THE OPTIC NERVE 



647 



cerebral cortex in the lower animals and pathological observations on 

 the human subject, it has been shown that fibres from the apparent 

 origin of the optic tracts pass backward to the gray matter of the occipi- 

 tal lobes of the cerebrum. They then pass to the bulb, extend down 

 as far as the decussation of the pyramids and probably are concerned 

 in the reflex movements of the iris. 



The two roots of each optic tract unite above the external corpus 

 geniculatum, forming a flattened band, which takes an oblique course 

 around the under surface of the crus cerebri 

 to the optic commissure. 



The optic commissure, or chiasm, is 

 situated just in front of the corpus cine- 

 reum, resting upon the olivary process of 

 the sphenoid bone. As its name implies, 

 this is the point of union between the nerves 

 of the two sides. At the commissure the 

 fibres from the optic tracts take three direc- 

 tions ; and in addition, the commissure con- 

 tains filaments passing from one eye to the 

 other, which have no connection with the 

 optic tracts. The four sets of fibres in 

 the optic commissure are the following: 



1. Decussating fibres, passing from the 



optic tract on either side to the eye of the Fi g- 164. optic tracts, commissure 



. ., . r . and nerves (Hirschfeld). 



opposite side. The greatest part of the fibres 



. i, infundibulum ; 



take this direction. Their relative situation reum 

 is internal. 



2. External fibres, fewer than the pre- 

 ceding, which pass from the optic tract to 1;" 



the eye On the Same Side. II . facial nerve; 12, auditory nerve; 



T^., . , . . 13, nerve of Wrisberg; 14, glosso- 



3. Fibres situated on the posterior pharyngealnerve; I5ipneum ogastric; 

 border of the commissure, which pass from l6 - s P inal accessory; 17, subiingual 



nerve. 



one optic tract to the other and do not 



go to the eyes. These fibres are scanty and sometimes wanting. 



4. Fibres situated on the anterior border of the commissure, greater 

 in number than the preceding, which pass from one eye to the other and 

 which have no connection with the optic tracts. 



The fibres of the optic tracts on the two sides are connected with 

 distinct portions of the retina. This fact is illustrated in cases of hemi- 

 anopsia, which show that the decussating fibres have the following 

 directions and distribution : 



From the left side of the encephalon, fibres pass to the right eye, 



2, corpus cine- 



3, corpora albicantia ; 4, cere- 

 bral peduncle ; 5, pons Varolii ; 6, optic 

 tracts and nerves, decussating at the 

 commissure, or chiasm ; 7, motor oculi 



