92 ANATOMY AND PHYSIOLOGY. 



retains perfect sensation in it, or where the reverse of this happens 

 that is, where the power of motion remains while sensation is 

 gone. Of such cases, the following may be taken as an example : 

 Francisco Caasario, living at Rio Janeiro, fell from a scaffold twenty 

 feet high. On recovering from the shock, it was found that his left 

 side, from the shoulder downwards, was deprived of all power of 

 motion, but that sensation remained in it ; whereas, on the right 

 side, his powers of motion were perfect, but sensation was then and 

 afterwards so completely gone, that a lancet might be thrust deep 

 into the flesh without giving him the slightest pain. From the mid- 

 dle of the neck upwards, motion and sensation on both sides were 

 uninjured, and the line of demarcation was so exactly drawn, that 

 it might be defined by a thread surrounding; the neck. 



o J O 



Now, of such cases as the above, Sir Charles Bell's experiments 

 afford a most satisfactory explanation ; for though a limb is de- 

 prived both* of the power of motion and sensation by dividing the 

 spinal nerves that go to it, Sir Charles Bell showed, that by tracing 

 these nerves to their origin, they are each found to be composed of 

 two parts, one of which comes from the anterior column of the 

 spinal cord, while the other comes from the posterior column, and, 

 as represented, has always a small ganglion or swelling on it. He 

 further showed, that if the anterior root be cut, the power of motion 

 in the part supplied by the nerve is extinguished, as is also sensa- 

 tion, by dividing its posterior root. In his experiments, when the 

 posterior or sensitive roots of the nerve in a newly killed animal 

 were irritated with a sharp instrument, no effect was produced ; 

 but when the anterior or motive roots were irritated, the parts of 

 the body to which these nerves went, were thrown into convulsions. 

 An ass was killed, and immediately the motive nerve which sup- 

 plies the muscles of the jaws was irritated. The muscles con- 

 tracted strongly, and closed the jaw with a snap ; but when the 

 same nerve was divided in a living animal, the jaw fell relaxed. 



These explanations will render intelligible the account we shall 

 now give of the different nerves derived from the brain and spinal 

 cord.* They come off in pairs, as represented in Fig. 33. Figure 

 32 shows, as already mentioned, a longitudinal section of the brain 

 and medulla oblongata. No. 1 (in both Figures) is the 1st or olfac- 

 tory nerve, which goes to the nose, and gives the sense of smell ; 



* The derivation of the nerves from the brain is considered only apparent, many 

 physiologists believing that they can be traced to the spinal cord. 



