THE MEANING OF EVOLUTION 273 



The brain and cord are a switch-board of unimagin- 

 able complexity, so that an efferent impulse entering 

 it from, say, the eye, can be shunted on to one nerve 

 path after another, so that it may affect any muscle 

 in the whole body. This is no fiction : it may actually 

 be the case. In normal respiration a centre in the 

 hind-brain is stimulated to rhythmical activity by the 

 presence of carbon dioxide in the blood, and from it 

 efferent impulses originate which stimulate the muscles 

 of the chest wall and diaphragm. But in the distress 

 of asphyxia every muscle of the body may be stimulated 

 to activity in the effort to accelerate the oxygenation 

 of the blood, and these are not spasmodic movements 

 of the muscles of limbs, etc., but purposeful contractions 

 having for their object the increased intake of air into 

 the lungs. The central nervous system is, therefore, 

 a switch-board — so mechanistic physiology teaches, 

 neglecting any idea of an operator. But the whole 

 trend of modern investigation is to show that every 

 increase of specialisation in the evolution of the higher 

 animal adds to the complexity of this nervous apparatus 

 by increasing the number of alternative paths that an 

 impulse originating anywhere in the body may take 

 before it issues from the brain or spinal cord. Yet 

 with all this increase of complexity it is nevertheless 

 the case that in the higher animal the various parts of 

 the central and peripheral nervous system are more 

 and more integrated, so that in the actions of the 

 animal it becomes more and more the organism as a 

 whole that acts. 



All other organs in the animal body — excepting 

 always the reproductive apparatus — are accessory to 

 the sensori-motor system. The alimentary canal and 

 its glands dissolve the food-stuffs ingested ; the meta- 

 bolic organs, that is, the cells of the wall of the intestine, 



s 



