CEREBELLUM, PONS, AND MEDULLA. 239 



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 

 in the cerebellum 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 imphed by the results of Luciani's experi- 

 ments, 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 cerebellum, if we assume that the effect of this organ 

 on muscular tonicity is adaptive, that is to say, varies in a definite 

 way in the different muscles according to the character of the af- 

 ferent impulses received from the muscles, joints, labyrinth, etc. 

 That an adaptive tonicity of the muscles actually occurs is demon- 

 strated by experiments (see p. 409), and we can understand that a 

 regulated 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 invahdate this conclusion. In the first place, the 

 recovery in such cases is not entire!}' complete, since some ataxia 

 is still manifested in vigorous or hurried movements, and, in the 

 second place, the amount of restoration of normal activity which is 

 obtained may be referred to an adaptation or training in the 

 cerebral portion of the mechanism. The relative parts taken by the 

 cerebellum and the cerebrum in such movements vary probably in 

 different animals and in different movements in the same animal. 

 Removal of the cerebrum from a pigeon leaves an animal with 

 almost perfect power of controlling its equilibrium. In the dog a 

 similar operation is followed by a longer period of inability to con- 

 trol perfectly the movements of locomotion, and it is probable 

 that in man after such an operation the power of locomotion would 

 be acquired more slowly, if at all. On the other hand, the violent 

 effect upon such movements caused by the removal of the cerebel- 

 lum in the pigeon is less evident in the dog, and, if we may judge 

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



