194 HUMAN PHYSIOLOGY 



vessels. They there form clumps, and from these clumps 

 fibrin threads are seen to shoot out. They thus appear to 

 play an active part in clotting, possibly by yielding thrombin. 

 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. 



3. Erythrocytes Red Cells. Shape. All mammals ex- 

 cept 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. A 

 nucleus is not present in the fully developed mammalian 

 erythrocyte. In birds, reptiles, amphibia and fishes, the 

 corpuscles are elliptical biconvex bodies, with a well-marked 

 central nucleus. The size of the human erythrocytes is fairly 

 constant on an average 7 '6 micro-millimetres in diameter. 

 The number of red cells in health is about 5,000,000 in 

 men and 4,500,000 in women, per cubic millimetre ; but in 

 disease it is often decreased. 



The number of corpuscles per cubic millimetre is estimated 

 by the Haeinocytometer. This consists of (1) a pipette by 

 which the blood may be diluted to a definite extent with 

 normal salt solution, and (2) a cell ruled in squares each con- 

 taining above it a definite small volume of blood so that the 

 number of corpuscles in that volume may be counted under 

 the microscope. (Experiment.) 



The pale yellow colour of the individual corpuscles is due 

 to the pigment they contain being spread out in so thin a 

 layer. This pigment may be dissolved out by various agents 

 e.g. salts of the bile acids, distilled water, dilute acids, or 

 alkalies, &c., leaving a colourless shadow-like corpuscle, which 

 seems to be composed of a sponge-like stroma in the inter- 

 stices of which the pigment is held. 



Chemistry. The stroma of the erythrocytes is made up of 

 a globulin-like substance, in connection with which lecithin 

 and cholesterin occur in considerable quantities. Potassium 

 is the base most abundantly present. This stroma seems to 

 form a capsule round the cell, and through this capsule water 

 can pass under the influence of osmotic pressure. It is well 

 known that, if two solutions of different molecular concen- 



