others, is not a muscle-pain; it is produced by pressure on the nerves of the muscle-sheath, or of some neighbouring nerve.[1] And I have proved that no sign of pain is elicited by burning or pricking the muscle itself.
Now, if we are justified in attributing muscular sensibility to the anterior nerves, it is obvious that these nerves, when irritated, can only excite muscular sensations—no others. It is further obvious that the signs of such sensations must be very different from those of other sensations. Irritation of the root can only produce that sensation which precedes or accompanies adjustment of the muscles, or one of the vague diffusive sensations which the muscles contribute to the general consciousness. The direct response to such a sensation would be an adjustment of the muscles to which the particular nerves were sent; but this cannot take place, because the connexion between the nerves and the muscles is cut off. None, therefore, of the ordinary signs could be manifested. But is this a proof that the muscle-nerves are not sensory? To say so, would be to say that the optic nerve is not sensory, because it may be divided without the animal's manifesting any sign.
A few paragraphs back, allusion was made to the fact, that one of the nerves of the fifth pair has an "insensible branch"—will not transmit sensitive impressions (as the phrase is usually understood) more than a motor nerve will. Claude Bernard says: "Examinant chez le chien le nerf nasopalatin qui va à la membrane muqueuse du nez nous avons eté très surpris de le trouver en apparence complétement insensible, tandis que la branche principale, la sous orbitaire, nous offrait tous les signes d'une sensibilité vive." Here are two branches of the same nerve (both sensory), yet one of them completely without response to the stimulus which excited the others. Did Bernard thence conclude that the naso-palatine was not sensory? By no means. "Cette insensibilité d'un rameau appartenant à la cinquiéme paire porterait à penser qu'elle renferme des filet de sensibilité speciale; Majendie ayant prouvé que les nerfs de sensations speciales sont complétement insensibles aux irritations mécaniques."[2]
That all nerves may be, and that most are, double in function, the muscle-nerve being predominantly motor, because distributed to motor organs, whereas the skin-nerve is only distributed to the minute muscles of the skin, may be inferred from the anatomy of the invertebrata, in whom no double roots exist. Let us examine the ventral chord of a bee. From each ganglion one nerve-trunk issues, to supply both skin and muscles of each side. The first time I made a preparation of the bee's nervous system, I was forcibly arrested by this unity of motor and sensory nerves. But as I then believed in the classical doctrine, the explanation quickly suggested itself that, in the bee, there had not yet taken place that specialization into motor and sensory, which was found in vertebrata. True enough: but what is the specialization? Is it the introduction of a new kind of nerve, or only the assignment of one
