THE BLOOD 811 



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 homo- 

 geneous, but consists of a number of 

 corpuscles of different kinds sus- 

 pended in a light yellow transparent 

 fluid. In order to make out the 

 characters of these corpuscles the 



blood should be diluted with some v - x jj 



1 normal ' fluid, such as 0-9 per cent, 

 sodium chloride. It is then seen to 

 contain two classes of corpuscles. 

 Much the most numerous are the 



( -, , , mi ., A'ft ar . FlG - 357 - Non-nucleated red blood-discs 



red corpuscles. of human blood> On the right of the 



appearance according as the blood figure the corpuscles are seen on edge. 



. -, . n . . ( SWALE VINCENT. ) 



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. 357), 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 /x 



Thickness (at periphery) . . . 2-5 /m 

 Thickness (at centre) . . . . 1-0 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 amoeba, have the power of ingesting granules of 

 carmine, food, or dead bacteria with which they may come in contact. 



In addition to these two classes, a third body is generally described 

 under the name of ' blood-platelets ' or hsematoblasts. These are especially 

 well seen when the blood has been received directly into an excess of osmic 

 acid. It is still doubtful whether they are pre- existent in the circulating 

 blood or are formed in the plasma by a process of precipitation. 



