746 THE NERVOUS SYSTEM 



SECOND OR OPTIC NERVES 



The optic nerves a])pear at the base of the brain as a pair of round white 

 cords, which arise from the optic chiasma. From the posterior aspect of the latter 

 structure, two somewhat similar cords, the optic tracts, pass backwards and out- 

 wards. Each optic nerve is continuous, through the chiasma, with both optic 

 tracts. It will be convenient to trace tirst the central connections of the optic nerves 

 through the optic tracts, and afterwards to follow the nerves forwards from the optic 

 chiasma to the eyeball. 



The optic chiasma or commissure rests upon the optic groove, on the superior 

 surface of the sphenoid l)one. It is in relation above with the third ventricle, 1)eing 

 separated only b}' a thin layer of grey matter from that cavity. The internal car- 

 otid arteries are close to its outer sides, behind it is the tuber cinereum, and in front 

 the lamina cinerea, tlie anterior cerebral and the anterior conmiunicating arteries. 



Constitution of the optic chiasma. — Three sets of fibres are described in the 

 optic chiasma, namely: (a) Filires from one optic tract to the optic nerve of the 

 same side; (b) fibres from one optic tract to the optic nerve of the opposite side; 

 (6-) commissural fibres passing from one optic tract to its fellow of the opi)Osite 

 side, (a) The uncrossed fibres proceed from the lateral or temporal half of the 

 retina of the same side. They occupy the outer part of the chiasma. (b) The 

 crossed fibres, which form the largest constituent of the chiasma, are derived from 

 the mesial or nasal half of the retina, and i)ass into the optic tract of the oi>posite 

 side. (c) The commissural fibres occupy the back part of the chiasma. They 

 have nothing to do with vision. Two sets are described under the names of Gudden's 

 and Meynert's commissures. 



The optic tract passes backwards and outwards between the outer side of the 

 tuber cinereum and the anterior perforated space, then it passes under cover of the 

 temporal lobe, Avinding around the crus cerebri. As it applies itself to the latter, 

 it adheres to it, and becomes flattened from above downwards. It next inclines 

 upwards, and divides into an external and an internal root. These roots j^roceed 

 towards the external and internal geniculate bodies, respectively. The internal 

 root contains the filires of Gudden's commissure. 



A portion of the external root becomes connected with the nerve-cells in the 

 external geniculate body; another part enters the pulvinar of the optic thalamus, 

 while a third part passes in the superior brachium to the superior quadrigeminal 

 body of the same side. 



The fibres of the optic nerve terminate therefore amidst or in the ganglion 

 cells of the external geniculate body, the pulvinar of the optic thalamus, and the 

 superior quadrigeminal body; hence, these three bodies have been said to contain 

 the nuclei of origin of the optic nerve. Other fil)res, however, collectively termed 

 the optic radiation, emerge from these three })odies, and pass through the most 

 posterior part of the inner capsule to the occipital lobe, and thus the cortical origin 

 of the optic nerve is estal^lished. The exact area of the cortex devoted to the 

 sense of sight is still a matter of dispute, l)ut the weight of evidence seems to l)e 

 in favour of the cuneus being the cerebral centre of vision. 



Other roots of the optic nerve are described, of which the direct cortical root 

 (Wernicke and Gudden) is perhaps the most important. This root leaves the optic 

 tract as the latter is crossing the crus cerebri, and runs upAvards in the inner 

 capsule to join the optic radiati(^n. Another root is described which passes from 

 the optic chiasma into the central grey matter of the third ventricle. The 

 descending root (Stilling) enters the crus and is variously distril)Uted; a jiortion of 

 its fil)res become connected with the oculo-motor nucleus, and thus a jiath for 

 refiex movements of the iris and ciliary muscle is probably established. 



Gudden's commissure. — Fibres emerge from the posterior (|uadrigeminal 

 liody, constituting its brachium, and enter the internal geniculate body. After 

 ])artial interruption in the nerve-cells of the latter, these fibres are continued into 

 the optic tract and pass across to the opposite side, forming the back part of the 

 optic chiasma, to enter into a similar relation to the internal geniculate and posterior 

 <juadrigeminal bodies of the opposite side. 



