THE CEREBELLUM. 



1095 



lished being best marked opposite the superior colliculi. Above this clecnssation, which, how- 

 ever, does not involve all of its fibres, since some ascend on the same side, the cerebello-tegmental 

 tract is in large measure interrupted in the red nucleus (nucleus tegmenti ruber), that lies within 

 the upper part of the tegmental area of the cerebral crus (page 1114). The fibres not ending 

 around the cells of this nucleus are continued through the subthalamic region into the thalamus, 

 in relation to the cells of which they terminate. 



Of those ending within the red nucleus, the majority transfer their impulses to fibres that 

 arise from the rubral neurones and thence proceed to the thalamus in company with the unin- 

 terrupted fibres. From the 



thalamus the impulses are p IG 6 



carried by the thalamo-cortical 

 paths (page 1122) to the cere- 

 bral cortex, the cells of which 

 are thus influenced by the 

 coordinating reflexes of the 

 cerebellum. 



A considerable part of 

 the impulses conveyed to the 

 red nucleus is diverted by the 

 axones of some of its neurones 

 into an entirely different path, 

 namely, the rubro-spinal tract, 

 which decussates and carries 

 impulses from the cerebellum 

 through the brain-stem and 

 antero-lateral column of the 

 cord to the anterior root-cells 

 of the spinal nerves. 



From the foregoing 

 descriptions it is evident that 

 by means of its peduncles the 

 cerebellum receives no small 

 part of the sensory impulses 

 collected by the spinal and 

 cranial nerves and, in turn, 

 issues the impulses necessary 

 to maintain coordination and 

 equilibrium. Such impulses 

 may be entirely reflex, as in 

 the case of movements per- 

 formed automatically, in which 

 instance the circuit is (a) 

 from the spinal cord and the 

 medulla, directly or indirectly, 

 to the cerebellum chiefly by 

 way of the tracts within the 

 inferior cerebellar peduncles ; 

 (b) from the cerebellum to the 

 motor root-cells within the 

 brain-stem and the cord by 

 way of the cerebello-vestibulo- 

 spinal tract and the cerebello- 

 rubro-spinal tract. 



When the necessity 

 arises for voluntary efforts 

 in maintaining equilibrium, 

 the circuit includes impulses 

 from the cerebral cortex, in 

 which case the cerebello- 

 rubro - thalamo - cortical tract 



Pos 



Diagram illustrating chief components of cerebellar peduncles ; fibres 

 passing by inferior peduncle (IP) are red ; those by superior peduncle (SP) 

 are blue; those by middle peduncle (MP) are black; C, cerebrum; T, 

 thalamus; 1C, internal capsule ; R, red nucleus; Cb, cerebellum ; d. dentate 

 nucleus; p, pontine nucleus; v, 1, o, vestibular, lateral, and inferior olivary 

 nuclei; s, reception nuclei of sensory nerves; Sg spinal ganglion; i. 2, 

 cerebello-rubral fibres, one of which (4) is continued downward as rubro- 

 spinal tract; 3, cerebello-thalamic ; 5, rubro-thalamic ; 6, thalamo-cortical; 

 7, fronto-pontine ; 8, temporo-occipito-pontine ; 9, 10, ponto-cerebellar fibres. 



and the cortico-spinal tract form the most direct path. As accessory to this an indirect path, 

 impulses by way of the cortico-ponto-cerebellar and the cerebello-rubro-spinal tracts, may be 

 assumed as probably taking part in securing the necessary motor balance. 



