392 HEALTH AND DISEASE 



combine to form the commissure or chlasma of the optic nerves, which 

 is lodged in a depression at the base of the cranium the optic fossa. 



It is important to understand the behaviour of these nerves on reaching 

 the commissure. To look at, it would almost seem as if the nerves pro- 

 ceeding from the optic tracts had crossed in their course, and had gone 

 to the eye on the side opposite to that on which they first appear before 

 the chiasma is reached, but this is not exactly the case. 



As a matter of fact, the great bulk of the nerves do cross, but a certain 

 number of filaments continue on the original side and pass into the eye 

 on that side. We find, therefore, that fibres from the left side mingle 

 with fibres from the right, and together form the right optic nerve, and 

 vice versa. It should also be noticed that some of the fibres are believed 

 to cross from right to left in the optic chiasma, and to pass backward 

 through the opposite optic tract to the brain. 



When the nerve leaves the commissure it passes out of the cranium 

 through the optic foramen, and reaches the orbital cavity to pierce the 

 sclerotic coat of the lower part of the globe of the eye, and after passing 

 through the choroid coat, opens out and forms a thin nervous expansion 

 termed the retina. 



The function of the optic nerve is to transmit to the brain the 

 impressions made upon it by external objects, or, in other words, it is 

 the nerve of sight. 



Third Pair, Motores Oculorum. These nerves have their super- 

 ficial origin in the under surface of the cerebral peduncles. From this 

 point they proceed in a forward direction, and enter the orbit through the 

 foramen lacerum orbitale. 



This nerve supplies the elevator muscle of the upper eyelid, the internal 

 rectus, the superior and inferior rectus, the retractor oculi, and the small 

 oblique muscle. 



It also gives motor branches to the lenticular ganglion, and through it 

 supplies the ciliary muscle and the circular fibres of the iris. It is essen- 

 tially a motor nerve, and may cause all the parts named to contract. 



Fourth Pair, Pathetici. A very thin, long, and slender nerve 

 originating behind the corpora quadrigemina, from which it descends in 

 a forward and downward direction to the supra-sphenoidal fissure. By 

 this it is conducted to the pathetic foramen, through which it passes into 

 the orbit, and thence to its ultimate destination, the superior oblique 

 muscle of the eye. It is a nerve of motion. 



Fifth Pair, Trigeminal. This is much the largest of the cranial 

 nerves, and the variety and importance of its functions imbue it with 

 more than ordinary interest. 



