THE CENTRAL NERVOUS SYSTEM 835 



affected (James), and the proportion seems to be much higher among 

 congenital deaf-mutes. Kreidl and Bruck, too, have found that 

 abnormalities of locomotion and equilibration are much more 

 common in deaf-and-dumb children than in others. Now, in these 

 cases the defect is usually in the internal ear. We must conclude, 

 then, that the co-ordination of muscular movements necessary for 

 equilibrium is achieved in some centre, to which afferent impulses 

 pass from the internal ear by the vestibular branch of the auditory 

 nerve, and from which efferent impulses pass out to the muscles. If, a's 

 there is strong reason to believe, this centre is situated in the cerebellum, 

 the efferent path is, as already suggested (p. 832), partly an indirect 

 one (perhaps by commissural fibres to the Rolandic area, and then 

 out along the pyramidal tract), or more probably to lower centres, 

 perhaps in the posterior portion of the optic thalamus, which control 

 such massive co-ordinated movements as those concerned in walking 

 and the maintenance of the normal attitude, and thence out along 

 certain tracts that connect the thalamus to the spinal cord (p. 781). 



Ewald has made an observation which illustrates the peculiar 

 relation of the semicircular canals to the muscular system namely, 

 that the labyrinth (in rabbits) influences the course of rigor mortis 

 in the striped muscles. Rigor does not come on so soon on the side 

 from which the labyrinth has been removed (p. 675). He attributes 

 to the labyrinth, as one of its functions, the maintenance of a certain 

 tonus in the entire skeletal musculature. 



(2) 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 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 arc not associated with a distinct im- 

 pression in consciousness, but which 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. 811), and disorders 

 of equilibrium are the result. 



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

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

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

 equilibrium. When the soles of the feet are rendered insensitive by 

 local anaesthesia or by cold, and the person is directed to close his eyes, 

 he staggers and sways from side to side. The disturbance of equili- 

 brium in locomotor ataxia must be partly attributed to the loss of 

 these tactile sensations, for numbness of the feet is a frequent 

 symptom, and the patient asserts that he does not feel the ground. 

 An interesting illustration of the importance of afferent impulses 

 from the skin in the maintenance of equilibrium is afforded by the 

 behaviour of a frog deprived of its cerebral hemispheres. Such a 

 frog will balance itself on the edge of a board like a normal animal, 

 but if the skin be removed from the hind-legs, it will fall like a log. 



In birds and lower vertebrates the cerebellum is only represented 

 by the worm. Yet in many of these animals the same characteristic 



532. 



