THE BLOOD OF EQUIXES. 245 



In this i^aper a few properties of healthy blood will be dealt 

 with. 



The blood consists of a fluid part in which are suspended 

 numerous minute semi-solid bodies, the so-called " formed ele- 

 ments." These formed elements are somewhat heavier than 

 the fluid part, and if blood, prevented from clotting, be allowed 

 to stand, they gradually sink and settle in the bottom of the 

 vessel. The upper or fluid part, called plasma, is now seen to be 

 more or less transparent and to vary in colour from a pale to a 

 dark amber. In the bottom of the vessel the mass is dark red, 

 and above this there is a very thin whitish or greyish layer. 

 Microscopic examination shows that both the red and grey layers 

 sue composed of cells, and tliese cells are called respectively the 

 i-ed corpuscles or erythrocytes, and the white corpuscles or 

 leucocytes. 



The Erythrocytes. 



These are the cells more particularly concerned in the 

 transiwrtation of oxygen from the lungs to the tissues. The 

 red colour of the er*ythrocyte is due to haemoglobin, and it is this 

 substance which actually carries the ogygen. In the capillaries 

 of the lungs the erythrocytes are only separated from the air in 

 the alveoli by a thin membrane through which oxygen can pass 

 easily. Here then the haemoglobin of the erythrocytes combines 

 with the oxygen of the air to form a compoiuid " oxy haemoglo- 

 bin," which can be broken down very easily again. In the tissue 

 capillaries the state of affairs is partially reversed, the oxyhaemo- 

 globin being dissociated and the oxygen so liberated being used 

 by the tissues. 



The amount of haemoglobin in the blood is thus the important 

 factor determining the amount of oxygen that can be can-ied to 

 the tissues under any given set of circumstances, and since the 

 hsemoglobin is contained in concentrated form in the erythro- 

 cytes, their number is an index of the amount of oxj-gen that 

 can be carried. 



It is quite easy to estimate the amount of haemoglobin in 

 tlie blood, and also to count the number of red cells per unit 

 volume. From these data it has been ascei'tained that in health 

 ths amount of haemoglobin per erj^throcyte is fairly constant, 

 although some variations occur. 



The numher of erythrocyte h per luiit volume of himian blood 

 is fairly constant during health, and approximates to 5 millions for 

 men and 4^ millions for women in every cubic millimeter of 

 blood. Variations from this normal figure are often noted, how- 

 ever, and in general it is found that the figure is higher for robust 

 active individuals, and lower for obese. In the case of children 

 it was found that the counts of erythrocytes were as. a rule higher 

 after long vacations than at the end of the school term. But 

 those variations are not great — at any rate, they do not extend 

 into millions. 



