62 HUMAN PHYSIOLOGY 



the behaviour of the blood is explained by the 

 fact that the switching removes fibrin from the 

 blood, this substance being found in the shape of 

 whitish jelly-like threads attached to the twigs. 

 Blood, therefore, may be said to consist of two parts, 

 a fluid part to which the name of plasma is given, 

 and solid parts represented by the corpuscles of the 

 blood which are of two kinds, red and white. The 

 plasma of the blood may be regarded as the real 

 blood, for it contains the substances which are 

 essential for the nourishment of the body. The 

 corpuscles, however, have also their highly impor- 

 tant uses. A speck of blood pressed between two 

 thin plates of glass and examined 

 microscopically, shows the clear 

 plasma of the blood liquid, whilst 

 floating in it are innumerable cor- 

 puscles (Fig. 17). Blood may be 

 regarded therefore as really a 

 clear fluid, seeing that the blood 

 Fig. 17. BLOOD liquor, or " plasma," is itself colour- 

 CORPUSCLES. less. It owes its red colour to the 

 (a) red corpuscles immense number of red blood cor- 

 in rouleaux; (c puscles which float in the blood 



lic * uid - Blood mi ^ ht in this wav 

 be compared to water in a ditch in 



summer which appears green in colour to the eye. 

 It is not really green, because if some of this water 

 be viewed in a tumbler the clear water itself is 

 seen, and the green colour is then perceived to 

 be due to the infinite number of small green plants 

 which float in it. So with the blood; its colour 

 is due to the immense number of red corpuscles, 

 and it is only when the blood is microscopically 

 treated as described, that the eye is capable of 



