THE SPECIAL NERVOUS MECHANISMS 123 



roots of the cranial and spinal nerves. That means, obviously, 

 that there must be paths, confined to the central nervous system 

 itself, along which impulses may travel from the nuclei of special 

 sense in the mid-brain to the motor nuclei in the medulla and 

 cord. The principal intracerebral and intraspinal paths, as 

 well as those which pass from cord to brain, and vice versa, are 

 represented in Fig. 28. Therefore our " simple " reflex, or other 

 action, will (in the intact, normal animal) include all the paths, 

 or analogous ones, represented in Fig. 33, and also another 

 series of paths between the organs of special sense and the mid- 

 brain, and between the latter and the medullary and spinal 

 motor nuclei. Thus the main paths in use in the case of a man 

 walking in a crowded street and " mechanically " avoiding other 

 people must be somewhat as indicated in Fig. 35. 



Here we do not represent the afferent paths between the 

 acting muscles and the nuclei in the cord, and, of course, no 

 details of the very imperfectly known paths between the mid- 

 brain and spinal centres. 



The Mechanism of Co-ordination. 



Nor do we ever suggest the all-important apparatus of co- 

 ordination. In such a complex series of actions as those involved 

 in walking a very great number of muscular systems are at work. 

 Practically all the muscles of the legs are active and immediately 

 concerned in the production of the movements, but the body is 

 also carried erect and balanced, and this is the work of antag- 

 onistic muscle systems belonging to the trunk. The arms swing 

 about. At each step, and with every deviation from a straight 

 line, the centre of gravity of the whole body changes its position. 

 Practically every nucleus, or ganglion, in the whole spinal cord 

 must be concerned in the generation of the impulses going out 

 along the efferent nerves to the muscles, and in receiving the 

 afferent impulses generated in the acting muscles and joints, and 

 giving information from instant to instant as to what is going 

 on there. Now it is unlikely that the relatively simple reflex 

 mechanisms constituted by the sensory and other afferent nerves 

 entering into synapses with the motor nerves is competent for 

 this work of co-ordination, and there is much evidence that an 

 additional nervous apparatus is involved. 



This additional mechanism is the cerebellum and its connec- 

 tions. We have seen that there are tracts of nerve fibres passing 



