CH. XXVII.] OXYGEN WANT 399 



rendered less efficient, as may be done by the use of drugs. This is 

 in agreement with a series of researches on the heat given out by 

 frogs' muscle, which shows that the heat given out in a single 

 contraction varies directly with the tension in the muscle. The 

 efficiency of the heart regarded as a machine seems to be low (about 

 2 per cent.), owing perhaps to the anatomical disposition of the strands 

 of fibres. 



Intensity of Respiration. Most of the figures, relating to gaseous 

 metabolism given in the preceding paragraphs were obtained from 

 the examination of the tissues and organs of the dog. If all the 

 tissues were examined in turn, and their relative weights known, 

 an average might be struck which would give the gaseous met- 

 abolism for the body taken as a whole, and this might be expressed 

 as the amount of oxygen used per minute per gramme of body- 

 weight. An easier and more practicable method, however, is to weigh 

 the animal, and then from the composition of the inspired and 

 expired air and the amount of oxygen taken in and given out, 

 calculate how much is retained and utilised. In the dog, the 

 amount is about 0-016 c.c. of oxygen per minute per gramme of 

 body-weight. This figure, however, is not the same in all animals, 

 and the size of the figure will indicate what we may term the 

 intensity of respiration. Thus in cold-blooded animals, especially 

 fishes with their small supply of oxygen, the figure is very much 

 smaller. Among warm-blooded animals great variations are also 

 seen ; the intensity of respiration, for instance, is much greater in 

 birds than in mammals. Among the mammals, the intensity of 

 respiration varies, roughly, inversely with the size of the animal ; 

 thus, in the mouse, an animal that breathes with extreme rapidity, 

 the intensity is probably ten to fifteen times greater than in the 

 dog, and in the elephant very much less. In man, the average is 

 about half that in the dog, that is, 0-008 c.c. of oxygen per 

 gramme of body-weight per minute. 



Oxygen Want. 



The balance between the demand for oxygen and the supply 

 may be upset either by increasing the demand or decreasing the 

 supply. 



Oxygen want may therefore take place either as the result of 

 violent exercise or of diminished oxygen pressure. Normally a man 

 takes into his blood about 400 c.c. of oxygen per minute, in sleep 

 about half this, and in such exercise as is afforded by the pushing of 

 a motor bicycle up a steep hill, 3000 c.c. per minute or more are 

 absorbed by the blood. During the violent contraction of a muscle, 

 the blood-now through it is momentarily retarded, though over a 

 considerable interval of time it is much increased. Nevertheless 



