62 ANIMAL BIOLOGY. [Parti. 



blood has to pass through the lungs. But not all has to pass 

 through either the alimentary canal to be enriched with tissue 

 nutriment or through the kidneys to be deprived of nitrogenous 

 tissue waste. This will best be seen by the diagram (Fig. 23). 

 the blood which leaves the left ventricle may either pass to 

 the alimentary canal, thence to the liver, where it undergoes 

 complex and highly important changes, and thence to the right 

 auricle ; or it may go to the kidney, and thence to the right 

 auricle ; or it may go to the senso-motor arc, and thence to the 

 right auricle. 



Thus to enable the senso-motor arc to continue its functions, 

 there must be a circulation of blood, and processes of digestion 

 and absorption, of arterialisation, and of excretion, to say 

 nothing of those vital processes which go on within the liver. 

 These, together with the reproductive function, are the essen- 

 tials of the general physiology of such an animal as a rabbit or 

 a pigeon. But in addition to this essential physiology there is 

 a vast amount of accessory physiology, of which more hereafter. 



