THE SECOND OR OPTIC NERVE 



977 



Applied Anatomy. Destruction of the olfactory tract of one side causes loss of smell (anos- 

 mia) on the side of the injury, because the olfactory tract is practically uncrossed. In severe 

 injuries to the head the olfactory bulb may become separated from the olfactory nerves, thus 

 producing loss of the sense of smell, 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 



FIG. 726. Extent of true olfactory mucous membrane, (v. Brunn.) 



simultaneously exciting the sense of smell. When the sense of smell is lost, an individual 

 cannot distinguish the flavor of food, but he can distinguish that a substance is salt, or sweet, 

 or bitter, or acid. The most usual cause of injury to the olfactory nerve is fracture of the base 

 of the skull, the line of fracture passing through the cribriform plate of the ethmoid bone, but a 

 blow upon the face, forehead, or back of the head which does not produce fracture may injure 

 the nerves. 



THE SECOND OR OPTIC NERVE (N. OPTICUS). 



The fibres of the optic nerve, the special nerve of the sense of sight, are the central 

 processes of the retinal ganglion cells which, after converging to the optic papilla, 

 leave the eyeball by piercing its fibrous and vascular tunics as a cylindric cord. 

 The point of emergence is situated a little mesad (3 to 4 mm. -^ to ^ inches) 

 of the posterior pole of the globe. Behind the eyeball the nerve passes back- 

 ward and inward through the orbital fat and optic foramen to enter the middle 

 fossa of the cranium. The total length of the nerve averages 45 to 50 mm. 

 (1-|- to 2 inches). The two nerves converge to decussate partially, forming the 

 chiasm. 



The optic chiasm (chiasma opticum) (Figs. 727 and 728 and p. 911) is somewhat 

 quadrilateral in form, rests upon the olivary eminence and on the anterior part of 

 the diaphragma sellae, being bounded above by the lamina terminalis; behind by 

 the tuber cinereum; on either side by the anterior perforated substance. Within 

 the commissure the optic nerves of the two sides undergo a partial decussation 

 (Figs. 668 and 728), described in detail on pages 911 and 912. 



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