326 THE NERVOUS SYSTEM 



fifth of the basis pedunculi; and similar fibers from the temporal lobe descend 

 through the sublenticular part of the internal capsule and the lateral one-fifth 

 of the basis pedunculi. These fibers, together with the corticospinal tract, 

 form the longitudinal fasciculi of the pons; and, along with collaterals from that 

 tract, they end within the nuclei pontis in synaptic relations with the neurons 

 of the second order (Figs. 106, 242). 



Neuron II. Arising from cells in the nuclei pontis, the transverse fibers of 

 the pons cross the median plane and run by way of the brachium pontis and 

 white substance of the cerebellum to the cerebellar cortex of the opposite side. 



THE CEREBELLO-RUBRO-SPINAL PATH 



The cerebello-rubro-spinal path is the conduction system through which the 

 cerebellum contributes its important share to the control of the primary motor 

 neurons of the spinal cord in the interest of muscular coordination, equilibration, 

 and the maintenance of muscle tone. Other efferent connections of the cerebel- 

 lum have been discussed on page 211. 



Neuron I. From the Purkinje cells of the cerebellar cortex fibers run to 

 terminate in the central nuclei of the cerebellum, especially the dentate nucleus 

 (Fig. 242). 



Neuron II. Arising chiefly, if not entirely, from the cells of the dentate 

 nucleus, fibers run through the brachium conjunctivum, undergo decussation 

 in the tegmentum of the midbrain ventral to the inferior colliculi, and end in the 

 red nucleus and thalamus (Figs. 242, 243). 



Neuron III. From cells in the red nucleus arise the fibers of the rubrospinal 

 tract, which cross the median plane in the ventral tegmental decussation, and 

 descend through the reticular formation of the brain stem and the lateral funic- 

 ulus of the spinal cord. Here this tract occupies a position just ventral to the 

 lateral corticospinal tract, and its fibers end in the anterior gray column in 

 relation to the primary motor neurons. 



We have learned that the cerebellum is the chief center of the proprioceptive 

 system and is concerned with the maintenance of the proper tonicity of the 

 muscles, the coordination of their contractions, and especially with those re- 

 actions necessary to maintain or to re-establish that evenly balanced spacial 

 orientation known as equilibrium. The cerebello-rubro-spinal path is the con- 

 duction system primarily concerned in these reactions. 



What is perhaps the first direct experimental evidence of the function of this 

 system has been given by Weed (1914). The extensor rigidity, so characteristic 



