162 THE INDUCTIONS OF BIOLOGY 



the higher molluscs, in which this simple tube is replaced 

 by a system of branched tubes, that deliver their contents 

 through their open ends into the tissues at distant parts; 

 and on coming to those advanced types of animals 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 to fulfil its 

 office without the help of offices that are quite separated from 

 its own. The heart is now a complex pump, worked by 

 powerful muscles that are excited by a local nervous system ; 

 and the general nervous system also, takes a share in regu- 

 lating the contractions both of the heart and of all 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 force 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 

 certain nerves; and that the heart is duly propelling the 

 blood to be aerated. From instant to instant digestion pro- 

 ceeds only on condition that there is a supply of aerated blood, 

 and a due current of nervous energy through the digestive 



