846 



THE CENTRAL NERVOUS SYSTEM 



lateral nucleus, a collection of grey matter in the lateral portion of the 

 spinal bulb. But its main strand runs on unbroken through the 

 medulla, in front of the restiform body, and behind the olive, and after 

 reaching the upper part of the pons bends back over and in company 

 with the superior peduncle as the ventral spino-cerebellar bundle, to 

 end in the worm of the cerebellum (Fig. 353). 



A few fibres of Cowers' tract may pass by the middle peduncle to the 

 opposite cerebellar hemisphere. Some of its fibres do not go to the 

 cerebellum at all. One group can be followed to the corpora quadri- 

 gemina (spino-tectal fibres], and another by way of the tegmentum of the 

 crus cerebri to the optic thalamus (spino-thalamic fibres) . 



Through the relay of the gracile and cuneate nuclei, the postero- 

 internal and postero-external columns of the cord are further con- 

 nected on the one hand with the cerebrum, and on the other with the 

 cerebellum. The cells of the nuclei give off fibres (internal arcuate 

 fibres) which, sweeping in wide arches across the mesial raphe to the 



opposite side, take up 

 a position behind the 

 pyramid in the tract of 

 the fillet, a bundle of 

 fibres which becomes 

 more compact, and 

 therefore more distinct, 

 as it passes brainwards. 

 Receiving fibres from 

 other sources on its 

 way, and also giving 

 off fibres, it runs up- 

 wards through the dor- 

 sal or tegmental portion 

 of the pons. In the mid- 

 brain it divides into 

 two portions, the lateral 

 fillet, also called the 

 lower fillet or fillet of Reil, and the intermediate, also called the upper 

 fillet. The lateral fillet contains mainly fibres arising in the cochlear 

 nucleus of the auditory never, and ends in grey matter of the pos- 

 terior corpus quadrigeminum, and partly in the mesial geniculate 

 body. It appears to be a path for the conduction of auditory 

 impulses. The intermediate fillet contains chiefly the fibres that 

 come off from the gracile and cuneate nuclei, but is enlarged by the 

 accession of fibres from the sensory nuclei of the cranial nerves. It 

 terminates in the lateral nucleus of the optic thalamus by forming 

 synapses with nerve-cells, whose axons, passing through the posterior 

 limb of the internal capsule and the corona radiata, continue the 

 afferent path to the cerebral cortex. 

 Not all of the axons from the cells of the cranial sensory nuclei run 



Fig. 342. Diagrammatic Transverse Section of 

 Crura Cerebri and Aqueduct of Sylvius, a, an- 

 terior corpora quadrigemina b, aqueduct; c, red 

 nucleus; d, fillet; e, substantia nigra;/, pyramidal 

 tract in the crusta of the crura cerebri; g, fibres 

 from frontal lobe of cerebrum; h, fibres from tern- 

 poro-occipital lobe ; i, posterior longitudinal bundle. 



