THE BLOOD. 141 



The red corpuscles have no nuclei, although, in their usual state, the 

 unequal refraction of transmitted light gives the appearance of a cen- 

 tral spot, brighter or darker than the border, according as it is viewed 

 in or out of focus. Their specific gravity is about 1088. 



The corpuscles of all mammals with the exception of the camelidse 

 are circular and biconcave. In the camelidae they are oval and bicon- 

 vex. In all mammals the corpuscles are non-nucleated, and in all other 

 vertebrates (birds, reptiles, amphibia, and fish), the corpuscles are oval 

 biconvex and nucleated (fig. 121). 



Numbers. The normal number of red blood cells in a cubic milli- 

 metre of human blood was estimated by Welcker, in 1854, to be 5,000,000 

 in men and 4,500,000 in women. Eecent observations, however, have 

 shown that these estimates are a little low, especially in men, and the 

 average number has been placed by different authorities at various points 

 between 5,000,000 and 5,500,000, or even 6,000,000. Still the original 

 numbers as given by Welcker are accepted at the present day as being 

 sufficiently accurate for ordinary purposes. It has also been shown that 

 there are many distinct physiological variations in the number, depend- 

 ing on the time of day, digestion, sex, and pregnancy.* The number 

 of red cells usually diminishes in the course of each day, while the 

 leucocytes increase in number. It has been suggested that this is due to 

 the influence of digestion and exercise. 



It has generally been found that within half an hour or an hour 

 after a full meal the number of red cells begins to diminish, and that 

 this keeps up for from two to four hours, when it is followed by a 

 gradual rise to the normal. The usual fall is 250,000 to 750,000 per 

 cubic millimetre. These results are most marked after a largely fluid 

 meal, and are probably due to dilution of the blood as a result of the ab- 

 sorption of fluids. In animals the number of red cells is increased by 

 fasting, but in man the results are variable, some authorities claiming 

 an increase and others a decrease. In childhood there is no difference 

 between the sexes in the number of red cells per cubic millimetre, but 

 after menstruation is established, a relative anasmia develops in women. 

 Welcker's original estimate placed the difference at 500,000 per cubic 

 millimetre, and these figures have been generally accepted, though one 

 observer (Leichtenstein) asserted that the difference was 1,000,000. 

 Recent- investigations, however, seem to show that Welcker's estimate is 

 a little too great. 



Menstruation in healthy subjects has practically no effect, as not more 

 than 100-200 cubic centimetres are lost normally in the course of several 

 days. Under such circumstances the normal diminution of red cells per 

 cubic millimetre is probably less than 150,000, though one observer 

 (Sfameni) has placed the loss at about 225,000. Some observers, on the 



