768 



SPECIAL SENSES. 



the sense of sight is completely abolished by destruction of the tubercula quadrigemina 

 (called bigemina, in birds), show that the origin of the filaments that preside over vision 

 is, in all probability, from these bodies. 



The two principal roots of the optic nerves 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. This is usually called the optic tract, 

 in contradistinction to the optic nerve, which is de- 

 scribed as arising from the optic commissure. 



The optic commissure, or chiasm, is situated just in 

 front of the corpus cinereum, 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 directions ; and, in addition, 

 the commissure contains 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 upon either side to the eye of the opposite 

 side. The greatest part of the fibres take this direc- 

 tion. Their relative situation is internal. 



2. External fibres, much less numerous than the 

 preceding, 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, more numerous than the 

 preceding, which pass from one eye to the other and which have no connection with the 

 optic tracts. 



It is probable, reasoning chiefly from cases of cerebral injury or disease, that the fila- 

 ments from the optic tracts upon the two sides are connected with distinct portions of 

 the retina ; and two pathological cases have lately been reported by Drs. Keen and 



Thomson, of Philadelphia, which go to show that this is 

 the fact, and which illustrate certain interesting points 

 in connection with the decussation of the nerves. One 

 was a case of gunshot-wound of the head, with severe 

 injury of the brain-substance. This case presented, im- 

 mediately after the injury, unconsciousness and partial 

 paralysis of the right arm and right leg, which lasted two 

 or three months. About a year after, the paralysis had 

 almost entirely disappeared, but the memory was some- 

 what impaired. Upon careful examination of the eyes, it 

 was ascertained that the field of vision was divided in 

 each eye by a vertical line passing through its centre. 

 In the right eye, the inner half of the retina, beginning on 

 a line with the inner border of the macula lutea, was entirely insensible to light. In 

 the left eye, the outer half of the retina, beyond the macula, was insensible to light. No 

 pathological appearances were observed upon examining the retinas with the ophthalmo- 



FIG. 240. Optic tracts, commissure, and 

 nerves. (Hirschfeld.) 



1, infundibulum ; 2, corpus cinereum ; 3, 

 corpora albicantia; 4, cerebral pedun- 

 cle ; 5, tuber annulare ; 6, optic tracts 

 and nerves, decussating at the commis- 

 sure, or chiasm; 7, motor oculi com- 

 munis ; 8, patheticus ; 9, fifth nerve ; 10, 

 motor oculi externus; 11, facial nerve; 

 12, auditory nerve ; 13, nerve of Wris- 

 berg; 14, glosso-pharyngeal nerve; 15, 

 pnemnogastric; 16, spinal accessory; 17, 

 sublingual nerve. 



FIG. 241. Diagram of the decussation 



at the optic commissure. 

 The dotted lines show the four direc- 

 tions of the fibres. 



