544 



THE BRAIN 



The nerve fibres which traverse the formatio reticularis run both in the 

 transverse and in the longitudinal direction. The transverse fibres are the 

 internal arcuate fibres. The longitudinal fibres are derived from different 

 sources in the two fields. In the lateral part of the formation they represent 

 the fibres of the lateral funiculus (after the removal of the cerebello-spinal 

 and the lateral cerebro- spinal fasciculi), which are continued up under cover 

 of the olive. In the medial part, or formatio alba, two longitudinal strands 

 take origin, viz., the lemniscus and the median longitudinal fasciculus. 

 Both lie close to the raphe. The lemniscus is placed immediately posterior 

 to the pyramid, and is formed by the internal arcuate fibres after their 

 decussation. The median longitudinal fasciculus takes form in the upper 

 part of the medulla oblongata, immediately subjacent to the grey matter 



Anterior medullary velum 



Ventricle iv. . 

 Mesencephalic root of fifth nerve 



Medial longitudinal 

 bundle 



Formatio reticularis 



Nucleus of 

 lateral lemniscus - 



Trigeminal 

 nerve 



Grey matter on floor 

 , of ventricle iv. 

 Brachium 



' conjunctivum 



Lateral lemniscus 



Commencing decus- 

 sation of brachium 

 conjunctivum 



Medial lemniscus 



Transverse 

 V fibres of 



Pyramidal bundles (cerebro-spim 

 fasciculi) 



FIG. 231. Transverse section through the upper part of the Pons 

 of the Orang. 



of the floor of the fourth ventricle. It is formed by longitudinal fibres of 

 the formatio alba which come from the fasciculus anterior proprius of the 

 spinal medulla. 



Internal Structure of the Pons. When transverse sections are made 

 through the pons, it is seen to consist of two well-defined parts, viz., a 

 ventral and a dorsal. Broadly speaking, the ventral part, pars basalts, 

 corresponds to the pyramidal parts of the medulla, and the basal parts of 

 the pedunculi cerebri ; whilst the dorsal part corresponds to the formatio 

 reticularis of the medulla oblongata and the tegmental parts of the pedunculi 

 cerebri. 



The basal part of the pons is the larger of the two subdivisions. It 

 is composed of a large number of transverse bundles of fibres, through 

 the midst of which coarse longitudinal bundles of fibres proceed down- 

 wards from the bases of the pedunculi cerebri to form, in the medulla 

 oblongata, the two pyramids. Scattered amongst these transverse and 

 longitudinal bundles of fibres, and filling up the interstices between them, 



