no THE MECHANISM OF LIFE 



fibres, and thus an impulse arising from the stimulus of some 

 receptor owing to a change in the environment becomes con- 

 verted into another kind of impulse that stimulates muscles to 

 contract and relax, and so enables the animal to make an appro- 

 priate (or adaptive) response. But why " appropriate " ? "We 

 discuss this question (but do not answer it) in a later chapter. 



The lower brain, cerebellum, and spinal cord, are therefore 

 centres (or ganglia) where afferent impulses become converted 

 into efferent ones. They are simply the loci of synapses. 



The last development of sensation is the becoming aware of 

 the external changes that stimulate the receptors. It must not 

 be thought that the development of consciousness of the environ- 

 ment, or of the body itself, is the sole, or even the main, function 

 of the nervous system. "What the latter does in the lower 

 animals almost entirely, and what it mainly does, even in man, 

 is to convert a sensory stimulus set up by some change in the 

 environment into an appropriate response. Consciousness and 

 psychical life doubtless accompany this conversion in all animals, 

 though their intensity is the dimmer the lower in the scale of 

 evolution the organism is placed. In ourselves this final develop- 

 ment of sensation is the function of the cortex cerebri. 



General Body Sensation. — Kemembering, then, that the impulses 

 arising from the stimulation of a receptor organ need not, and 

 usually do not, give rise to changes of consciousness, we may con- 

 sider, first, the paths by which the afferent impulses coming from 

 the skin, muscles, and joints of the limbs and trunk reach the brain. 



All such impulses enter the cord by the dorsal (or posterior) 

 or sensory roots of the spinal nerves, but the paths along which 

 they afterwards travel depend upon their nature. Those that 

 arise as the stimulation of receptors in the deeper muscles and 

 joints enter the grey matter of the cord, and are received by the 

 synapses of nerve cells there. From these nerve cells axons pass 

 out into the white matter, and these form nervous tracts on each 

 side that go up through the medulla, enter the inferior peduncles 

 of the cerebellum, and end in the grey matter of that part of the 

 brain. At the same time other nerve fibres, carrying similar im- 

 pulses, enter the cord by the sensory roots, and travel directly up in 

 the white matter without first forming synapses in the grey matter. 



Thus a large number of nerve fibres coming from the receptor 

 organs in the deeper muscles and joints deliver their impulses 

 into tracts of fibres in the cord, which then run up through the 



