10 



CHAPTER II. 



BLOOD. 



9. UNDER the microscope blood appears as a trans- 

 parent fluid, the liquor sanguinis or plasma, in which 

 float vast numbers of formed bodies, the blood cor- 

 puscles. The great majority of these are coloured : 

 a few of them are colourless. The latter are called 

 white or colourless blood corpuscles, or leucocytes. 

 The former are called red or coloured blood corpuscles, 

 or blood-discs. They appear red only when seen in a 

 thick layer ; when in a single layer they appear of a 

 yellow-greenish colour, more yellow if of arterial, 

 more green if of venous blood. The proportions of 

 plasma and blood corpuscles are sixty-four of the 

 former and thirty-six of the latter in one hundred 

 volumes of blood. By measurement it has been found 

 that there are a little over five millions of blood cor- 

 puscles in each cubic millimetre ( 15 ^ 25 of a cubic inch) 

 of human blood. There appears to be in healthy 

 human blood one white corpuscle for 600-1200 red 

 ones. In man and mammals the relative number of 

 blood corpuscles is greater than in birds, and in birds 

 greater than in lower vertebrates. 



10. The red blood corpuscles (Fig. 6) of man 

 and mammals are homogeneous bi-concave discs (except 

 in the camelidse, where they are elliptical), and do not 

 possess any nucleus. Being bi-concave in shape, they 

 are thinner and more transparent in the centre than 

 at the periphery. In other vertebrates they are oval 

 and more or less flattened from side to side, and each 

 possesses a central oval nucleus. 



The diameter of the human red blood corpuscles is 



