CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 271 



Arrived at the chiasma, the outer fibres of each tract continue 

 onwards to the optic nerve of the same side beyond the chiasma, 

 while the inner fibres cross over in the chiasma, and continue on in 

 the optic nerve of the opposite side. Some fibres appear to cross 

 from one optic tract to another, along the posterior part of the 

 chiasma, and others to cross, along its anterior part, from one o ( ptic 

 nerve to another. 



From the chiasma each optic nerve extends through the optic 

 foramen in front of it diverging widely from its fellow of the 

 opposite side. Emerging from the optic foramen, it is surrounded 

 by the recti muscles of the eyeball, the hinder part of which it 

 enters a little to the inside of its middle, piercing the two outer 

 coats of the eyeball, and expanding within it (to form the Retina), 

 as will be hereafter noticed in describing the eye. 



The optic nerve is made up of a number of separate bundles of 

 fibres, enclosed in prolongations of the dura mater. In the middle 

 of these bundles runs a small artery called the arteria centralis retinal. 



11. The THIRD pair of nerves (oculo-motor) arise deeply from a 

 grey nucleus in the floor of the iter a tertio ad quartum ventri- 

 culum, close to the origin of the fourth nerve. They issue from the 

 cerebral surface in the interpeduncular space between the 

 crura and cerebri, and immediately in front of the pons Yarolii 

 (Fig. 128, ///). 



Each nerve traverses the dura mater and sphenoidal fissure, and, 

 after receiving one or two fine branches from the sympathetic, 

 divides, and goes to supply the superior, inferior, and internal recti 

 muscles of the eyeball, as also its inferior oblique muscle, and that 

 of the elevator of the eyelid. 



The FOURTH pair of nerves, called also the TROCHLEAR or (from 

 their function of raising the eyeball) PATHETIC, arise deeply from 

 one grey nucleus in the floor of the iter a tertio ad quartum ventri- 

 culum, and from another in the floor of the fourth ventricle close to 

 the origin of the fifth nerve. They issue from the cerebral surface, one 

 on the outer side of each of the crura cerebri (Fig. 128, IV), im- 

 mediately in front of the pons, but each may be traced back round 

 the crus to a spot below and behind the corpora quadrigemina in 

 the valve of Vieussens. It goes through the sphenoidal fissure to 

 the upper oblique muscle of the orbit. 



12. The FIFTH pair of nerves, called also the TRIGEMINAL, 

 arise by two roots, one large sensory root, and one small root called 

 motor, because its branches minister not to sensation, but to 

 muscular contraction. 



The large root takes its deep origin from behind the olivary body, 

 if not from the floor of the fourth ventricle, and emerges from the 

 surface of the encephalon at the side of the pons Varolii, near its 

 upper and anterior margin, just where its fibres extend upwards 

 and backwards to form the middle crus of the cerebellum. 



The small root also takes its deep origin from the medulla, 

 oblongata, and possibly from the floor of the fourth ventricle. It 



