THE ENCEPHALIC OR CRANIAL NERVES 629 



logic stimulus to the retinal visual cells is the impact of the undulations of 

 the ether. In general it may be said that, at least for the same color, the 

 intensity of the objective undulation or vibration determines the intensity of 

 the sensation. 



Owing to the partial decussation of the optic nerve-fibers at the chiasma, 

 the impressions made on the temporal side of the retina are received by the 

 cuneus of the same side, while the impressions made on the nasal side are re- 

 ceived by the cuneus of the opposite side. Each eye is, therefore, associated 

 with the cuneus of each cerebral hemisphere. 



Pupillary Fibers. The optic nerve also contains nerve-fibers some- 

 what larger in caliber than the usual visual fibers, which are sup- 

 posed to form the afferent path for those nerve impulses which excite 

 reflexly a contraction of the sphincter pupillcB muscle, thus varying the size 

 of the pupil. These fibers, termed pupillary fibers, come from all portions 

 of the retina but most abundantly from the posterior pole in and around 

 the macula. The existence of these fibers is confirmed by pathologic find- 

 ings. In a manner similar to that of the visual fibers they, too, undergo a 

 decussation in the optic chiasm, so that in the optic tract there are pupil- 

 lary fibers which come from the temporal side of the eye of the corresponding 

 side, and fibers which come from the nasal side of the eye of the opposite 

 side (Fig. 264, page 633). The central termination of these fibers is not 

 positively known. 



Hemiopia and Hemianopsia. Division of the optic nerve between the 

 eyeball and the optic chiasm is followed by complete blindness in the eye of 

 the corresponding side. Owing to the partial decussation of the fibers in 

 the chiasm, division of an optic tract is followed by a loss of sight in the outer 

 two-thirds of the eye of the same side and in the inner third of the eye of the 

 opposite side. To this loss of visual power in the retina the term hemiopia 

 is given. In consequence of this loss of visual power in the retina there is 

 a corresponding obscuration or total obliteration of nearly one-half of the 

 visual field, 1 to which the term hemianopsia is given. If, for example, the 

 right optic tract is divided there will be hemiopia in the outer two-thirds of 

 the right eye and the inner third of the left eye, with left lateral hemianopsia, 

 and as the portions of the retina which are affected are associated in vision 

 the loss of the visual fields is spoken of as homonymous hemianopsia (Fig. 

 266). A destructive lesion of the cerebral visual area, the cuneus and 

 the adjacent gray matter on the right side, is also followed by left lateral 

 hemianopsia. 2 



1 The visual field comprises that portion of the external world from which, with the eyes 

 stationary, rays of light pass to the retinae and is the area included between the extremes of 

 the visual lines entering the pupil. The center of the visual field is the area the rays of light from 

 which are focalized on the fovea centralis. The visual field is somewhat irregular in outline by 

 reason of the position of the eyeball in the orbit cavity, and the consequent interference with 

 the entrance of light by the bridge of the nose, the cheek bones, and the eyebrows. The hori- 

 zontal diameter of the visual field for the right eye is about 150, of which 90 pertain to the 

 temporal and 60 to the nasal portion. The vertical diameter is about 115, of which 45 pertain 

 to the superior and 70 to the inferior portion. By reason of the position of the eyes in the orbit 

 cavity the two visual fields, viz.: that of the right and of the left eye, overlap to a variable extent in 

 their nasal divisions. 



a It should be borne in mind that in both instances the retina itself is unaffected. The impact 

 of light generates, as usual, nerve impulses which proceed as far backward as the point of division 

 or destruction. In consequence those portions of the cerebral cortex stimulation of which evokes 

 the sensation of light remain unaffected and the individual does not become aware, through 

 sensation, of the presence of a luminous body in the left side of the visual field. 



