CHAPTER IV 

 THE BLOOD 



Nature of the Blood. The blood is a liquid with a 

 large number of minute solid bodies floating in it. The solid 

 bodies, the corpuscles, are of two kinds the red and the 

 colourless. There are about 5,000,000 red corpuscles in a 

 cubic millimetre l of blood, and there are about 500 times as 

 many red corpuscles as colourless corpuscles. The colour of 

 the blood is due to the red corpuscles in it. The liquid in 

 which the corpuscles lie, the plasma, is water, containing a 

 number of substances in solution ; so that it is " thicker " than 

 water, and since the corpuscles are only a very little heavier 

 than the plasma, they will not easily settle. By a method 

 which will be described later on, they can be made to settle, 

 and the plasma is then seen to be almost without colour, 

 having only a pale yellow tint. 



The Red Corpuscles. When a drop of blood is ex- 

 amined with the microscope the red corpuscles appear of a 

 reddish-yellow colour, and the plasma in which they are lying 

 appears quite colourless. A red corpuscle is round and flat 

 like a coin, but thinner in the middle than near the edge, so 

 that each side is slightly concave. They are all about the 

 same size, ^W^ 1 ^ an mc ^ across > an( ^ about a quarter of 

 that in thickness. They are very numerous, so that they are 

 often too crowded together to be studied carefully, and they 

 are prone to run face to face into rows like a pile of coins. 

 Some will be rolling over, so that they will be seen on edge ; 

 others will be seen on the flat. In the latter case they will be 

 darker or lighter in the centre, according to the focus of the 

 microscope. They are soft and flexible, changing their shape 



1 See p. xi. 



