104 MESENCEPHALON. 



station in a special group of nerve-cells (upper nucleus ofthefilleC) at the level of the 

 inferior corpora quadrigemina. Some of the lateral fibres of this middle portion, 

 however, pass to the upper corpora quadrigemina (upper fillet), and even extend 

 beyond the superior quadrigeminal region to the subthalamic region (Flechsig), 

 eventually reaching the parieto-occipital part of the cerebral hemispheres by the 

 posterior part of the internal capsule (v. Gudden). Both the upper and lower fillet 

 receive fibres from the antero-lateral columns of the spinal cord, for after hemisection 

 of the cord degenerated fibres are seen on the same side, both in the lateral fillet of the 

 inesencephalon, where they are mingled with the mass of undegenerated fibres which 

 have been derived from the nucleus gracilis and nucleus cuneatus (see p. 53 and 

 below), and in the bundle of the upper fillet, which enters the superior corpora quadri- 

 gemina ; a few degenerated fibres are also seen in corresponding positions on the oppo- 

 site side. Traced downwards the fibres of the lower fillet pass, as we have seen, a few 

 by means of the trapezium towards the auditory nucleus of the opposite side, a few 

 to the antero-lateral column of the medulla oblongata and cord (antero-lateral 

 ascending tract), but most are traceable to the anterior column of the medulla 

 oblongata dorsal to the pyramids, and passing across the raphe, proceed as internal 

 arched fibres to the nuclei of the posterior columns, from the cells of which they in 

 all probability arise. 



To sum up : The fillet as a whole is composed of ascending fibres, most of which 

 are derived from the nuclei of the opposite posterior columns of the medulla 

 oblongata, but a few from other sources. The fibres of the antero-lateral ascending 

 tract which enter the fillet may, according to Edinger, be derived from cells of the 

 opposite posterior horn of the spinal cord ; if so, all the fillet-fibres may be regarded 

 as having crossed over from the opposite side, some in the cord, others at the 

 superior or sensory decussation in the medulla oblongata. The fact that they are 

 thus derived is however by no means certain. Superiorly the fillet may be described 

 as being prolonged upwards above the pons in three parts, of which one the lower 

 fillet goes to the inferior of the corpora quadrigemina of the same side, a second 

 upper fillet to the superior corpora quadrigemina and occipital region of the 

 cerebral hemisphere, and a third, mesial fillet, to the base of the cerebrum through 

 the crusta. 



DORSAL PAET OF THE MID-BRAIN: CORPORA QUADRIGEMINA. 



As before stated the Sylvian aqueduct is covered on its dorsal aspect by the 

 quadrigeminal lamina, bearing the bodies of the same name. The median part of 

 the lamina is marked by a comparatively wide groove (sulcus longitudinalis, s. 

 saffittalis), shallower inferiorly, which serves to separate the corpora quadrigemina 

 of opposite sides (fig. 77). This grooved surface, which is raised above the level of 

 the upper medullary velum, is connected with the velum by a small median strand 

 of longitudinal fibres termed the frcenulum veli (fig. 77, fr). In front of the upper 

 (anterior) pair of corpora quadrigemina the groove is interrupted by a transverse 

 white prominence the posterior commissure ; but both this and the upper end of 

 the median groove are in the natural condition concealed by the pineal body (p), 

 which projects backwards and downwards from the posterior wall of the third 

 ventricle and rests between the upper pair of quadrigeminal bodies. A well-marked 

 narrow transverse groove (sulcus transversus) which commences a short distance 

 from the middle line, and is curved round the lower border of the upper tubercle, 

 separates this from the lower tubercle of the same side. 



The corpora quadrigemina are two pairs of rounded eminences which are 

 mainly composed of grey matter, although covered externally by and containing in 

 their interior many white fibres. The upper or anterior tubercles (_fig. 77, c.q.s.} 



