LEWES ON THE SENSORY AND MOTOR FUNCTIONS OF NERVES. 177 



or use ? Flexor, extensor, and sphincter muscles are not held to be 

 different kinds, but the same kind applied to different uses ; the peculiar 

 property of muscles, Contractility, is found in all three, but this pro- 

 perty is put to different uses, when the anatomical connexions of the 

 muscles differ. In like manner, as I conceive, the nerves which are 

 distributed to sensory surfaces, to muscles, and to glands, are all of the 

 same kind, and have the same properties ; but differ in their uses, as 

 they are different in their anatomical connexions. 



The champion of Bell must prove, first, that the motor nerve is of 

 such a structure as to be incapable of transmitting a stimulus to a sensi- 

 tive centre ; or he must prove, secondly, that its anatomical distribution 

 is such that no sensitive centre can be reached by it. One of these two 

 conclusions must be established. No other alternative is possible. Let 

 us examine both points. 



It is admitted that motor and sensory nerves are of the same struc- 

 ture. The conclusion, therefore, that they must be identical in pro- 

 perty seemed to me self-evident, the more so as, to guard against 

 superficial objections, I added the qualification, "under similar condi- 

 tions." If any one chooses to maintain that two substances identical 

 in structure, under identical conditions, can have different properties, I 

 must leave him to the indulgence of his "right of private judgment;" 

 meanwhile, the axiom remains, that identity of structure implies identity 

 of property. There is, indeed, another avenue of attack. It has been 

 said, " You do not know that the two nerves are identical; there may 

 be differences important, yet too minute for appreciation at present. 

 Of two bars of steel, one may have a magnetic property, and the other 

 none, yet you cannot on mere inspection detect any difference in their 

 structure. Of two tissues, one may be dead, and the other living, yet 

 you cannot detect a difference." AVe do not know that two muscles, or 

 two secreting cells, may not be so different in structure as to have 

 different properties ; but until this is proved, we are not warranted in 

 assuming it. All we do know of the nerves points to their identity : 

 they have the same physical, electrical, and chemical properties ; and, 

 till the contrary be proved, we must assume them to be identical in all 

 their properties. The two bars of steel have precisely similar properties, 

 according to their similarities of structure ; but, over and above these, one 

 of them possesses a magnetic property, in consequence of its having been 

 submitted to peculiar conditions ; under the like conditions, the second 

 bar of steel would be magnetic. The same may be said of dead and 

 living tissues; they have in common, structure and property, and as long 

 as they are under similar conditions there .will be no difference between 

 them ; but under the group of conditions known as " life" and " death," 

 there will of course be differences. 



Cut off the leg of a frog, and resect its sciatic nerve, the muscles 

 will for some time retain their property (Contractility), and will con- 

 tract on being stimulated. The nerve also retains its property (]S r eu- 

 rility), and on being stimulated, it will excite the muscles to contraction. 

 We know that muscles will contract without the intervention of nerves, 



VOL. I. — N. H. R. 2 A 



