Page:Natural History Review (1861).djvu/189

Rh 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 property 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 sensitive 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 structure. The conclusion, therefore, that they must be identical in property seemed to me self-evident, the more so as, to guard against superficial objections, I added the qualification, "under similar conditions." 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." We 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 contract on being stimulated. The nerve also retains its property (Neurility), and on being stimulated, it will excite the muscles to contraction. We know that muscles will contract without the intervention of nerves,