548 THE NEKVOUS SYSTEM. 



information regarding the constitution of the restiform body. Thus, the posterior 

 spino-cerebellar tract (Fig. 481), from the lateral column of the spinal medulla, 

 can be traced into it ; and large numbers of fibres which take a curved course on 

 the surface of the medulla oblongata may likewise be followed into it ; these are 

 the external arcuate fibres. Numerous other fibres enter the restiform body on its 

 deep aspect, but these will be studied at a later stage. 



Fibrae Arcuatse Externae. The external arcuate fibres enter into the constitu- 

 tion of the restiform body, after pursuing a longer or shorter course on the 

 surface of the medulla (Fig. 481). 



They are more particularly seen in the neighbourhood of the olive, over the 

 surface of which they may be observed coursing in the form of a number of fine 

 curved bundles or as a continuous sheet of fibres. They vary greatly in number 

 and in distinctness, and they are sometimes so numerous that they cover the 

 olive almost entirely. An attentive examination will show that the majority of 

 them come to the surface in the median fissure between the pyramids, and also. 

 not infrequently, in the groove between the pyramid and olive, or through the 

 substance of the pyramid itself. The anterior median fissure in its upper part is 

 often almost completely blocked up by these emerging fibres. The external arcuate 

 fibres, reaching the surface of the medulla in this manner, turn backwards, and 

 the great majority enter the restiform body and form a considerable part of its' 

 outer portion. 



Other arcuate fibres arise in the cuneate and gracile nuclei, and enter the 

 restiform body of the same side. Van Gehuchten, however, denies this. 



There is frequently present, especially upon the left side, a bundle of fibres 

 that is usually mistaken for a group of arcuate fibres. It is the fasciculus circmn 

 olivaris pyramidis (Fig. 481). It consists of a bundle of varying size which emerge; 

 from the pyramid, bends backwards, curving round the inferior border of the olive 

 and then passes obliquely upward and backwards to end in a fusiform ridge of gra] 

 matter, the corpus ponto-bulbare (Essick), which crosses the restiform body ver 

 obliquely (Fig. 481, the ridge immediately posterior to the fila of the vagus nerve^ 



These structures are of great morphological interest, and will be referred I 

 again in the succeeding pages (see Fig. 499, p. 566). 



PONS. 



The pons (O.T. pons Varolii) is a marked white prominence on the basal aspec 

 of the brain which is interposed between the medulla oblongata and the peduncu 

 cerebri, and lies in front of the cerebellum. It is convex from side to side, and froi 

 above downwards, and transverse streaks on -its surface show that, superficially e 

 least, it is composed of bundles of nerve-fibres, most of which 1 course transversel; 

 On each side these transverse fibres are collected together in the form of a lar 

 compact strand, which sinks in a backward and lateral direction into the whil 

 matter of the corresponding hemisphere of the cerebellum. This strand is terme 

 the brachium pontis, and the term " pons," applied to the entire structure, express* 

 in an admirable way the arch-like manner in which this portion of the brai 

 bridges across the interval between the two cerebellar hemispheres. 



The ventral surface of the pons is in relation to the basilar part of tl 

 occipital bone and the dorsum sellse of the sphenoid bone. It presents a medie 

 groove (sulcus basilaris), which gradually widens as it is traced upwards: 

 lodges the basilar artery. This median depression is produced by the prominen' 

 which is caused on each side by the passage of the cerebro- spinal fasciculus dow: 

 wards through the pons. The trigeminal nerve, with its large entering sensory ro 

 and its small emerging motor root, is attached to the side of the anterior aspect of t 

 pons, nearer its superior than its inferior border (Fig. 481). It is usual to restri 

 the term " pons " to that portion of the structure which lies between the two t 

 geminal nerves, and to apply the designation of brachium pontis to the part whi 

 extends beyond the nerve into the hemisphere of the cerebellum. The abduce 

 nerve, the facial nerve, and the acoustic nerve are attached to the brain at the infer: 

 border of the pons. The abducens nerve emerges at the inferior border of the pc 



