THE OPTIC NERVE. 1223 



THE OPTIC NERVE. 



The optic nerve (n. opticus) is, as conventionally described, part of the pathway 

 which includes additionally the optic commissure and the optic tract and transmits 

 the visual impulses received by the retina to the primary centres within the pulvinar 

 of the optic thalamus and the external geniculate and superior quadrigeminal bodies. 

 The retina, the nervous tunic of the eye (page 1462), comprises three fundamental 

 layers — (a) the percipient visual cells, {b) the receptive ganglion retince and (r) the 

 cerebral layer. The latter contains the neurones, the axones of which constitute the 

 nerve-fibres that converge towards the optic disc and, piercing the vascular and 

 fibrous coats, form the greater part of the optic nerve, commissure and tract. 



In addition to the fibres of retinal origin, which alone carry visual impulses, the optic 

 nerve contains a considerable number of supplementary fibres, which are only indirectly con- 

 cerned in sight. Some of these fibres, distinguished by their small diameter, pass towards the 

 retina, originating within the brain from the cells of the primary visual centres or from sympa- 

 thetic neurones, and probably transmit vasomotor inipJulses controlling the retinal blood- 

 vessels. Other supplementary fibres, perhaps by way of a centre situated within the medulla, 

 pass from the retina and are regarded as conveying indirectly to the oculomotor nucleus the 

 impulses resulting in reflex pupillary movements. 



The optic nerve (Fig. 1198) extends from the eyeball, which it leaves about 

 3 mm. to the medial side of the posterior pole, to the optic commissure. Leaving 

 the eyeball, the nerve pursues a slightly sinuous course backward, inward and up- 

 ward towards the apex of the orbit, where, surrounded by the origins of the recti 

 muscles, it traverses the optic foramen in the sphenoid bone in company with the 

 ophthalmic artery, which lies to its outer and lower side. On gaining the interior 

 of the cranium, it converges towards the nerve of the opposite side with which it 

 joins to form the major part of the optic commissure in the vicinity of the olivary 

 eminence, medial to the internal carotid artery. The entire length of the optic nerve 

 is from 30-40 mm., of which the intraorbital part includes from 20-30 mm., thus 

 allowing for changes in the position of the eyeball without undue stretching of the 

 nerve. Its diameter is from 3-4 mm. Within the orbit the nerve is embedded in the 

 orbital fat and surrounded by the ocular muscles and, near the eyeball, by the ciliary 

 vessels and nerves. It is crossed above and from without inward by the ophthalmic 

 artery and the nasal nerve, and, about 10 mm. from the eyeball, is penetrated by the 

 central artery of the retina, which, with its companion vein, continues its intra- 

 neural course as far as the optic disc. In addition to a sheath from the pia mater 

 and a delicate one from the arachnoid, the optic nerve receives a robust tubular pro- 

 longation from the dura at the optic foramen. These sheaths, with the intervening- 

 subarachnoidal and subdural lymph-spaces, are continued on the nerve as far as the 

 eyeball, where they blend with the sclerotic coat. 



The optic commissure (Fig. 1046), formed by the meeting of the converging 

 optic nerves in front and the diverging optic tracts behind, is somewhat flattened and 

 transversely oblong and measures about 12 mm. where broadest. It rests upon the 

 olivary eminence, is embraced at the sides by the internal carotid arteries, and lies 

 beneath the floor of the third ventricle in advance of the tuber cinereum in close rela- 

 tion with the inferior surface of the brain. It divides posteriorly into the two optic 

 tracts. On reaching the commissure, or chiasm, as it is sometimes called, the optic 

 fibres, estimated at upwards of half a million (Salzer), undergo partial decussation, 

 those from the nasal or inner half of each retina crossing to the mesial part of the 

 opposite optic tract, while those from the temporal or outer half continue into the 

 lateral part of the tract of the same side. The existence of a commissural loop con- 

 necting the two optic nerves has not been established, although formerly accepted. 



Occasional instances have been encountered in which the decussation of the 

 optic fibres was complete, thus repeating in man the condition that normally obtains 

 in all nonmammalian vertebrates, as well as in a few rodents (mouse, guinea-pig). 

 Rarely the optic commissure has been absent, the optic fibres passing directly into 

 the tract of the same side. 



