THE BLOOD. 53 



becomes much brighter in color, because owing to .the 

 shrinkage of these corpuscles, the light which is reflected 

 from them is correspondingly concentrated. 



3. Number. Careful and repeated counts of the num- 

 ber of red corpuscles contained in human blood have been 

 made showing that in a cubic millimeter of blood (small 

 drop) there are in males about five millions, and in females 

 about four and one-half millions. This number varies a 

 little; it is decreased after a hearty meal, after severe hem- 

 orrhages, after prolonged fasting, or in such sickness as 

 leukaemia, in which the decrease in number explains the 

 general paleness of complexion. The number to the cubic 

 millimeter varies, however, greatly in different animals. In 

 the form proteus with its huge corpuscles, there are thirty- 

 six thousand ; in the frog four hundred thousand ; in birds 

 three million six hundred thousand, while in the llama, of 

 South America, it reaches the enormous number of fourteen 

 millions. Compared with the white corpuscles, in human 

 blood there are from four to five hundred red to one white. 



4. Surface. In spite of their small size such large 

 numbers give a combined surface which is surprisingly 

 large. Taking the amount of blood in the average body as 

 about six pints, the total surface of all the contained cor- 

 puscles would not be far from four thousand square yards. 

 This would be a surface that would require more than 

 eighty steps to walk across it at its shortest distance. It is 

 a surface over twenty-five hundred times greater than the 

 entire surface of the body. As about one hundred and sev- 

 enty-six cubic centimetres of blood pass into the lungs in 

 each second of time, it means that a corpuscle surface of 

 not much less than one hundred square yards is exposed to 

 the action of the oxygen in this short space of time. Does 

 this not help one to understand the rapidity with which the 

 oxygen is taken up and distributed as well as the amount 

 carried? 



5. Composition. The essential element of the red blood 

 corpuscle is the red haemoglobin imbedded, so to speak, in 



