68O OCULOMOTOR NERVE. 



bodies, the geniculate bodies and the pulvinar, but not into the conducting path 

 to the psycho-visual center. The secondary degenerations following destruction 

 of the cortical visual center are discussed on p. 787. 



The optic nerve is the nerve of vision the physiological stimulation 

 of which occurs only through conveyance of the vibrations of the 

 luminiferous ether to the rods and cones of the retina. Every other 

 form of irritation of the nerve, either in its course or at its center, 

 causes a sensation of light. Division or degeneration of the nerve gives 

 rise to blindness. Irritation of the optic nerve causes also reflex con- 

 traction of the pupils through the oculomotor nerve, and marked irri- 

 tation, also closure of the lids and flow of tears. 



As the optic nerve has separate connections both with the psycho-visual center 

 and with the pupil-contracting center it will be readily understood that under 

 pathological conditions, on the one hand, blindness with preservation of the iris- 

 reaction, and, on the other hand, loss of the movement of the iris, with preservation 

 of vision, have been observed. 



Gudden, in 1882, found two different kinds of fibers in the optic nerve, namely 

 fine or visual fibers, whose center is situated in the quadri geminate body, and 

 coarse or pupillary fibers, whose origin can be traced to the external geniculate 

 body. Destruction- of the visual fibers causes blindness, that of the pupillary fibers 

 gives rise to marked dilatation of the pupils. 



Pathological. Irritation in the range of the entire nervous apparatus may 

 cause excessive sensitiveness of the visual organs (optic hyperesthesia} , and also 

 visual sensations of varied kind (photopsia, chromopsid) , which, in case the irrita- 

 tion extends to the psycho-visual center, may even become actual visual hallu- 

 cinations. Material alterations and inflammatory processes in the nervous appa- 

 ratus are often followed by nervous impairment of vision (amblyopia) or even by 

 blindness (amaurosis). Nevertheless, both conditions may occur as the signs of 

 disorder in other organs, so-called sympathetic symptoms, being often probably 

 due to alterations in the circulation of the blood through irritation of the vaso- 

 motor nerves, and readily undergoing retrogression. Remarkable intermittent 

 forms of amaurosis are the day-blindness (hemeralopia, for example in connection 

 with diseases of the liver) and the night-blindness (nyctalopia} . Disorders of the 

 cortical visual center are considered on p. 787. 



III. OCULOMOTOR NERVE. 



The fibers of the oculomotor nerve arise as neurites of the ganglion cells of 

 the oculomotor nucleus situated in the gray matter beneath the aqueduct of 

 Sylvius. Several groups of cells can be distinguished in this nucleus: (ij The lat- 

 eral chief nucleus, consisting principally of large ganglion cells and passing below 

 the aqueduct of Sylvius on each side close to the middle line. (2) Between the two 

 lateral nuclei lies the single, smaller, large-celled central nucleus, and (3) in front 

 of this on each side, a smaller, small-celled nucleus. The fibers from the posterior 

 portions of the lateral and central nuclei decussate. In apes the nerves for the 

 external ocular muscles arise from the chief nucleus of the same and the opposite 

 side, those for the internal muscles from the accessory nuclei. 



Frbm the angular gyrus of the cerebral cortex, the psychomotor center for 

 the voluntary movements of the eyes, and probably also from the visual sphere 

 (for the involuntary adjustment of the eyes for direct vision), fibers that undergo 

 partial decussation in the raphe of the tegmentum pass to the oculomotor nucleus, 

 with whose cells they come in contact by means of terminal branches. Not far 

 from the pons the nerve appears in the midst of the inner bundle of fibers of the 

 peduncle as a median and a posterior lateral group of fibers. 



The oculomotor nerve contains : i . The voluntary motor fibers for all 

 of the external ocular muscles, with the exception of the external rectus 

 and the superior oblique, and for the elevator of the upper lid. The 

 coordinated movement of both eye-balls is, however, independent of the 

 will. 2. The fibers for the sphincter muscle of the pupil that are active 

 through reflex stimulation from the retina. 3. The fibers for the muscle 



