BLOOD AND LYMPH 209 



amount of fat are present, while the chief inorganic con- 

 stituents are potassium salts. 



2. Blood Platelets. These are small circular or oval dis- 

 coid bodies, about one-third the diameter of a red blood cor- 

 puscle. Some observers have stated that they contain a 

 central nucleus. They are very sticky and mass together 

 when blood is shed and adhere to a thread passed through 

 blood or to any rough point in the lining of the heart or 

 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. 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. Erythroeytes Red Cells. 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. 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 erythrocytes is fairly constant in each species 

 of animal. In the horse they are on an average 6 to 6'5 

 micro-millimetres in diameter. The number of red cells in 

 health is about 7,000,000 per cubic millimetre in the horse ; 

 but in disease it is often decreased. 



The number of corpuscles per cubic millimetre is estimated 

 by the Hsemocytometer. 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 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 to form a capsule round the cell. This pigment may 

 be dissolved out by various agents, and the action is termed 

 Haemolysis. It may be brought about in different ways 1st. 



