750 THE NERVOUS SYSTEM 



matter of the floor of the fourth ventricle, but it also receives the whole or the 

 greater part of a long tract of fibres, -wliich passes downwards from the mesen- 

 cephalon, and which is termed the descending root. 



The majority of the fibres of the portio major spring from the nerve-cells of the 

 Gasserian ganglion; they enter the pons at the base of the middle peduncle and 

 near its upper l»order. In the suljstance of the pons they i)ass Ijackwards and 

 downwards, and the smaller numl)er teiininate in ar])orisations around the cells of 

 the upper sensory nucleus of the fifth nerve, an ovoid group of nerve-cells situated 

 in the upper and lateral part of the floor of the fourth ventricle external to the 

 motor nucleus and continuous ])elow with the substantia gelatinosa Rolandi, which 

 is called the lower sensory nucleus. The greater number of the filjres of the portio 

 major pass downwards in the pons, medulla, and spinal cord, forming a crescentic 

 bundle, known as the ascending root of the fifth nerve, which lies external to the 

 sul)stantia gelatinosa of Rolando. It diminishes in size as it descends, and it 

 finally terminates in the upper part of the cervical region of the cord. Its fibres 

 pass into the substantia gelatinosa and probably terminate in ar]:iorisations. In the 

 pons the ascending root of the fifth nerve is deeply placed between the auditory and 

 facial nerve-roots (fig. 430), and in the upper part of the medulla it is covered by 

 the restiform body (fig. 432), but in the lower part of the medulla and the upper 

 part of the cord it lies close beneath the surface. 



The descending root arises from a column of nerve-cells Avhich is embedded in 

 the grey matter of the aqueduct above and external to the nucleus of the third 

 nerve. The root commences at the level of the upper part of the nates, and increases 

 in size as it descends, forming a white Inmdle of fibres crescentic in section, some 

 of these fibres being connected with the nerve-cells which are placed on their inner 

 side (fig. 444, v). It passes through the grey matter of the upper part of the 

 floor of the fourth ventricle, close to the outer side of the substantia ferruginea, 

 and reaches the upper part of the motor nucleus of the fifth nerve. Here it is joined 

 by the principal part of the motor root, which arises from the motor nucleus, and 

 by a few fibres from the motor nucleus of the opi^osite side. It then passes down- 

 wards and forwards through the pons to emerge as the portio minor of the fifth 

 nerve. At its emergence it is placed a little in front of the sensory portion, and is 

 separated from the latter by some of the transverse fibres of the pons. The motor 

 nucleus is an ovoid mass of cells consideraVjly shorter than the accessory sensory 

 nucleus, on the inner side of which it is placed. It is on the same line as the 

 nucleus of the facial nerve, and is immediately in front of that nucleus. 



According to some observers some fibres of tlie descendiug root enter the sensorj' root and are 

 said to eventually pass into the ophthalmic division. 



The sensory and motor roots of the nerve pass downwards and fonvards towards 

 an aperture in the dura mater which is placed under cover of the tentorium 

 cerebelli, a little external to the apex of the petrous portion of the temporal l)one 

 in the posterior fossa of the skull. In this course the motor root takes a half- 

 spiral turn around the sensory root, passing first to the inner side, and then beh^w 

 the latter. Both roots then pass through the aperture above mentioned to enter 

 Meckel's space in the middle fossa, between the supporting and periosteal layers 

 of the dura mater. The motor root passes out of the skull through the foramen 

 ovale, accompanied by a larger sensory bundle from the Gasserian ganglion, and 

 joins with the latter outside the skull to form the mandiljular (or inferior max- 

 illary) division of the fifth nerve. The sensory root spreads out into a flattened, 

 somewliat fan-slia])('(l, plcxiform lumdle, and enters the Gasserian ganglion. 



The Gasserian ganglion is a reddish-grey band of ganglionic matter, with its 

 long axes slightly curved so as to present a convexity forwards and outwards. The 

 upper and lower surfaces of the ganglion are also slightly convex and are some- 

 what adherent to the dura mater, the upper surface being more firmly attached 

 than the lower. It rests in a depression on the petrous bone and, in front of this, 

 on the cartilage which occupies the foramen lacerum medium. From the convex 

 antero-external liorder of tlie ganglion, three large bundles of nerve-fibres arise. 

 The first or oplithalmic division enters the orbit tln'ough the sphenoidal fissure. 

 The second or maxillary ( sujx'rior maxillary) division leaves the skull through the 



