130 A MANUAL OF VETERINARY PHYSIOLOGY 



been termed intramolecular oxygen. It there loses the properties 

 of free oxygen, and forms a complex substance which readily 

 yields carbon dioxide and other matters on decomposition, and 

 this passes into the bloodvessels of the muscle and is carried 

 away to be got rid of at the lungs. The changes which the oxygen 

 undergoes from the time it leaves the blood until it reappears 

 as carbon dioxide in the tissues are completely unknown. It 

 has been suggested that the nature of the oxidative changes 

 occurring in living tissues may be due to the presence of oxidising 

 ferments in the body — oxydases, as they have been termed. 

 Such ferments have been demonstrated to exist in nearly all 

 the tissues. 



The phenomena of tissue respiration may be studied in living 

 tissues by ascertaining the composition of the blood before and 

 after circulating through them. Such an experiment has been 

 carried out on the horse by Chauveau and Kaufman, utilising 

 the levator muscle of the upper lip during feeding. The con- 

 sumption of oxygen and the production of carbon dioxide was 

 many times greater during activity than during rest. Glands 

 such as the submaxillary have been similarly examined, while the 

 respiratory changes occurring in the lungs during rest and work 

 have formed the subject of many experiments. All tell the same 

 story, though not all in the same degree. One remarkable 

 example of this is furnished by L. Hill's examination of the 

 respiratory exchange in the living brain, which, compared with 

 skeletal muscle, was found to be very low. 



It is impossible here to examine further the question of respira- 

 tion in the tissues, but the question will be referred to in detail 

 when its importance warrants it, as in respiratory exchange, 

 muscular work, animal heat, and general body metabolism. 



Deficiency in Oxygen. — When an animal is compelled to breathe 

 the same air over and over again, there is a gradual loss of 

 oxygen and an 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. This is further evidence, if any 

 be needed, that the oxygen is not simply absorbed by the blood, 

 and that its absorption does not obey the ordinary laws of pres- 

 sure. Experimental inquiry has proved that animals may live 

 in an atmosphere containing only 14 per cent, of oxygen, that 

 distress appears at 11 per cent., for at or below this pressure the 

 haemoglobin cannot take up its full amount of oxygen, and rapid 

 asphyxia follows when the oxygen falls to 3 per cent. 



In poisoning by carbon monoxide, the latter gas turns the 

 oxygen out of the blood-cells, yet although the whole of the red 

 cells are converted into carriers of carbon monoxide, the animal 



