324 NERVES. 



The olfactory, optic, and common and internal motors of the 

 eye, arise from the cerebrum or mesocephalon ; the rest from 

 the chorda oblongata. From the chorda spinalis, thirty-one pairs 

 of nerves, double in substance and function, like the trigeminum, 

 proceed on each side, by an anterior and a posterior root ; 

 eight pairs in the neck the first above the first cervical ver- 

 tebra, the last below the last cervical vertebra ; twelve in the 

 back ; and five in the loins, the last below the last lumbar verte- 

 bra. The anterior root of these double nerves is smaller than the 

 posterior, and each begins by many filaments, which unite in 

 their passage out. The posterior root forms a ganglion, and the 

 nerve externally to this unites with the anterior nerve. 



The five pairs of the lumbar portion, proceed, enclosed in mem- 

 brane, together with five or six other pairs, from the bulbous 

 extremity of the chord, and pass through the foramina of the 

 sacrum. This splitting of the chord is termed the chorda equina. 

 Besides these, a pair arises at about the seventh or eighth cervical 

 pair, called accessory, running up into the cranium through the 

 foramen magnum, and coming in contact with the pneumonogas- 

 tric nerve ; and it passes out again through the foramen lacerum. 



Many nerves unite : for instance, twigs of the portio dura with 

 twigs of all the branches of the trigeminum ; and twigs of the 

 ninth with the lingual branch of the trigeminum. Many nerves 

 unite to separate again, forming what are termed plexuses ; and 

 the nerves running into and from a plexus may be different in 

 number. (Cut, p. 315.) On some nerves we observe nodules of 

 various shapes, called ganglions ; and sometimes more than one 

 nerve have the same ganglion. We have seen that Gall applies 

 the word ganglion to masses of nervous substance also in the 

 encephalon and spinal chord ; and other anatomists, in a similar 

 manner, apply it to the enlargements of the fifth cerebral nerve 

 and of the posterior spinal nerves. 



Nerves are collections of white filaments contained in delicate 

 membranes, and united into fibres like those of the brain, and 

 all invested with another membrane, called neurilema, which again 

 is enclosed in a firm white membrane. M. Raspail has lately 

 examined them, and finds them to be aggregations of solid cylin- 

 ders, each invested, like muscular fibrils, with a fine membrane, 

 and the whole with a common covering to form a trunk. 1 He de- 

 clares that no tube exists in them, as many have asserted. A 

 1 Nouveau Systfrne, 513. sqq. 



