1436 A MANUAL OF ANATOMY 



wards, and then occupies the dorso-lateral part of the tegmentum, 

 lying almost beneath the internal geniculate body. 



The fibres of the main or mesial fillet (chief sensory tract) termi- 

 nate in two ways, (i) Some enter the upper quadrigeminal body, 

 and these probably terminate in the stratum lemnisci. (2) Others 

 traverse the subthalamic tegmental region and enter the anterior 

 part of the optic thalamus, within which they terminate in arbori- 

 sations around the cells of the anterior nuclear area (chief sensory 

 nucleus). From these cells relays of thalamo-corticipetal fibres 

 proceed to the cerebral cortex. 



It is convenient to refer to the main or mesial fillet as the sensory 

 fillet. 



Lateral Fillet. — ^The main or mesial fillet being the sensory fillet, 

 it is convenient to refer to the lateral fillet as the acoustic fillet. 

 The fibres of this fillet are derived from the following sources: 

 (i) The corpus trapezoides, the fibres of which come from the 

 ventral cochlear nucleus, the nucleus trapezoides, and the superior 

 olive of the opposite side, as well as from the superior olive of the 

 same side; (2) the striae acusticse, which are derived from the lateral 

 cochlear nucleus of the opposite side; and (3) the nucleus of the 

 lateral fillet. 



The fibres of the right and left lateral fillets decussate across the 

 median plane. Having crossed to the opposite side, the fibres 

 become longitudinal and form a well-marked ascending tract in 

 the dorsal part of the pons Varolii, which takes up a position on the 

 lateral or outer side of the main or mesial fillet. In this part of 

 its course the lateral fillet encounters a collection of grey matter, 

 called the nucleus of the lateral fillet. Some of its fibres end in this 

 nucleus. Others pursue their upward course, and are reinforced 

 by relays of fibres which arise from the nerve-cells of the nucleus. 

 On leaving the pons Varolii the lateral fillet enters the tegmentum, 

 and its fibres terminate in (i) the nuclei of the lower quadrigeminal 

 body, and (2) the cells of the internal geniculate body. The fibres 

 destined for the lower quadrigeminal body, having curved round 

 the outer aspect of the superior cerebellar peduncle, become super- 

 ficial on the outer side of the tegmentum. The fibres destined for 

 the internal geniculate body reach it through the inferior brachium, 

 partly directly and partly through the intervention of the lower 

 quadrigeminal body. The axons of the cells of the internal genicu- 

 late body form corticipetal fibres which pass to the cortex of the 

 first or superior temporal gyrus of the temporal lobe. 



The lateral fillet, as previously stated, is associated with the 

 auditory apparatus. It is chiefly composed of ascending fibres. 

 There are, however, some descending fibres which are probably 

 derived from the lower quadrigeminal bodies. 



In addition to the foregoing tracts there other tracts. 



Fasciculus Retroflexus. — The fibres of this bundle, already described, arise 

 from the cells of the ganglion habenulcB. They descend in the upper part of 

 the tegmentum, internal to the red nucleus, and they terminate in arboriza- 

 tions around the cells of the interpeduncular gangUon. 



