156 THE NERVES. 



firmness. Thirty-six pairs of spinal nerves are given off 

 from the medulla spinalis by appropriate vertebral fora- 

 mina. 



The cerebral nerves. — The first pair, or olfactory nerves, 

 are very large continuations of the substance of the cere- 

 brum ; and have a vacancy or sinus within their interior. 

 They appear to be formed of both the cortical and medul- 

 lary portions of the brain ; and take their origin from the 

 anterior of the organ {Plate V. Fig 1 . 1, 1), to be passed 

 out of the cranium by the very numerous foraminpe in the 

 cribriform portion of the ethmoid bone ; after which they 

 divide into filamentary twigs ; which traversing the ethmoi- 

 dal holes, are finally ramified over the surface of the pitu- 

 itary membrane ; by which its exquisite sensibility of smell 

 is furnished. However moisture is essential to this sense, 

 which departs when the nostrils are perfectly dry. There 

 is a connexion between this nerve and the muscles of respi- 

 ration ; as sneezing after taking snuff sufficiently proves. 

 This sympathy, which has been long known, has lately 

 been very aptly termed reflex action ; because the irritation 

 of a sentient nerve is carried to the brain or to a ganglion, 

 and thence reflected to a motor branch. 



The second pair, or optic {Plate V. Fig 1.2), arise from 

 four eminences proper to them, called corpora quadrigemina ; 

 which are situated behind the lateral ventricles. In their 

 progress forwards they issue posteriorly to the third pair of 

 nerves ; and unite anterior to the pituitary gland ; as that 

 body was formerly called, from its being imagined to be of 

 glandular structure (which it decidedly is not) ; in order to 

 supply the nose with moisture. Having joined together 

 they subsequently separate ; and leave the skull by their 

 proper holes in the cranium, each being enclosed in a 

 sheath furnished by the dura mater ; proceeding with the 

 ophthalmic artery to enter the orbit; when, penetrating the 

 globe of the eye, the nerve is expanded into fine network ; 

 and is called the retina, to which we are indebted for the 

 special sense of vision (see the Eye) . 



The third pair, or motores oculorum {Plate V. Fig I . 3, 3), 

 are small, and arise from the crura cerebri ; being behind the 

 second pair : they accompany a branch of the fifth pair 

 out of the skull into the orbit; and, dividing into various 



