THE SECOND OR OPTIC NERVE. 



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ciated with the temporal lobe, where it probably includes the gyrus hippocampi, 

 uncus, and hippocampus major. It is further described as comprising the part 

 of the callosal convolution which lies below the genu and rostrum of the corpus 

 callosum, and also the posterior part of the orbital surface of the frontal lobe. 



Surgical Anatomy. In severe injuries to the head the olfactory bulb may become sepa- 

 rated from the olfactory nerves, thus producing loss of the sense of smelling (anosmia), and with 

 this a considerable loss in the sense of taste, as much of the perfection of the sense of 

 taste is due to the sapid substances being also odorous and simultaneously exciting the sense of 

 smell. 



The Second Nerve (Fig. 389). 



The Second or Optic nerve (n. options), the special nerve of the sense of sight, 

 is distributed exclusively to the eyeball. The nerves of opposite sides are connected 

 together at the commissure, and from the back 

 of the commissure they may be traced to the 

 brain, under the name of the optic tracts. 



The optic tract, at its connection with the 

 brain, is divided into two bands, external and 

 internal. The external band is the larger ; it 

 arises from the external geniculate body and 

 from the under part of the pulvinar of the optic 

 thalamus, and is partly continuous with the 

 brachium of the anterior or upper quadrigeminal 

 body. The internal band curves round the 

 crusta, and passes beneath the internal genicu- 

 late body, Avith which it is connected, and then 

 appears to lose itself in the brachium of the 

 posterior or inferior quadrigeminal body. The 

 fibres by which it is connected to the internal 

 geniculate body are merely commissural, form- 

 ing part of Gudden's commissure. From this 

 origin the tract winds obliquely across the under 

 surface of the crus cerebri, in the form of a 



flattened band, and is attached to the crus by its anterior margin. It then assumes 

 a cylindrical form, and, as it passes forward, is connected with the tuber cinereum 

 and lamina cinerea. It finally joins with the tract of the opposite side to form the 

 optic commissure. 



The commissure or chiasma, somewhat quadrilateral in form, rests upon the 

 olivary eminence and on the anterior part of the diaphragms sellae, being bounded, 

 above, by the lamina cinerea ; behind by the tuber cin- 

 ereum ; on either side by the anterior perforated space. 

 Within the commissure, the optic nerves of the two 

 sides undergo a partial decussation. The fibres which 

 form the inner margin of each tract and posterior 

 part of the commissure have no connection with the 

 optic nerves. They simply pass across the commissure 

 from one hemisphere of the brain to the other, and con- 

 nect the internal geniculate bodies of the two sides. 

 They are known as the commissure of Gudden. The remainder and principal 

 part of the commissure consists of two sets of fibres, crossed and uncrossed. The 

 crossed, which are the more numerous, occupy the central part of the chiasma, and 

 pass from the optic tract of one side to the optic nerve of the other, decussating in 

 the commissure with similar fibres of the opposite tract. The uncrossed fibres 

 occupy the outer part of the chiasma, and pass from the tract of one side to the 

 nerve of the same side. 1 



1 A specimen of congenital absence of the optic commissure is to be found in the Museum of the 

 Westminster Hospital. See also Henle, Ntrventehre, p. 393, ed. 2. 

 46 



FIG. 389. The left optic nerve and optic 



tracts. 



f the fibres 



