644 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



in air. The cells of which our bodies are composed live, like 

 the amoebae, in a fluid — the intercellular fluid or lymph in 

 which they are bathed ; and our skin may be compared to the 

 glass in which the amoebae and the water in which they swim 

 are contained. There is, however, a very great difference 

 between the amoeba and the cells composing the bodies of the 

 higher animals, for it can swim about freely, whereas they 

 are for the most part fixed. The amoeba can thus obtain fresh 

 supplies of nutriment and oxygen by moving through the 

 water in which it lives, while in higher organisms the fluid 

 moves over the cells. This fluid, or lymph, is the liquor 

 sanguinis, which passes out of the capillaries and supplies 

 all the tissues with nutriment and oxygen, at the same time 

 that it removes from the system carbonic acid and the pro- 

 ducts of tissue-waste. 



The interchange of oxygen and carbonic acid between the 

 tissues and the blood is termed internal respiration. 



But this interchange would soon remove all the oxygen 

 from the blood and load it with carbonic acid, unless it had 

 some means of absorbing oxygen and giving off carbonic acid 

 to the atmosphere. This is effected in the lungs, and the 

 interchange of gases between the blood and external air is 

 termed extern/il respiration. 



The blood, therefore, acts as an oxygen-carrier between the 

 lungs and the tissues. A certain amount of the oxygen taken 

 up by the blood is simply dissolved in it; but the amount 

 of this is not sufficient to supply the wants of the tissues, and 

 the greater part of the oxygen which they require is carried 

 to them by means of the haemoglobin, or colouring matter of 

 the blood. This substance forms a loose compound with 

 oxygen in the lungs, and gives it off to the tissues when it 

 reaches the capillaries, and on again passing through the 

 lungs it takes up a fresh supply. The carbonic acid which is 

 formed in the tissues by their oxidation is taken up by the 

 blood in the capillaries and given off in the lungs ; but it 

 seems to be carried from the tissues to the lungs, not by the 

 haemoglobin, but by some one or other of the salts in the blood. 

 Both internal and external respiration are essential for the 



