THE CENTRAL NERVOUS SYSTEM 731 



opposite cerebral hemisphere, and that a lesion in one half 

 of the cerebellum affects chiefly the co-ordination of the 

 movements of the same side of the body, that is to say, of 

 the side connected with the opposite cerebral hemisphere. 



We do not as yet know the full significance of this extraordinarily 

 free communication of the grey matter of the cerebellum with every 

 part of the central nervous system. But it is evident that by the 

 broad highway of the restiform body, or the cross-country routes 

 from cerebral cortex to cerebellum, impulses may reach it from 

 every quarter ; while impulses passing out from it along its peduncles 

 may influence the motor discharge either indirectly through the 

 Rolandic cortex and the pyramidal tract, or more directly through a 

 descending spinal path that brings it 

 into relation with the nuclei of origin 

 of the motor nerves. It is an organ 

 so connected that is suited to take cog- 

 nizance of the multitudes of afferent im- 

 pressions concerned in the co-ordination 

 of movements and the maintenance of 

 equilibrium, and to regulate the outflow 

 of efferent impulses in correspondence 

 with the inflow of afferent. This is a 

 convenient place to consider a little 

 more in detail the nature and peri- 

 pheral sources of the most important 

 of these afferent impressions. 



(1) Afferent Impressions from the 

 Muscles. Muscles are richly supplied 

 with afferent fibres, for about half of 

 the fibres in the nerves of skeletal 

 muscles degenerate after section of 

 the posterior roots beyond the ganglia 



(Sherrington). Various kinds of impressions may pass up these 

 muscular nerves : (a) Impressions giving rise to pain, as in 

 muscular cramp and in experimental excitation of even the finest 

 muscular nerve-filament ; (b) impulses causing a rise of blood- 

 pressure; (c) impulses which are not associated with a distinct 

 impression in consciousness, but enable us to localize the position 

 of the limbs, head, eyes, and other parts of the body ; (d) impulses 

 which inform us as to the extent and force of muscular contraction, 

 and seem to underlie the so-called muscular sense. It is the last 

 two kinds if, indeed, they are distinct which must be concerned 

 in equilibration. In locomotor ataxia such impressions are blocked 

 by degeneration in a part of the afferent path (p. 703), and disorders 

 of equilibrium are the result. 



(2) Afferent Impressions from the Skin. Of the various kinds of 

 nerve-impulses that arise in the nerve-endings of the skin, only those 

 of touch and pressure seem to be concerned in the maintenance of 



FIG. 263. THE SEMICIRCULAR 

 CANALS (DIAGRAMMATIC). 



H, horizontal or external ; 

 superior ; P, posterior. 



S, 



