124 



Cranial Nerves. 



TrocAtea 



Tendo 

 rect. sup. 



I. N OLFACTOR.^iu 



M. Levant, palp 

 supe 



Foram. optic. 



IV. N. TROCHLEA 



V: N. TR1BEMIN. 



468. The Olfactory and Optic Nerves. 



View from above. 



The second pair of cranial nerves is the optic nerve, Xcrmts 

 opt/nix; under the name of optic tract, Tractus opticux, it winds obliquely 

 across the under surface of the Cms ccrcbrl^ from without inwards and 

 downwards, converging with the nerve of the opposite side, to form 

 with it in front of the Tultcr ehicrenm the chiasma or optic commis- 

 sure;. The greater number of the fibres of the two nerves decussate in 

 the commissure, but some are continued from the tract to the nerve of 

 the same side. The optic nerves, as they arise from the chiasma, are 

 rounded cords, diverging from each other; each passes through the cor- 

 responding optic foramen of the sphenoid bone to the orbit, and continues 

 its oblique course, inclining at the same time somewhat downwards, to 

 the eyeball, which it enters on its posterior aspect somewhat internal 

 to its center. Here the nerve perforates the sclerotic and choroid coats 

 and terminates by expanding in the retina. The fibrous layer of the 

 retina is formed by the fibres of the optic nerve (see Fig. 307). The 

 optic nerve is surrounded by a sheath from the dura mater, which finally 

 becomes blended with the texture of the sclerotic. 



