ECONOMY OF THE BODY 133 



at work. Metschnikoff found some which were transporting 

 pigment from a feather, and others which were eating away 

 at the brain of an over-aged parrot — eating away at part 

 of the body to which they themselves belonged. 



The bird's red blood corpuscles are elliptical sHghtly 

 biconvex discs, gradually pointed at the ends. According 

 to Venzlaff (i9ii),they vary in number from 1,715,000 to 

 5,400,000 per cubic centimetre, tending to be more 

 numerous when they are small. For man the number is 

 about 5,000,000 per cubic centimetre, and 4,500,000 for 

 woman. They arise in part from the cells in the medulla 

 inside the bones and in part from the spleen. There is 

 considerable diversity in the dimensions of the red-blood 

 corpuscles in birds, even in the same species. A common 

 size, according to Magnan (191 1), is i3*3/>t by 6*5ju,. Some of 

 the large birds of prey have distinctly larger corpuscles, 

 ^•S- ^7'7l^ by 8/x in the vulture {Gyps fulvus), but there does 

 not seem to be any general correlation between size of 

 bird and size of corpuscle. For comparison we may quote 

 a few measurements of the diameters of mammalian red 

 blood corpuscles — elephant, g'^fSi ; man, y^jx ; dog, jfx ; 

 horse, G'^jx ; ox, 5"5/x ; Javanese chevrotain, Zjx. It is 

 plain that the red blood corpuscles of birds are on the whole 

 larger than those of mammals, but the difference of shape 

 must be borne in mind. By /x is meant a micron, i.e. one 

 thousandth of a millimetre. 



The blood of birds may be called the richest blood in 

 the animal kingdom, for it seems to have more red blood 

 corpuscles per ounce than in any other animal. This corre- 

 sponds to the intensity of their life and to their aerial 

 activities. People who go to live at high altitudes acquire 

 perhaps 20 per cent, more red blood corpuscles — an 

 adjustment to the increased difficulty of breathing which 

 follows from a decrease in the percentage of oxygen in the 

 atmosphere. 



The cells of the blood float in a serum — a complex 

 mixture of salts, dissolved food-materials, and fluid nitro- 

 genous waste. Biologically considered, the salts are 



