lo Oct., 1908]. Elements of Animal Physiology. 637 



breathing is forced or laboured, then expiration must be facilitated and 

 this is accomplished by the muscles of the abdominal wall contracting and 

 therefore pressing the abdominal viscera against the diaphragm. Further, 

 the ribs move in the reverse direction to that which takes place in inspiration 

 and so the capacity of the thorax is reduced. 



The amount of air which is taken in (or giVen out) in quiet breathing 

 has been called "tidal" air but, as no definition of q^uiet breathing can 

 be given, the term is valueless. The maximum volume of air which can be 

 expired after the deepest inspiration possible, is called vital capacity. 

 The amount varies in man from 3 to 3.8* litres and is supposed to vary 

 in a horse from 25 to 30 litres. After the most violent expiration there 

 always remains a considerable volume of air in the lungs ; this is called 

 RESIDUAL AIR and its volume is given as 1.5 litres for man and 7 to 17 litres 

 for a horse. Once air has been admitted to the lungs it cannot be 

 completely got rid of by mechanical means ; in consequence the lungs of 

 a mammal, that has breathed only once, will float in water, whilst the lungs 

 of one still-born, or killed before a breath has been taken, will sink in 

 water. 



THE CHEMISTRY OF RESPIRATION. 



A clue to what is happening in the lungs is given by the chemical 

 analysis of ordinary air as compared with that of expired air. 



Air inspired. Air expired. 



Oxygen 20 9 per cent. ... ... 15 '9 per cent. 



Nitrogen and Argon 79 per cent. ... 79 per cent. 



Carbon-Dioxide 004. per cent. ... 4-4 per cent. 



Water vapour in varying amount ... Saturated with water vapour. 



Dust and bacteria present ... ... No dust or bacteria. 



Traces of liydrogen and marsh 

 gas present. 



The loss of dust and bacteria is due to the trapping action of the 

 moist surfaces of the breathing passages, particularly those in the nose, 

 but all the other changes we may refer to the lungs. It is evident that 

 oxygen is absorbed and an almost equal amount of carbon-dioxide added 

 to the air. The traces of hydrogen and marsh gas are due to these sub- 

 stances being formed in the bowel by fermentation and being in part 

 absorbed by the blood and liberated in the lungs. The nitrogen it will be 

 observed undergoes no alteration. The main change concerns the oxygen 

 and carbon-dioxide. Now it is found that if blood be placed under an air- 

 pump, gases will escape in quantity sufficient to measure and analyze. 

 The following table furnishes us with the next piece of evidence: — 



The air-pump can extract from 100 volumes of blood — 



Enterint;- the luns's. Leaving the luiiy.s. 



(Venous.) (Arterial.) 



Oxygen 6 • 5 vols. ... ... i2'8 vols, 



Carbon-Dioxide 4-6- 5 vols. ... 39 6 vols. 



Plainly, therefore, the blood as it passes through the lungs takes up 

 oxygen and gives off nearly an equal volume of carbon-dioxide. When we 

 next consider how oxygen is held by the blood, we shall find that the 

 haemoglobin of the corpuscles is responsiBle for this faculty. If blood be 

 bereft of its corpuscles and shaken with air, 100 volumes of the blood can 



* A litre = 61 cubic inches = i| pints. 



