THE NERVES AND THEIR FUNCTIONS. 1-15 



This cut represents a portion of nerve, showing the filaments of 

 which it is composed, and one of them drawn out. 



3. First, or olfactory. These nerves take their name 

 from " olfactus" the smell, as they are essential to this sense. 

 They are the softest nerves in the body, and more closely 

 connected with the hemispheres of the brain than any othei> 

 They lie immediately under the anterior lobes of the brain ; 

 and as they pass on, they swell into a bulb, from which 

 numerous fibres issue, and pass into the nose through a plate 

 of one of the bones of the skull. This plate is pierced with 

 so many holes for transmitting these twigs of the nerve, that 

 it has the appearance of a seive ; and from this circumstance 

 is called the ethmoid bone. The olfactory nerve is spread 

 out upon the lining membrane of the nose, and is merely 

 defended by a thin layer of mucus. It was necessary that 

 these nerves should come to the surface, in order to come in 

 contact with the vapours inhaled by the nostrils. In some 

 animals these nerves are very large, especially in ruminants, 

 but in others they are entirely wanting, as in whales. In 

 fishes, you may see the bulb of the olfactory placed immedi- 

 ately under the cup-like nostril. From the bulb, the nerve 

 runs backward along a canal filled with transparent fluid, 

 enters the skull, and joins the brain. 



4. The second, or optic. -Z These are the largest of the 

 cerebral nerves, and can be traced as far back as the medulla 

 obfongata at the base of the brain. They pass along the base 

 of the brain, and just before they enter the orbits they unite 

 with each other^ or seem to decussate, or cross each other. 

 In some fishes, these nerves evidently cross without union ; 

 but in man, it is now pretty well established, that a semi- 



decussation takes place ; that is, one half of the right optic 

 nerve crosses to the opposite side, and joins a half of the 

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