562 ANTERIOR AND POSTERIOR ROOTS OF THE SPINAL NERVE. 



bands is pierced with a great number of openings, through which 

 the vessels run from the pia mater into the cineritious nucleus. 

 The medulla spinalis is divided, as we have seen, into two 

 symmetrical halves, by the anterior and posterior median 

 fissures. Each half of the cord has connected with it upon 

 the side, thirty-one pair of nerves, called the spinal nerves ; 

 each nerve communicates Vith the medulla, (or, as is more 

 commonly said, arises from it,) by two roots, called the anterior 

 and posterior roots of the spinal nerves. Viewing the spinal 

 cord and nerves in connexion with their functions, it would be 

 more correct to say, that the anterior roots only arise from the 

 cord and run outward, and that the posterior terminate in the 

 cord ; for it has been satisfactorily proved, as has been before ob- 

 served, that the anterior roots are the nerves of motion, that is, 

 conductors of the will to the voluntary muscles, and that the 

 posterior are the nerves of sensation, conveying the impressions 

 from without, through the medium of the spinal marrow to 

 the brain, which is the seat of consciousness. 

 The fasciculi of the posterior roots are much larger than 

 those of the anterior. They pass through a small oblong 

 ganglion, just before their connexion with the cord, while 

 upon the anterior roots there is no ganglion. Upon the outer 

 side of the ganglion the filaments of both roots are inextricably 

 interwoven. The fasciculi of both roots of the different spinal 

 nerves, form flattened pyramids, at the base of which they 

 communicate with the medulla spinalis, and the pyramids 

 formed by each root nearly touch each other at their upper 

 and lower margin, throughout the whole length of the spine. 

 The anterior arise from a superficial fissure on the external 

 surface of the medulla, which extends throughout its whole 

 length, and is exactly opposite the termination of the anterior 

 horn of cineritious matter. This is called the anterior lateral 

 fissure. Between the bottom of this fissure and the anterior 

 horn of cineritious matter, there is nothing but a very thin 

 lamen of medullary matter. 

 The connexion of the posterior roots, is by a fissure much 



