NERVUS OPTICUS.
The fibres of the optic nerve arise within the retina and are the axones of • the ganglion cells located within the ganglion-cell layer of the nervous tunic of the eye.
They extend to the chiasm. Here, one part of the fibres passes to the tractus opticus of the opposite side, the other part passes direct to the tract of the same side. The fibres end within the corpus geniculatum laterale, the pulvinar and the superior colliculus ; these end-stations constitute primary visual centres. From the lateral geniculate body and the pulvinar, fibres pass through the hindmost part of the posterior limb of the capsula interna to the secondary or cortical visual centres within the cortex of the cuneus, thereby forming the optic radiation of Gratiolet. Fibres also pass in the opposite direction from the cortical visual centre to the primary centres. It must be noted fur ther, that fibres exist, which arise within the primary centres and end within the retina.
The visual fibres proper terminate within the corpus geniculatum laterale • and the pulvinar thalami and probably do not invade the superior colliculus of the corpus quadrigeminum, at least in the higher vertebrates. The optic fibres which pass to the superior colliculus are concerned with a special duty. Stimuli carried by these fibres to the superior colliculus are transferred to the deeper lying oculomotor nucleus, resulting in the liberation of the pupillary reflex. These optic fibres ending in the superior colliculus are known, therefore, as pupillary fibres.
The pupillary reflex consists in a contraction of the pupil in response to the entrance of light. The reaction is exhibited by both eyes ; that is, when the light falls on only one eye, the contraction occurs not only in the stimulated eye (direct reaction), but also in the other eye (consensual reaction).
The intercentral paths of the pupillary reflex are not positively established, although it may be regarded as certain, that the entire reflex-tract is made up of a sequence including several neurones. It may be assumed that the stimulus passes : a. From the retina to the superior colliculus ; b. From the superior colliculus to the nucleus of the oculomotor nerve ; c. From the oculomotor nucleus to the ciliary ganglion ; d. From the ganglion ciliare to the sphincter pupillae muscle.
Since the illumination of one eye causes uniform contraction of both pupils, the re flex being, therefore, homo- and heterolateral, it follows that the impulse from one colliculus must be transferred to both oculomotor nuclei, or one nucleus must be able to stimulate both the right and left sphincter pupillae muscles. The particular fibre-bundle, by means of which the impulse is transferred from the superior colliculus to the oculomotor nucleus, is not definitely determined. In Fig. 16o, the reflex path is schematically represented, with the as sumption, that the fibres proceeding from the superior colliculus reach both oculomotor nuclei.
A knowledge of the course of the fibres of the optic nerve, particularly their semi decussation, supplies the explanation of one of the most important disturbances of vision hemiopia (half seen) or hemianopsia (half not seen). If the conduction of one optic tract, for example the left, be interrupted by a lesion, the stimuli coming from the left retinal halves of the two eyes can no longer be transmitted to the cortical centres in the left hemisphere, the right halves of the visual fields are lost and only the left halves of fixed objects are still seen (Fig. 158). This condition is spoken of as homolateral or homony mous hemianopsia or hemiopia. Lesion of the left tractus leads to right-sided homonymous hemianopsia or to left-sided hemiopia ; lesion of the right tract leads to left-sided homonymous hemianopsia or to right-sided hemiopia.
Homonymous hemianopsia follows, of course, not only lesion of the tractus opticus, but also lesion of the secondary paths connecting the primary and sec ondary centres, that is within the optic radiation, or lesion of the cortical centre.
In relation to the diagnosis of the seat of the lesion, the pupillary reaction pos sesses a certain significance. If in ho monymous hemianopsia the light-reflex is lost when the insensitive half of the retina is illuminated, the seat of the lesion is the tractus (Wernicke's hemianopsic pupillary rigidity or hemiopic pupillary reaction). If, on the contrary, the light-reflex of the pupil is intact, then the lesion lies higher, for example, in the internal capsule or in the cerebral cortex. In the majority of cases of homonymous hemian opsia, we have to do with tumors of the occipital lobe, more rarely with lesions of the tractus opticus. Complications associated with hemianopsia, such as hemiplegia, hemiparesis, con tractions and aphasic disturbances (with right-sided hemianopsia), must also be borne in mind.
The same-sided or homonymous hemianopsia is the opposite of the heteronymous hemianopsia, which occurs more rarely than the homonymous. When the temporal halves of both visual fields are wanting, such heteronymous hemianopsia is known as temporal hemianopsia. In such cases, the lesion is situated within the chiasm, either in the middle or in the anterior or the posterior angle of the chiasm, whereby the decussating fibres are involved. Temporal hemianopsia is observed, for example, in acromegaly, in which the enlargement of the hypophysis cerebri concurrently affects the chiasm. When the nasal halves of both visual fields are wanting, the condition is spoken of as nasal hemianopsia and is produced by involvement of the uncrossed fibres, as when the chiasm is subject to pressure on both sides in the lateral angle by enlarged carotids.