892 



THE NERVE SYSTEM 



The inferior peduncle (postpedunculus; corpus restiforme) is the continuation of 

 the restiform body of the medulla oblongata. It contains both afferent and 

 efferent fibres, connecting the cerebellar cortex with structures situated caudad : 

 (1) The dorsal or direct spinocerebellar tract, composed of axones arising in Clarke's 

 column (spinal cord) and terminating in the cortex of the superior vermis on both 

 sides of the median line, but chiefly on the opposite side. (2) The olivocerebellar 

 tract, composed of axones arising in the (inferior) olivary nuclei principally 

 from the contralateral or opposite olive, and terminating in the cortex of the 

 superior vermis and adjacent hemispheral portions, as well as in the dentate 

 nucleus. (3) External arcuate fibres from the homolateral and contralateral 

 nuclei of the gracile and cuneate funiculi. (4) The nucleocerebellar tract, composed 

 of axones from the recipient nuclei of certain cranial nerves (vestibular, trigeminal). 

 (5) The cerebellospinal (descending) tract, terminating in relation with the ventral 

 horn cells at various levels of the spinal cord. 



The middle peduncles (medipedunculi; brachia pontis) are the largest of the 

 three pairs. They consist of a mass of curved fibres comprising the pons and enter- 

 ing either cerebellar hemisphere between the parted lips of the ventral end of the 



peduncular fissure, just ectad of the 

 inferior peduncle. Each peduncle con- 

 tains axones coursing in opposite direc- 

 tions and in large part may be con- 

 sidered as purely commissure! fibres. 

 Some of the axones terminate, however, 

 in the nuclei pontis to convey impulses 

 to the cells therein; these in turn send 

 their axones (tractus pontocerebellures) 

 into the opposite middle peduncle, and 

 therefore constitute interrupted com- 

 missural systems. A few fibre systems 

 in the middle peduncles establish rela- 

 tions with certain other structures in 

 the brain stem, notably the nuclei of the 

 oculomotor, trochlear, and abducent 

 cranial nerves. 



The superior peduncles (prcpedunculi; 

 brachia conjunctiva) emerge cephalad- 

 from the cerebellum entad of the middle peduncles. As they extend cephalad 

 they converge to form the lateral boundaries of the fourth ventricle and partly 

 roof it in. On trans-section they appear of oval outline, somewhat concave 

 toward the cavity of the ventricle. The superior medullary velum or valvula, a 

 thin lamina of white substance, spans the interval between the converging superior 

 peduncles, and thus completes the roofing-in of the cephalic portion of the fourth 

 ventricle. 



The superior peduncles consist almost wholly of axones arising from the cells 

 of the dentate nucleus, the ectal part of the nucleus fastigii of the same side, and 

 niesal part of the nucleus fastigii of the opposite side. In their course, converging 

 cephalad, these bundles pass into the tegmentum of the mid-brain ventrad of 

 the inferior corpora quadrigemina, and decussate almost wholly. The fibres of 

 each peduncle terminate in the red nucleus of the opposite side, although a few 

 continue to enter the thalamus. 



This system of fibres is also called the tractus cerebellotegmentalis, and axones 

 of inverse functional direction have been included therein. 



The ventral spinocerebellar tract (Growers' tract) is in relation with the superior 

 peduncle and superior medullary velum. Unlike the dorsal or direct spinocere- 



FIG. 657. Semidiagram of the three cerebellar 

 peduncles. (Testut.) 



