DISSECTION OF THE BRAIN, OR ENCEPHALON. 261 



valve, the nerve winds round the crus cerebri, and appears in front of 

 the pons. It leaves the cranium by the minute pathetic foramen, 

 and reaches the back of the orbit. It is the smallest of the cranial 

 nerves. 



The Trifacial, Trigeminal, or 5th nerve, springs out of the side of 

 the pons by two roots. The outer and larger of these is termed the 

 sensory root ; and its fibres are traceable to cells of the grey matter of 

 the medulla, pons, and locus caeruleus, and possibly also to the 

 cerebellum. This root near its origin expands into a large ganglion 

 — the Gasserian ganglion, beyond which it divides into three branches, 

 viz., the ophthalmic, superior maxillary, and inferior maxillary divi- 

 sions. The inner or motor root of the 5th nerve is traceable to grey 

 matter of the pons. It joins the inferior maxillary division of the 

 sensory root. The superior maxillary division leaves the cranium by 

 the foramen rotundum, the ophthalmic division by the foramen lacerum 

 orbitale, and the inferior maxillary division by the forepart of the 

 foramen lacerum basis cranii. The trifacial is the largest of the 

 cranial nerves. 



The Abducent or 6th nerve. This nerve springs from the anterior 

 part of the medulla, in line with the faint groove that limits outwardly 

 the inferior pyramid. Some of its fibres issue from the groove beneath 

 the pons and the medulla, while others penetrate the trapezium. The 

 fibres of the nerve are traceable to a group of nerve cells in the medulla. 

 The nerve reaches the orbit by passing through the foramen lacerum 

 orbitale. 



The Facial or 7th nerve springs out of the medulla, close behind 

 the pons, its fibres seeming to continue outwards the trapezium. Its 

 rootlets are traceable to nuclei of grey matter in the medulla. The 

 nerve is joined by a delicate filament — the portio intermedia — which 

 appears between the roots of this and the next nerve. The 7th nerve 

 enters the internal auditory meatus in company with the 8th nerve. 

 Separating from that nerve, it passes along a canal in the petrous 

 temporal bone — the aqueduct of Fallopius — from which it emerges 

 by the stylo-mastoid foramen, under the parotid gland. Within the 

 aqueduct of Fallopius the nerve forms a knee-shaped bend, and at 

 that point it shows a minute ganglion — the geniculate ganglion — from 

 which proceed the great and small superficial petrosal nerves (pages 

 194 and 220). 



The Auditory or 8th nerve springs from the medulla, close behind 

 the pons, and immediately external to the root of the 7th. It is here 

 compounded of two roots — a superior and an inferior. The superior root 

 (Plate b5) passes over the restiform body to the grey matter at the floor 

 of the 4th ventricle. The inferior root springs out of the side of the 

 restiform body, its fibres arising from nerve cells of that body or of the 



