CH. XLIV.] TRANSVERSE SECTIONS OF MID-BRAIN 637 



up, where the floor of the ventricle is again narrowing. At last, in 

 the region of the mid-brain, we once more get a canal (Sylvian 

 aqueduct) which corresponds to the central canal of the spinal cord. 



Fifth and Sixth sections are taken through the mid-brain, and 

 are drawn on a smaller scale than the others we have been examin- 

 ing ; they represent the actual size of the sections obtained from the 

 human subject. 



Near the middle is the Sylvian aqueduct, with its lining of ciliated 

 epithelium. In the grey matter which surrounds it are large nerve- 

 cells from which the fourth nerve, and higher up the third nerve, 

 originate ; the fibres of the third nerve are seen issuing from these in 

 fig. 465, B., III. The reticular formation of the pons is continued up 

 into the mid-brain, and is called the tegmentum. It is composed of 

 both longitudinal and transverse bundles of fibres intermingled with 

 grey matter. Its transverse fibres include those of the superior 

 peduncles of the cerebellum which decussate in the middle line (fig. 

 465, A., S.C.P.). These originate from the cells of the dentate nucleus 

 of the cerebellum; after decussation they bifurcate, the ascending 

 branches being lost in the collection of nerve-cells in the tegmentum 

 known as the tegmental or red nucleus, while the descending branches 

 turn downwards in the reticular formation. The axons from the cells 

 of the red nucleus run downwards and form Monakow's bundle, or 

 the prepyramidal tract which we have already seen in the spinal cord. 



Another important longitudinal bundle in the tegmentum is the 

 fillet. This, we have seen, is the longitudinal continuation of the 

 internal arcuate fibres, which, starting from the cells of the posterior 

 column nuclei of the opposite side, form the second relay on the 

 sensory path ; to these fibres others are added which originate from 

 other masses of grey matter in bulb and pons. In the mid-brain the 

 fillet splits into three bundles, termed the lateral, the upper, and the 

 mesial fillet. 



(1) The lateral fillet is chiefly formed by fibres derived from the accessory audi- 

 tory, the inferior olivary, and trapezoid nuclei of the opposite side. Some of its 

 fibres terminate by synapses around a new collection of cells (the lateral fillet 

 nucleus) ; their axons pass inwards towards the raphe. The rest of its fibres can be 

 traced to the grey matter of the inferior corpora quadrigemina. 



(2) The upper fillet consists of fibres which go to the superior corpora quadri- 

 gemina and partly to the tegmental region of the mid-brain and optic thalamus. 



(3) The mesial fillet goes on through the tegmentum of the crus cerebri, and its 

 fibres terminate around the cells of the optic thalamus, and the subthalamic region. 

 From here fresh axons forming a new relay continue the afferent impulses to the 

 cortex of the cerebrum. 



The mesial fillet is the important link in this region between 

 the sensory spinal nerves and the part of the brain which is the seat 

 of those processes we call sensations. But most of the fibres which 

 continue the sensory path of the cranial nerves form another less 

 well-defined tract (the central tract of the sensory cranial nerves) which 



