouring it off again. Blood behaves, therefore, 

 like an " opaque colour " (Rollett), as its colouring-matter is suspended 

 in the form of fine particles the blood-corpuscles. 



Hence, it is possible to separate the colouring-matter from the fluid part of the 

 blood by filtration. This is accomplished by mixing the blood with fluids which 

 render the blood-corpuscles sticky or rough. If mammalian blood be treated with 

 one- seventh of its volume of solution of sodic sulphate, or if frog's blood be mixed 

 with a two per cent, solution of sugar and filtered, the shrivelled corpuscles, now 

 robbed of part of their water, remain upon the filter, 



