THE CEREBELLUM. 517 



muscles connected with it. That this position may be maintained all 

 the different groups of antagonistic but cooperative muscles must be 

 accurately coordinated in their actions. Any failure in this respect is at 

 once attended by a disturbance of the equilibrium and displacement. 



In progression, walking, running, dancing, etc., the body is trans- 

 lated from point to point by the alternate action of the legs. Whether 

 the direction of the translation be linear or curvilinear, as the legs 

 change their position from moment to moment, the center of gravity 

 also changes, and at once the equilibrium is menaced. If it is to be 

 maintained and displacement prevented there must be a prompt 

 readjustment in the relation of all parts of the body so that the line 

 of gravity falls again within the base of support. The more com- 

 plicated the movements of progression, or the narrower the base of 

 support, the greater is the danger to the equilibrium, and hence the 

 necessity for rapid and compensatory changes in coordinated muscle 

 activity. All movements of this character, in man at least, are pri- 

 marily volitional and require for their performance the constant exercise 

 of the attention. With frequent repetition they gradually come to 

 be performed independently of consciousness and fall into the cate- 

 gory of secondary or acquired reflexes. 



Though coordinating power is exhibited by the spinal cord, 

 medulla, and basal ganglia, it is only in the cerebellum that this 

 power attains its highest development and differentiation. To it 

 is assigned the power of selecting and grouping muscles, not in any 

 restricted part, but in all parts of the body, and coordinating their 

 actions in such a manner as to preserve the equilibrium. 



The Results of Experimental Lesions. If the cerebellum in 

 its totality, coordinates and harmonizes the action of the muscles 

 on the opposite sides of the body, any derangement of its structure or 

 its connections with the cord, medulla, pons, or basal ganglia should 

 at once be followed by incoordination of muscles and a want of har- 

 mony in their action. Experimental lesions of the cerebellum are 

 attended by such results. The phenomena observed are many and 

 complex. They differ in extent and character in different animals 

 and in accordance with the extent and location of the lesion, though 

 the note of incoordination runs through them all. 



Removal of one lateral half of the cerebellum in the dog is followed 

 by an inability to maintain the equilibrium necessary to the erect 

 position. On attempting to stand, the animal at once falls toward 

 the side of the lesion, the muscles of which at the same time contract 

 and give to the body a distinctly curved condition (Fig. 232). The 

 anterior limbs are extended to the opposite side. On making efforts 

 to regain the standing position, the animal may roll over around the 

 long axis of its body. Conjugate deviation of the eyes is frequently 

 observed as well as nystagmus. 



