THE SPECIAL NERVOUS MECHANISMS 111 



medulla and the inferior peduncles of the cerebellum to end in 

 the grey matter of the latter part of the brain. 



But a large number of fibres entering the cord via the sensory- 

 roots take a very different course. These do not form synapses 

 in the grey matter, but turn into the white matter at once, and 

 form the two great tracts of fibres represented in Fig. 28 as the 

 " great sensory tracts in the cord." These tracts end in the 

 medulla in four prominent nuclei — that is to say, the fibres form 

 synapses with the nerve cells in these ganglia. The axons passing 

 out from the latter cells are collected together to form the two 

 great sensory tracts called the lemnisci, and the latter, after 

 crossing, as indicated in Fig. 28, run up into the corpora quadri- 

 gemina and optic thalami, and end by forming synapses with the 

 cells in those nuclei. There they come into relation with 

 nervous tracts passing out from the brain, and serving as the 

 avenues along which motor impulses go out to the muscles of the 

 body. So far, then, as we have studied it, the great sensory 

 tract from the cord to the brain is one which carries impulses 

 arising in the muscles and joints up into the mid-brain centres, 

 via the ganglia in the medulla and the lemnisci tracts, and which, 

 again, is mainly or solely a means of muscular co-ordination. 

 In order that consciousness may be affected by these impulses, 

 another link must be added to the chain of paths, and this is, 

 we shall see, the tract of fibres passing up from the mid-brain 

 to the cortex cerebri. 



Lastly, impulses arising from the heat, cold, touch, and pain 

 receptors in the skin pass into the cord through fibres in the 

 sensory roots of the spinal nerves. These go into the grey 

 matter directly, and form synapses round the nerve cells there. 

 The axons of such cells go out again into the white matter of 

 the cord, but they do not form a continuous tract of fibres. 

 Instead of that, they run upwards only a short distance, turn 

 back into the grey matter, form other synapses, and then re-enter 

 the white matter to form another short tract. By a series of 

 such linkages the impulses reach the medulla, and then pass up 

 into the lower brain by a devious route. Finally, they become 

 connected with tracts passing up into the cortex, when they 

 undergo full development into psychical affections. 



Thus the paths or tracts " mediating " general sensibility of 

 the limbs and trunk lead up through the sensory roots of the 

 spinal nerves and through the grey and white matter of the cord 



