672 



SPECIAL SENSES. 



oblongata, extend down as far as the decussation of the pyramids, and proba- 

 bly are concerned in the reflex movements of the iris. 



The two roots of each optic tract unite 

 above the external corpus geniculatum, form- 

 ing 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 situ- 

 ated just in front of the corpus cinereum, 

 resting upon the olivary process of the sphe- 

 noid 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 directions ; and in ad- 

 dition, the commissure contains filaments pass- 

 ing 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 op- 

 tic tract upon either side to the eye of the op- 

 posite side. The greatest part of the fibres 

 take this direction. Their relative situation is 

 internal. 



2. External fibres, fewer than the preced- 

 ing, which pass from the optic tract to the eye 

 upon the same side. 



3. Fibres situated on the posterior boundary of the commissure, which 

 pass from one optic tract to the other and do not go to the eyes. These fibres 

 are scanty and are 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 upon 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 FIG. ^.-Diagram of the 



,1 i i i ,i tion of fibres at the optic com- 



pass to the right eye, supplying the inner, or na- missure. 

 sal mathematical half of the retina, from a ver- The d0 r^ c 1ioLTof 8 ?nTfibrls four *" 

 tical line passing through the macula lutea. Fi- 

 bres also pass to the left eye, supplying the outer, or temporal half of the 

 retina. The macula lutea, then, and not the point of entrance of the optic 

 nerve, is in the true line of division of the retina. 



FIG. 242. Optic tracts, commissure 

 and nerves (Hirschfeld). 



1, infundibulum ; 2, corpus cinereum ; 

 3, corpora albicantia ; 4, cerebral 

 peduncle ; 5, pons Varolii ; 6, optic 

 tracts and nerves, decussating at 

 the commissure, or chiasm ; 7, mo- 

 tor oculi communis ; 8, patheticus ; 

 9, fifth nerve ; 10, motor oculi ex- 

 ternus ; 11, facial nerve ; 12, aud- 

 itory nerve ; 13, nerve of Wris- 

 berg ; 14, glosso-p_haryngeal nerve; 

 15, pneumogastric ; 16, spinal ac- 

 cessory ; 17, sublingual nerve. 



