CH. XL1V.] 



THE CRUS. 



637 



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. 470, B III. The reticular formation of 

 the pons is continued up into the mid-brain, and is called the 

 tegmentum. Its transverse fibres include the decussating fibres of 

 the superior peduncles of the cerebellum. The fibres of the fillet 

 partly pass in an oblique manner to the side of the mid-brain, 

 and terminate in the grey matter of the corpora quadrigemina 

 (C Q) ; this is called the lateral fillet ; the rest of the fillet 

 (mesial fillet] goes on through the crus, and has been traced 



c.o. 



Fig. 470. 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 ; 8.N., substantia nigra shown only on one side ; T, tegmentum ; 

 8, Sylvian aqueduct, with its surrounding grey matter; L. O., lateral groove; 

 p./., posterior longitudinal bundle; d. V, descending root of the fifth nerve; 8.C.P., 

 superior cerebellar peduncle; F, fillet; in., 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 (P>/, are in the middle, and 

 the fronto-cerebellar (F.C.) and temporo-occipital cerebellar (T.O.C.) at the sides. 

 (After Schafer.) 



into the optic thalamus ; from here fresh nerve-fibres, forming 

 a new relay, continue the afferent impulses to the cortex of the 

 cerebrum. 



The pyramidal bundles of the pons are continued upwards, and 

 form the middle third of the crusta (er) or pes. The crusta and 

 tegmentum are separated by a layer of grey matter called the 

 substantia nigra (S N). There is also grey matter in the teg- 

 mentum itself, which is called the tegmental or red nucleus. 



The corpora quadrigemina are formed mainly of grey matter ; 

 from each superior corpus a bundle of white fibres passes upwards 

 and forwards to the geniculate bodies, eventually joining the 

 optic tract of the same side. The white layer on the surface of 

 the grey matter of the C. quadrigemina is derived from the optic 

 tract ; these fibres come from the retina, and terminate by arbo- 

 rising around the cells of the grey matter of the C. quadrigemina. 



