THE CRANIAL NERVES. 531 



The existence of an 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 jSJQJlJbe_ojLtJie..jMit side, the left halLof the object will 

 be invisible. The reason fortfiis will be apparent on reference to 

 Fig. 242. 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 optir 

 tract or 01 the cortira.1 ylsiml area Thf> g Pa f O f Wirm in ar.^ mV^r. 

 case^ is indicated by a peculiarity of the iris reflex pointed out by 

 Wernicke, which will be referred to in connection with the considera- 

 tion of the oculo-motor nerve. 



THIRD PAIR. THE OCULO-MOTOR. 



The third cranial nerve, the oculo-motor, consists of some 

 15,000 peripherally coursing nerve-fibers which serve to bring the 

 nerve-cells from which they arise into relation with a large portion of 

 the general musculature 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 re- 

 mainder come from a group of cells on the opposite side of the 

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



243)- 



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 have succeeded in locating the 

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

 difference 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 following order : 



1. The sphincter pupillae. 



2. The tensor chorioideae (the accommodation nucleus). 



