CHAP, i.] BLOOD. 35 



glass slide, in a metal frame, is ruled into minute rectangles, e.g. J mm. 

 by i mm., so as to give a convenient area of -g^th of a square mm. 

 Three small screws in the frame permit a coverslip to be brought to a 

 fixed distance, e.g. -J- mm., from the surface of the slide. The blood 

 having been diluted, e.g. to 100 times its volume, a small quantity of 

 the diluted (and thoroughly mixed) blood, sufficient to occupy fully the 

 space between the coverslip and the glass slide when the former is 

 brought to its proper position, is placed on the slide, and the coverslip 

 brought down. The volume of diluted blood now lying over each of the 

 rectangles will be yj^th (^ x ^) of a cubic mm. ; and if, when the cor- 

 puscles have subsided, the number of corpuscles lying within a rectangle 

 be counted, the result will give the number of corpuscles previously dis- 

 tributed through -j-jyoth of a cubic mm. of the diluted blood. This 

 multiplied by 1 00 will give the number of corpuscles in 1 cubic mm. of 

 the diluted blood, and again multiplied by 100 the number in 1 cubic 

 mm. of the entire blood. It is advisable to count the number of cor- 

 puscles in several of the rectangles, and to take the average. For 

 the convenience of counting, each rectangle is subdivided into a number 

 of very small squares, e.g. into 20, each with a side of ^th mm., and 

 so an area of ^J^th of a square mm. 



Since the actual number of red corpuscles in a specimen of 

 blood (which may be taken as a sample of the whole blood) is 

 sometimes more, sometimes less, it is obvious that either red cor- 

 puscles may be temporarily withdrawn from and returned to the 

 general blood current, or that certain red corpuscles are after a 

 while made away with, and that new ones take their place. We 

 have no satisfactory evidence of the former being the case in 

 normal conditions, whereas we have evidence that old corpuscles do 

 die and that new ones are born. 



26. The red corpuscles, we have already said, are continually 

 engaged in carrying oxygen, by means of their haemoglobin, from 

 the lungs to the tissues ; they load themselves with oxygen at the 

 lungs and unload at the tissues. It is extremely unlikely that this 

 act should be repeated indefinitely without leading to changes 

 which may be familiarly described as wear and tear, and which 

 would ultimately lead to the death of the corpuscles. 



We shall have to state later on that the liver discharges into 

 the alimentary canal, as a constituent of bile, a considerable quan- 

 tity of a pigment known as bilirubin, and that this substance has 

 remarkable relations with, and indeed may be regarded as a deriva- 

 tive of hcematin, which as we have seen ( 24) is a product of the 

 decomposition of haemoglobin. It appears probable in fact that the 

 bilirubin of bile (and this as we shall see is the chief biliary pigment 

 and the source of the other biliary pigments) is not formed wholly 

 anew in the body but is manufactured in some way or other out of 

 haematin derived from haemoglobin. This must entail a daily con- 

 sumption of a considerable quantity of haemoglobin, and, since we 

 know no other source of haemoglobin besides the red corpuscles, 

 and have no evidence of red corpuscles continuing to exist after 



32 



