THE SPECIAL NERVOUS MECHANISMS 129 



going over to the right-hand side of the medulla, and vice versa. 

 The pyramidal tracts are then continued down the spinal cord, 

 and there their fibres enter into the grey matter and form 

 synapses with the nerve cells which give origin to the axons 

 that go out into the ventral or motor roots of the spinal nerves. 



Thus the grey matter of the motor region of the cortex is, on 

 the one hand, in connection with fibres that run uninterruptedly 

 down into the spinal cord and control the nervous centres there 

 which set the muscles of the body and limbs in action. On the 

 other hand, it is in connection with the centres in the other parts 

 of the cortex which are the termini of the sensory paths. 



But it is also in connection with the cerebellum via another 

 specialised cortical region. Figs. 28 and 36 show a prominent 

 tract of fibres descending from the cortex and ending in the 

 region of the pons varolii — that is, the middle part of the middle 

 peduncles which unite together the two halves of the cerebellum. 

 Thus our cortical motor cells have communication with the 

 nervous mechanism of co-ordination. 



And just in the same way as the pyramidal tracts connect 

 together the cortical and spinal nuclei, so do other more com- 

 plicated paths connect the motor cortex with the medullary 

 nuclei which give origin to the nerves that supply the muscles 

 of the head and face. Thus every part of the motor system of 

 the whole body is under the control of the cortex cerebri. This 

 is particularly the case in man. In the fishes the great pyramidal 

 tracts are hardly to be recognised, and they become developed 

 to a progressively greater extent as we ascend the scale of evolu- 

 tion represented by the dog, monkeys, anthropoid apes, and 

 man. In the lower vertebrates the nuclei which mediate between 

 the organs of sense and the spinal ganglia are those in the mid- 

 brain, but in the higher mammals this nervous mechanism becomes 

 more and more replaced by the cortex and its connections. 



Ahnost all the cortex behind the motor area is, as Fig. 38 

 indicates, the seat of sensory functions. Those centres called 

 " visual," " auditory," " music," etc., contain pyramidal and 

 other cells, the dendrites of which form synapses with the tracts 

 of fibres coming up from the mid-brain nuclei in which the 

 nerves coming from the organs of sense terminate. Stimulation 

 of these cortical sensory centres is certainly essential to the 

 development into full consciousness of the impulses arising from 

 the stimulation of the sense organs. This is a matter to which 

 we return in the following chapter. 



9 



