132 A MANUAL OF VETERINARY PHYSIOLOGY 



of 3 -4 per cent, absorbed, so that the blood contains 23*4 per 

 cent, of oxygen instead of 20 per cent. The practical applica- 

 tion of this fact in the treatment of certain diseases by the 

 inhalation of oxygen is interesting. If we double the amount 

 of oxygen in the air, less than 1 per cent, of the extra addition 

 is absorbed. Either the small amount of extra oxygen thus 

 absorbed must be very valuable, or we must find some other 

 explanation of the undoubted advantage of oxygen inhalation 

 in disease. 



The physiology of the matter is, in effect, this : The air con- 

 tains 20 per cent, of oxygen, which is more than enough for the 

 needs of the body. Even the venous blood returns to the lungs 

 with from 10 to 12 volumes of oxygen per cent, unused, 

 while if the oxygen in the air be doubled, less than 1 per cent, 

 of the extra is absorbed. It may, however, be that the excess 

 of oxygen in the alveolar air of the lungs during oxygen inhala- 

 tion enables the tissues to obtain their normal amount more 

 easily. 



Apncea is the term applied to a condition in which no respira- 

 tory movements of any kind are made. It is in the main a 

 laboratory product, though it may also be met with in surgical 

 operations. Apncea may be produced artificially by blowing air 

 into and sucking it out of the lungs at a more rapid and forcible 

 rate than the ordinary respiratory rhythm of the animal. Two 

 conditions have now been acting, either of which separately will 

 produce apncea. In the first place there is a very free lung 

 ventilation, and in consequence a change in the composition of 

 the gases in the alveoli of the lungs, and so in the blood of the 

 medulla. The partial pressure of the carbon dioxide in the 

 blood will be lowered, and as a result of the natural stimulus of 

 the respiratory centre being withdrawn, the centre will remain 

 inactive. This is not due, as was at one time supposed, to an 

 excess of oxygen in consequence of free lung ventilation, as it 

 will occur when the lungs are distended with hydrogen. 



If the lowered pressure of carbon dioxide in the blood is the 

 source of the suspended breathing, the introduction of this gas 

 into the blood should abolish the apnceic condition. This is 

 found to be so. A blast of C0 2 cuts short apncea in the rabbit, 

 while a blast of air has no such effect. 



Another form of apncea results from stimulation of the in- 

 hibitory fibres of the vagus by the repeated distension of the 

 lungs. This condition cannot be produced if the vagi be divided. 



The real value of the experimental production of apncea lies 

 in the light which it throws on the normal respiratory process, 

 and it is very remarkable that the normal rate of breathing 

 suitable to the animal's immediate requirements should be 



