936 



THE CENTRAL NERVOUS SYSTEM 



highway of the restiform body, or the cross-country routes from cere- 

 bral 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 the antero -lateral de- 

 scending 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 cognizance of the multitudes of afferent impressions 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. 



Sherrington points out that all the modern theories of cerebellar 

 function harmonize with his conception of the cerebellum as the head 

 ganglion of the proprio-ceptive system (p. 934). The most influential 

 of the proprio-ceptive organs being the labyrinth, the central organ of 

 the whole proprio-ceptive mechanism is built up over the central con- 

 nections of the labyrinth. Thither converge connecting (internuncial) 

 paths from the central endings of proprio-ceptive neurons in all seg- 

 ments of the body (from joints, muscles, tendons, ligaments, viscera, 

 etc.). Thus a central organ is developed, which varies in size and 

 complexity in 



different kinds of ,. ...,-,. -^ 



animals accord- -J*W 



ing to the com- 

 plexity of their 

 habitual move- 

 ments. 



Fig- 375- The Semicircular Ca-ials (Diagrammatic) (after 

 Ewald). H, horizontal or external; S, superior; P, pos- 

 terior. The two horizontal canals lie in the same plane. 

 The plane of the superior vertical canal of one side is 

 parallel to the plane of the posterior vertical canal of the 

 opposite side. 



Afferent Im- 

 pulses concerned 

 in Equilibration 

 and Orientation. 

 This is a con- 

 venient place to 

 consider a little 

 more in detail 

 the nature and 



peripheral sources of some of the most important afferent impressions 

 concerned in equilibration and orientation. 



(i) Afferent Impulses from the Semicircular Canals. The semi- 

 circular canals are three in number, and lie nearly in three mutually 

 rectangular planes: the external canal in the horizontal plane, the 

 superior canal in a vertical longitudinal plane, and the posterior canal 

 in a vertical transverse plane. Each canal bulges out at one end into 

 a swelling, or ampulla, which opens into the utricular division of the 

 vestibule (Figs, 375, 455). The other extremities of the superior and 

 posterior canals join together, and have a common aperture into the 

 utricle, but the undilated end of the external or horizontal canal opens 

 separately. The utricle and the semicircular canals are thus connected 

 by five distinct orifices. The greater part of the internal surface of 

 the membranous canals, utricle and saccule, is lined by a single layer 

 of flattened epithelium. But at one part of each ampulla projects a 

 transverse ridge, the crista acustica, covered not with squamous, but 

 with long columnar epithelium. Hair-like processes (auditory hairs) 



