THE SECOXD OR OPTIC NERVE. 



793 



FIG. 477.- 

 tracts. 



-The left optic nerve and optic 



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. 



Second Nerve (Fig. 477). 



The Second or Optic Nerve, 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 arises from the ex- 

 ternal geniculate body and from the under 

 part of the pulvinar of the optic thalamus, and 

 receives most of the fibres of the brachium of 

 the superior Corpus quadrigeminum. The in- 

 ''/nal arises from beneath the internal genic- 

 ulate body, from which it derives fibres, and 

 joins with the other band to form the optic 

 tract. From this origin the tract winds ob- 

 liquely across the surface of the crusta in the 

 form of a flattened band, destitute of neuri- 

 lernma and attached to the crusta by its an- 

 tero-superior margin. It then assumes a cyl- 

 indrical form, and, as it passes forward, is 

 connected with the tuber cinereum and lam- 

 ina cinerea. It finally joins with the tract of the opposite side to form the optic 

 / rmmisture. 



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

 optic groove of the sphenoid bone, being bounded, above, by the lamina cinerea ; 



behind, by the tuber cinereum ; on either side, by the 

 anterior perforated space. Within the commissure the 

 optic nerves of the two sides undergo a partial decus- 

 sation. The fibres which form the inner margin (in- 

 'o optic nerve ferior commissure of Gudden) of each tract are con- 



y same side. . , , J , . , , , 



of opposite tinned across from one to the other side of the brain. 

 These may be regarded as commissural fibres (inter- 

 cerebraT) between the internal geniculate bodies. Some 

 fibres are continued across the anterior border of the 

 chiasma, and connect the optic nerves of the two sides, having no relation with 

 the optic tracts. 1 They may be regarded as commissural fibres between the two 

 retime (inter-retinal fibres). The outer fibres of each tract are continued into the 

 optic nerve of the same side. The central fibres of each tract are continued into 

 the optic nerve of the opposite side, decussating in the commissure with similar 

 fibres of the opposite tract. 2 



The optic nerves arise from the fore part of the commissure, and, diverging from 

 one another, become rounded in form and firm in texture, and are enclosed in a 

 sheath derived from the arachnoid. As each nerve passes through the correspond- 

 ing optic foramen it receives a sheath from the dura mater ; and as it enters the 

 orbit this sheath subdivides into two layers, one of which becomes continuous with 

 the periosteum of the orbit ; the other forms the proper sheath of the nerve and 



1 The presence of these fibres has been doubted by some observers, but they have been recently 

 asserted to exist by Stilling. 



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

 Westminster Hospital. (See also Henle, Nervenlehre, p. 393, ed. 2.) 



side. 



FIG. 478. Course of the fibres 

 in the optic commissure. 



