162 THE HUMAN BODY 



and the vestibule of the ear. Its sensory nerve-trunk is the ves- 

 tibular portion of the auditory nerve. Probably the sense of sight 

 has communication also with the cerebellum through its connec- 

 tions with the midbrain nuclei. So far as we know the other 

 senses do not come into afferent connection with the cerebellum. 

 The only afferent tract of the cerebellum remaining to be men- 

 tioned is the one by which it may receive impulses from the 

 cerebrum. This consists of pyramidal fibers from the motor cor- 

 tex which end in the pons Varolii in synaptic connection with 

 neurons which pass from this structure to the cerebellum by way 

 of its middle peduncles. 



The efferent paths from the cerebellum consist at the beginning 

 of fibers which arise from large characteristic cell-bodies of the 

 cerebellar cortex, the so-called " cells of Purkinje." These fibers 

 terminate in the dentate nuclei in synaptic connection with neurons 

 which continue the path to the red nuclei of the midbrain. From 

 the red nuclei proceed efferent fibers which terminate in the 

 same manner as do the pyramidal fibers from the cerebrum, in 

 synaptic connection with the motor cells of the ventral horn. 

 The efferent tracts from the red nuclei are interesting as forming 

 practically the only means of communication between the brain 

 and the motor organs aside from the pyramidal tracts. Since the 

 latter are presumably wholly given over to voluntary activities 

 all our involuntary motions must receive their impulses through 

 the tracts from the red nuclei. These tracts extend along the 

 cord in the lateral columns on the ventral side of the crossed 

 pyramidal tracts. They are often called the pre-pyramidal tracts. 



We have in the cerebellum a complex organ receiving impulses 

 specially from the senses of touch, equilibrium, muscle sense, 

 and sight, in connection, on its efferent side with the motor or- 

 gans of the Body, and subject through its afferent tract from the 

 cerebrum, to voluntary control. The most marked result of ex- 

 tensive injury of the cerebellum is muscular incoordination; this 

 is especially marked in acts of locomotion, and similar acts re- 

 quiring delicate interaction of many muscles. The senses which 

 act most directly on the cerebellum are the very ones which have 

 most to do with such an act as walking, where the movements 

 must be adjusted according to the guidance of the pressure of the 

 feet upon the ground, the condition of equilibrium of the Body, 



