THE CENTRAL NERVOUS SYSTEM 773 



fillet. The lateral fillet contains mainly fibres arising in the 

 cochlear nucleus of the auditory nerve, and ends in grey matter 

 of the posterior 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 

 axoas, passing through the posterior limb of the internal capsule 



FIG. 324. DIAGRAMMATIC TRANSVERSE SECTION OF CRURA CEREBRI AND 

 AQUEDUCT OF SYLVIUS. 



a, anterior 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 temporo-occipital lobe ; 

 i, posterior longitudinal bundle. 



and the corona radiata, continue the afferent path to the cerebral 

 cortex. 



Besides the ascending fibres, the bundle anatomically de- 

 scribed as the tract of the fillet contains some descending fibres 

 which on section of the tract degenerate below the lesion. They 

 lie to the mesial side of the intermediate fillet, and since their 

 cells of origin seem to be in the thalamus and their course is 

 towards the bulb, they are spoken of as a thalamo-bulbar tract. 



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

 nuclei run in the fillet. Many of them occupy a position in the 

 reticular formation of the tegmentum dorsal to the fillet as they 

 pass through the pons and mid-brain to end in the thalamus and 

 the region below it (sub-thalamic region). From the sensory 



