478 SIR CHARLES BELL ON THE BRAIN. 



The origin of the posterior roots of the Spinal Nerves, and their relation to the decus- 

 sation of the Posterior Column. 



The brain beings before us so as to present its posterior aspect, and the back part 

 of the spinal marrow, we raise the cerebellum and tear the pia mater, so as to expose 

 the fourth ventricle. We may divide the processes of the cerebellum and take that 

 body away. Having the parts thus prepared, we attend more particularly to the poste- 

 rior series of roots of nerves which run towards the uppermost spinal nerve. 



If we trace the line where the posterior roots of the spinal nerves arise, we find 

 that the posterior columns of the spinal marrow are behind these roots ; and if we 

 trace these posterior columns upwards, we see them diverging under the name of 

 corpora restiformia to the cerebellum. We strike a level by following the posterior 

 roots of the spinal nerves into the spinal marrow. In doing this we shall find it 

 necessary to lift the posterior column, and then, being able to trace the roots of the 

 nerves, we shall find them connected with a course of longitudinal filaments ; and 

 these, on further investigation, will be found to be continued from the point imme- 

 diately below the decussation of the posterior column of sensation, which I have de- 

 scribed above (Plate XX. fig. 2. c). 



Thus it will be found, that the posterior roots of the first, and consequently of all 

 the spinal nerves, are derived from that posterior column which descended from the 

 posterior division of the crus cerebri^ and that they are thus placed in the same re- 

 lation as the anterior roots with respect to the decussation of the prolonged medullary 

 matter of the cerebrum. 



The origin of the sensitive root of the Fifth Nerve, and its relation to the Spinal Marrow. 



In former papers I have proved the fifth nerve of the head, according to the ar- 

 rangement of Willis, to be the nerve of sensation to the head and face, thus distin- 

 guishing it from the nine nerves of the encephalon, and from the appropriate nerves 

 of the senses to the nose, and eye, and ear. 



I gave my reasons, at the same time, for distinguishing it as the nerve of mastica- 

 tion, and showed, in short, that it had all the characteristics of a spinal nerve. It 

 becomes now a subject of interest to observe in what respect it further resembles the 

 spinal nerves, and to inquire how its relations with the brain are formed. It is a hap- 

 piness in this inquiry, that although it be diflicult to trace the motor roots of nerves, 

 owing to the delicacy of their connexions with the brain, the sensitive root is followed 

 with ease into the brain or spinal marrow. 



We commence the dissection of the fifth nerve by distinguishing its grand divisions 

 as they emerge from the side of the pons, separated by a transverse band of fibres 

 (Plate XX. fig. 6, 7.)- 



Leaving, for the present, the scattered roots of the motor portion which pass between 

 the transverse cords of the pons, we shall proceed to follow the other in a retrograde 



