206 THE INDUCTIONS OF BIOLOGY. 



distant parts; and on coming to those advanced types which 

 have closed arterial and venous systems, ramifying minutely 

 in every corner of every organ; we find that the vascular 

 apparatus, while it has become structurally interwoven with 

 the whole body, has become unable properly to fulfil its office 

 without the help of offices that are quite separated from its 

 own. The heart, though mainly automatic in its actions, is 

 controlled by the nervous system, which takes a share in 

 regulating the contractions both of the heart and the 

 arteries. On the due discharge of the respiratory function, 

 too, the function of circulation is directly dependent : if the 

 aeration of the blood is impeded the vascular activity is 

 lowered; and arrest of the one very soon causes stoppage of 

 the other. Similarly with the duties of the nervo- 



muscular system. Animals of low organization, in which 

 the differentiation and integration of the vital actions have 

 not been carried far, will move about for a considerable time 

 after being eviscerated, or deprived of those appliances by 

 which energy is accumulated and transferred. But animals 

 of high organization are instantly killed by the removal of 

 these appliances, and even by the injury of minor parts of 

 them: a dog's movements are suddenly brought to an end, 

 by cutting one of the main canals along which the materials 

 that evolve movements are conveyed. Thus while 



in well-developed creatures the distinction of functions is 

 very marked, the combination of functions is very close. 

 From instant to instant the aeration of blood implies that 

 certain respiratory muscles are being made to contract by 

 nervous impulses passing along certain nerves; and that the 

 heart is duly propelling the blood to be aerated. From 

 instant to instant digestion proceeds only on condition that 

 there is a supply of aerated blood, and a due current of 

 nervous energy through the digestive organs. That the heart 

 of a mammal may act, its muscle substance must be con- 

 tinuously fed with an abundant supply of arterial blood.. 

 It is not easy to find an adequate expression for this double 



