9io THE CENTRAL NERVOUS SYSTEM 



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

 bellum, the efferent path is, as already suggested (p. 906), 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 con- 

 cerned in walking and the maintenance of the normal attitude, and 

 thence out along certain tracts that connect the thalamus to the 

 spinal cord (p. 858). 



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. 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-pres- 

 sure ; (c) impulses which are not associated with a distinct impres- 

 sion 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. 887), 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 

 equilibrium in locomotor ataxia must be partly attributed to the 

 loss of these tactile sensation's, 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;' 



