242 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



upon the neuromuscular apparatus of our so-called voluntary 

 movements. The precise nature of the regulating influence is in 

 dispute, and one who reads the literature finds it difficult at times 

 to separate clearly the different theories proposed, since some 

 authors are content with general statements and others attempt 

 a more specific analysis. On the whole, it seems desirable at 

 present to hold to the general idea, introduced by Flourens, that 

 the cerebellum is a central organ for co-ordination of voluntary 

 movements, particularly the more complex movements necessary 

 in equilibrium and locomotion. Instead, however, of assuming 

 with Flourens that the cerebellum contains a co-ordinating principle, 

 an expression that means nothing at present, we may assume that 

 it exerts its co-ordinating influence by virtue of the definite nervous 

 mechanisms contained in it that is, by nervous complexes which, 

 on the afferent side, are connected with the peripheral sensory 

 nerves to the vestibule of the ear, the muscles, joints, etc., and on 

 the efferent side are in direct or indirect relations with the motor 

 centers of the cord. Co-ordinated movements requiring the com- 

 bined and sustained activity of a number of muscles depend in 

 some way upon a combination of the activity of these mechanisms 

 with the discharging mechanisms farther forward in the brain 

 (cerebrum). Whether this co-activity consists in the addition of 

 a tonic element to the impulses proceeding from the cerebrum, as 

 would be implied by the results of Luciani's experiments, or whether 

 the cerebellum participates, through some form of representation 

 of these movements,* based upon the afferent impulses received 

 through the paths already described, cannot be settled at present. 

 Luciani's conception has the recommendation of being based upon 

 a large amount of experimental work, and it may be included or 

 utilized in a general theory of a co-ordinating function of the cere- 

 bellum, if we assume that the effect of this organ on muscular ton- 

 icity is adaptive, that is to say, varies in a definite way in the 

 different muscles according to the character of the afferent im- 

 pulses received from the muscles, joints, labyrinth, etc. That an 

 adaptive tonicity of the muscles actually occurs is demonstrated 

 by experiments (see p. 410), and we can understand that a regu- 

 lated tonicity of this kind may constitute the foundation upon 

 which the normal co-ordination of the muscles is effected. The 

 fact that in birds as well as in higher forms the animal eventually 

 learns to co-ordinate such movements after the loss of the cerebel- 

 lum does not invalidate this conclusion. In the first place, the 

 recovery in such cases is not entirely complete, since some ataxia 

 is still manifested in vigorous or hurried movements, and the 

 amount of restoration of normal activity which is obtained may 

 *See Horsley, "Brain," 1906, 446. 



