OETGIX OF NERVES. 567 



connected ■with the gyrus fornicatus, or to cross over to the opposite 

 side. 



Second or optic nerves. Each optic tract may be traced backwards 

 from the commissure, across the crus, to the under surface of the optic 

 thalamus, at the posterior extremity of which it ends by blending with 

 the corpora geniculata. Its fibres may be traced into the corpora geni- 

 culata, the optic thalamus, and the anterior of the corpora quadri- 

 gemina. Those which enter the thalamus pass, some through the 

 corpora geniculata, some beneath the inner corpus geniculatum, between 

 it and the crus (as the middle root of some authors) to reach the cells 

 of the lower stratum of the thalamus. A few fibres arise in the base of 

 the brain, from the lamina cinerea, and from a collection of grey matter 

 on the outer side of the tuber cincreum. 



Third or oculo-motor nerve. Each nerve arises from the inner 

 surface of the crus cerebri, immediately in front of the pons, by a 

 number of fasciculi which are attached to the surface in an oblique line. 



The fibres, diverging, pass backward through the substance of the teg- 

 mentum of the crus, some through the locus niger, some through the 

 tegmental nucleus, to reach the grey nucleus in which the majority of 

 them end. This is a column of multipolar nerve-cells, beneath the 

 grey floor of the aquedrict of Sylvius, below the corpora quadrigemina, 

 ■and extending beneath the upper part of the fourth ventricle. 



The fourth or trochlear nerve, which appears in the base at the 

 outer side of the crus cei'ebri, arises from the surface of the valve of 

 Vieussens, immediately behind the corpora quadrigemina, and close to 

 the middle line. 



In the substance of the valve, the fibres of each root divide into Ihree 

 groups. Of these, one, nsccndinij, passes obliquely forwards and up- 

 wards in the wall of the aqueduct of Sylvius, to end in the posterior 

 part of the nucleus beneath the corpora quadrigemina, the anterior 

 part of which gives origin to the fibres of the third nerve. A second 

 group of desrmding fibres pass on the outer side of the locus coeruleus 

 to the neighbourhood of the nucleus of the fifth nerve. A third group 

 of decussafitifi fibres cross to the other side, to join the ascending or 

 descending fibres of the other nerve. 



The fifth nerve {trifacial or trigeminal) arises from the side of the 

 pons Varolii, nearer to the upper than to the lower border. It consists 

 of two parts of unequal size, the smaller, motor, root being separated 

 fi'om the other by a few transverse fibres of the pons. 



Both roots curve backwards and downwards in the substance of the 

 pons, towards the outer angle of the floor of the fourth ventricle, near 

 the fovea centralis. The fibres of the sensory root turn outwards, most 

 of them to end in a collection of nerve-cells on their outer side, con- 

 tinuous below with the grey tubercle of Rolando. Some fibres pass 

 inwards beneath the floor of the fourth ventricle to the middle line. 

 Others descend in front of the nucleus, to the lower part of the medulla. 

 The fibres of the small root go to a group of large multipolar nerve- 

 cells, to the inner side of the fibres of the nerve, and near the outer 

 angle of the floor of the fourth ventricle. The prolongation down- 

 wards of this nucleus (seen at Vm, in fig. 359) extends to the lower 

 part of the medulla. 



The sixth or abducent nerve arises from the front of the ante- 

 rior pyramid, close to the lower edge of the pons, to which it is some- 



