BLOOD 485 



there form clumps, and in these they disintegrate, probably 

 liberating thromboplastin, and so start clotting. They are 

 present in the blood of mammals only. Their source is not 

 definitely known, but it has been suggested that they are the 

 extruded nuclei of developing erythrocytes, or that they are 

 derived from the giant cells of the bone-marrow (p. 500). 



3. Erythrocytes— Red Cells. 



1. Characters. — All mammals, except the camels, have 

 circular, biconcave, discoid erythrocytes, which, when the 

 blood is shed, tend to run together like piles of coins. The 

 camels have elliptical biconvex corpuscles. The fully 

 developed mammalian erythrocytes are without a nucleus. 

 In birds, reptiles, amphibia and fishes, the corpuscles are 

 elliptical biconvex bodies, with a well-marked central nucleus. 



2. Size. — The size of the human erythrocytes is fairly 

 constant — on an average 5 "5 micro-millimetres in diameter. 



3. Number. — The number of red cells in health is about 

 7,000,000 in the horse, but in disease it is often decreased. 

 The number of corpuscles per cubic millimetre is estimated by 

 the Haemocytometer. This consists of (1) a pipette by which 

 the blood may be diluted to a definite extent with a salt 

 solution of the same osmotic equivalent as the plasma, and 

 (2) a cell of definite depth ruled in squares,' each containing 

 above it a definite small volume of blood, so that the number 

 of corpuscles in that volume may be counted under the 

 microscope (Practical Physiology). 



The pale yellow colour of the individual corpuscles is due 

 to a pigment held in a fine sponge-like stroma which seems 

 also to form a capsule round the cell. 



4. Haemolysis. — This pigment may be dissolved out by 

 various agents, and the action is termed haemolysis. It 

 may be brought about in different ways — 



1st. By placing the erythrocytes in a fluid of lower 

 osmotic equivalent, i.e. of lower molecular concentration, 

 than the blood plasma and corpuscles. A solution of 0*9 

 per cent, of sodium chloride has the same osmotic equivalent 

 as the plasma and preserves the corpuscles unaltered ; in 

 more dilute fluid the corpuscles tend to swell up by 



