THE ENCEPHALIC OR CRANIAL NERVES 



617 



The existence of a homonymous hemianopsia becomes evident when 

 the individual is directed to focus the vision on an object placed directly in 

 front and with its center in the median plane of the body, when if the lesion 

 be on the right side, the left half of the object will be invisible. The reason 

 for this will be apparent on reference to Fig. 265. All the light rays emanating 

 from the left half of the object fall on the retina on the side of the injury, 

 and hence there will be no sensation. If, however, the object be moved to 

 the right without change in the position of the head, the entire object will be 

 visible, as all the rays fall on the normal side. If, on the contrary, the 

 object be moved to the left, it will be invisible for the opposite reason. 



Hemianopsia may be the result of 

 either destruction of the optic tract or 

 of the cortical visual area. The seat 

 of lesion in any given case is indicated 

 by a peculiarity of the iris reflex 

 pointed out by Wernicke, which will 

 be referred to in connection with the 

 consideration of the oculo-motor 

 nerve. 



THE THIRD NERVE. THE 

 OCULO-MOTOR 



The third cranial nerve, the oculo- 

 motor, consists of some 15,000 pe- 

 ripherally coursing nerve-fibers which 

 serve to bring the nerve-cells from 

 which they arise into relation with a 

 large portion of the general muscula- 

 ture of the eye. 



Origin. The axons composing 

 the third nerve arise from a series of 

 seven or eight groups of nerve-cells, 

 located in the gray matter beneath the floor of the aqueduct of Sylvius. 

 From each of these groups or nuclei, bundles of axons emerge, which after 

 a short course unite to form the common trunk. The large majority of 

 the fibers in the nerve come directly from the nuclei of the same side; the 

 remainder come from a group of cells on the opposite side of the median 

 line. There is thus a partial decussation of its fibers (Fig. 266). 



The different groups of cells, the nuclei of origin, are arranged in a serial 

 manner. The anatomic arrangement of these nuclei would indicate that 

 each nucleus is related to an individual member of the eye-group of muscles. 

 Clinical observation and the investigation of the results of pathologic processes 

 have not only shown that this is the case, but also succeeded in locating the 

 position of the nucleus for any given muscle. Though there is some differ- 

 ence of opinion in regard to the exact location of one or two of the nuclei, 

 the tabulation subjoined is approximately correct. 



Enumerating them from before backward, the nuclei occur in the follow- 

 ing order: 



1. The sphincter pupillae. 



2. The tensor chorioideae (the accommodation nucleus). 



FIG. 265. DIAGRAM TO SHOW THE EXIST- 

 ENCE or HEMIANOPSIA. The lesion is sup- 

 posed to be in the right optic tract. 



