672 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLVI 



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 



C.Q. / c.o. 



FIG. 414. Outline of two sections across the mid-brain : A, through the middle of the inferior ; B, 

 through the middle of the superior corpora quadrigemina, C.Q. Cr., crusta; S.N., substantia nigra 

 shown only on one side ; T, tegmentum ; S, Sylvian aqueduct, with its surrounding grey matter ; 

 L.G., lateral groove; p.l., posterior longitudinal bundle ; d. V., descending root of the fifth nerve ; 

 S.C.P., superior cerebellar peduncle ; F, fillet ; III., third nerve. The dotted circle in B represents 

 the situation of the tegmental nucleus. In B the three divisions of the crusta are indicated on one 

 side. The pyramidal fibres (Pi/) are in the middle, and the fronto-cerebellar (F.C.) and temporo- 

 occipital cerebellar (T.O.C.) at the sides. (After Schafer.) 



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 

 lies dorsal to the fillet, but terminates like it in the subthalamic 

 region and optic thalamus, whence a new relay carries on the 

 impulses to the cortex. 



Ventral to the tegmentum is a layer of grey matter, of which 

 the cells are deeply pigmented ; hence it is called the substantia 

 nigra (S.N.). This receives many collaterals from the pyramidal 

 bundles. 



The white matter on the ventral side of this is known as the 



