112 ANIMAL PHYSIOLOGY. 



It has already been pointed out that throughout the body 

 chemical changes are constantly taking place, in which oxygen 

 combines with organic matters, and that carbonic acid is 

 among the products. This oxygen is introduced in respiration, 

 and is carried by the blood in the arteries to the textures ; 

 while the blood which returns thence by the veins carries 

 with it, back to the lungs, the carbonic acid resulting from 

 the processes of oxidation which have taken place through- 

 out the body. The blood going to the textures, or what is 

 ordinarily known as arterial blood, has therefore more oxygen 

 in it than that which returns by the veins, and the venous 

 blood has more carbonic acid than the arterial. There is, 

 however, a considerable amount of oxygen left in venous 

 blood, except when the animal is killed by asphyxia, that 

 is to say, stoppage of respiration ; and the amount of car- 

 bonic acid given off by the lungs, is only a small proportion 

 of the total amount contained in the blood. 



82. The difference in the gaseous contents of the blood going 

 to the textures, and that which returns from them, is accom- 

 panied with a great difference of colour. When blood is 

 allowed to flow from a vein, it comes in a stream as dark as 

 claret, while the blood which comes from a superficial cut is 

 much lighter, and what spouts from a wounded artery is of 

 a bright scarlet. The dark blood from a vein, when spilt on 

 the ground, becomes bright in a few minutes, exposure to 

 the oxygen of the air sufficing to enable it to part with car- 

 bonic acid, and take up oxygen ; and scarlet blood exposed 

 to carbonic acid becomes dark. If a test tube be filled to 

 about a fourth from the top with defibrinated blood, such as 

 can be obtained by breaking down clot, and be shaken up a 

 few times so as to enable the air to mix with it, it will 

 become bright scarlet, and when allowed to stand for some 

 time it will get dark again ; when shaken a second time, it 

 will again grow bright; and this experiment maybe repeated 

 on the same specimen of blood day after day. The same 

 changes may be exhibited with a solution of the colouring 

 matter of the red corpuscles; for the corpuscles are destroyed 

 by addition of water, and their fluid contents are set loose, 

 and this solution alters its colour on exposure to oxygen and 

 carbonic acid alternately. 



