90 A Manual of Veterinary Physiology. 



liberates the oxygen in the tissues, and assists in liberating 

 the C0 2 in the lungs from the substances with which these 

 are chemically combined, viz., hemoglobin and carbonate 

 of soda. 



When an animal is compelled to breathe the same air 

 over and over again, there is a gradual loss of oxygen and 

 increase in carbonic acid, and though death will ultimately 

 ensue unless the air be renewed, it is remarkable that 

 before this occurs nearly the whole of the oxygen will 

 have been consumed from the atmosphere, which is further 

 evidence, if any be needed, that the oxygen is not simply 

 absorbed by the blood, and that it does not obey the laws 

 of pressure. Experimental inquiry has proved that animals 

 may live in an atmosphere containing only 14*8 per cent. 

 of oxygen, but that rapid asphyxia follows when the oxygen 

 falls to 3 per cent. 



By increasing the amount of oxygen in a mixture over 

 and above that contained normally in air, the blood cannot 

 be made to take up much more oxygen than if the normal 

 amount only were present ; a pressure of ten atmospheres 

 only causes an increase of 3*4 per cent, absorbed, so that 

 the blood contains 234 per cent, of oxygen instead of 

 20 per cent. The practical application of this fact in the 

 treatment of certain diseases by the inhalation of oxygen 

 is obvious : if we double the amount of oxygen in the air. 

 less than 1 per cent, of the extra addition is absorbed. 



Apnoea may be produced by distending the lungs several 

 times with air, and holding the breath ; during this period 

 the respiratory centre is controlled by certain impulses 

 occurring through the vagus. 



Dyspnoea is increased respiration due to an insufficient 

 quantity of oxygen in the blood. 



If the air supply be entirely cut off, asphyxia and death 

 rapidly ensue. Asphyxia has been divided into three stages. 

 hi the. first stage the attempts at breathing arc laboured ami 

 painful, deep and frequent, and all the respiratory muscles, 

 including the complemental ones, are brought into play : 

 convulsions occur, and the blood pressure rises. In the 



