916 PHYSIOLOGY 



excess. The changes are therefore kept within infinitesimal limits ; 

 in most cases they are within the limits of errors of analysis, and we 

 may therefore treat the blood as 'a fluid of approximately constant 

 composition and qualities. 



Blood obtained from a mammal is an opaque fluid varying in tint 

 according to the vessel from which it is derived, being scarlet when 

 taken from an artery, purplish in colour when taken from a vein, the 

 difference being determined by the degree of oxygenation of the blood. 

 On shaking venous blood with air it takes up oxygen and acquires the 

 scarlet colour characteristic of arterial blood. If examined in a thin 

 layer under the microscope its opacity is seen to be due to the fact 



that it is not homogeneous, but 

 ^^ consists of a number of corpus- 



cles of different kinds suspended 

 /in a light yellow transparent 

 ^*f9 flmrl Tr nrrloT* f.n malr-o rnif. -fVio 



fluid. In order to make out the 

 characters of these corpuscles 

 the blood should be diluted with 

 some ' normal ' fluid, such as 

 O9 per cent, sodium chloride. 



FIG. 354. Non-nucleated red blood-discs Xt is then Seen to contain two 

 of human blood. On the right of the classes of corpuscles. Much the 



6dge ' most numerous are the ' red cor- 



puscles.' These differ in appear- 

 ance according as the blood is derived from a mammal or from one of 

 the lower orders of vertebrates. In all the latter it is a nucleated cell. 

 In the frog, for instance, it is an oval bi-convex disc containing an oval 

 nucleus in the centre. In man and other mammals the red corpuscle 

 is a bi-concave circular disc (Fig. 353), varying in size in different 

 species. The average sizes of the corpuscles in man are given in the 

 following Table : 



Diameter . . . . . 7'1 to 7'8 ^ 

 Thickness (at periphery) . . 2-5^ 

 Thickness (at centre) 5 ; . TO to 2'0 ^ 



In the blood of man there is an average of five million red blood- 

 discs in every cubic millimetre of blood. 



The other kind of formed element, the white corpuscle, or leucocyte, 

 is present in much smaller numbers than the red corpuscle, there 

 being in human blood an average of one leucocyte to every 500 red 

 corpuscles. These leucocytes are colourless cells, somewhat larger 

 than the red blood-discs of man, presenting one or more nuclei and a 

 granular or hyaline protoplasm. When examined on the warm stage 

 they are seen to be amoeboid, and many of them, like the amosba, 



