636 STRUCTURE OF THE BULB, PONS, & MID-BRAIN. [CH. XLIV. 



ambiguus, also seen in this section, have been already alluded to 

 in our account of the origin of the cranial nerves (p. 628). 



Fourth section. This is taken through the middle of the 

 pons, and shows much the same kind of arrangement as in the 

 upper part of the bulb. The general appearance of the section 

 is, however, modified by a number of transversely coursing 

 bundles of fibres, most of which are passing from the cerebellar 

 hemispheres to the raphe, and form the middle cerebellar 

 peduncles. Intermingled with these is a considerable amount of 

 grey matter (nuclei pontis). The continuation upwards of the pyra- 

 mids (fig. 469, py) is embedded between these transverse bundles, 

 and separated by them from the reticular formation ; the deeper 

 transverse fibres, which form the trapezium (t), connect the supe- 

 rior olivary nucleus of one side to the accessory auditory nucleus 

 of the other side. The large olivary nucleus is no longer seen, 

 but one or two small collections of grey matter (o.s.) represent it, 

 and constitute the superior olivary nucleus. The nerves taking 

 origin in this region of the floor of the fourth ventricle are the 

 sixth, seventh, and eighth. The fifth nerve originates higher 

 up, where the floor of the fourth ventricle is narrowing, till at 

 last, in the region of the mid-brain, we once more get a canal 

 (Sylvia.n aqueduct) corresponding to the central canal of the 

 spinal cord. 



The reticular formation between the grey floor of the ventricle 

 and the trapezium is a continuation upwards of the reticular 

 formation seen in previous sections. It consists of white fibres 

 passing transversely in different directions, between which there 

 are fibres running longitudinally, and a considerable amount of 

 grey matter. In such a complex system of intercrossing fibres, it 

 is extremely difficult to separate any definite tracts, but there are 

 at least two longitudinal tracts of fibres in it which a little higher 

 up in the mid-brain are separated off from the rest ; one of these 

 is the fillet, the origin of which in the nuclei gracilis and cuneatus 

 of the opposite side we have already seen ; the other is the 

 posterior longitudinal bundle, which is stated by some to be a 

 continuation upwards of some of the fibres of the anterior column 

 of the cord ; it certainly contains fibres connecting the nuclei of 

 the third and sixth nerves of opposite sides. These are shown 

 in the 



Fifth and Sixth sections, which are taken through the 

 mid-brain, and are drawn on a smaller scale than the others we 

 have been examining ; they represent the actual size of the sec- 

 tions obtained from the human subject. 



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



