The Existence of an Adventitious Nerve-supply.—­While not exactly a sequel of the operation, the fact that it is not discovered until after the operation has been performed warrants us in mentioning it here.  It is not an uncommon thing in the lower operation to find that sensation and symptoms of lameness still persist after section of the nerve.  In many cases this has been traced to the existence of an abnormal nerve branch.  In the higher operation this is not so likely to be met with.  That it may occur, however, is shown by the following interesting case related by Harold Sessions, F.R.C.V.S.:[A]

[Footnote A:  Journal of Comparative Pathology and Therapeutics, vol. xii., p. 343.]

’In June of 1898 I saw a hunter suffering from navicular disease.  After carefully examining the leg, I advised the owner to have the operation of neurectomy performed upon him.  This he decided to do, and the horse was sent to me about the beginning of July.

[Illustration:  FIG. 62.—­DISSECTED EXTERNAL METACARPAL NERVE AND BRANCHES. a, Metacarpal; b, anterior plantar; c, extra branch (probably from the internal metacarpal), conveying sensation after division of the external metacarpal.]

’The operation was performed in the ordinary way, without any difficulty whatever.  The wounds healed nicely, but the horse still continued to go lame.  Careful examination showed that there was still sensation on the outside of the foot.  Thinking that possibly there might be two external metacarpal nerves, the horse was again cast, the operation being performed slightly lower down.  Only the main branch of the external metacarpal nerve could be found.  A piece of this was taken out, and the horse let up.  On examination, sensation was still found in the posterior part of the outside of the foot.  It was very evident that there was some abnormal distribution of the nerve, as sensation was still being conveyed to that part of the foot.

’As the horse was absolutely useless, and would have to be shot unless this piece of nerve could be found, he was again thrown, and after he had been anaesthetized I determined to follow the course of the nerve down, until I found where the accessory branch came from.  This I found a little below the fetlock, about 1/2 inch below the point where the anterior plantar nerve is given off from the metacarpal nerve.  It was about 1/2 inch below the spot where the anterior plantar nerve passes between the artery and vein of the foot, and it was somewhat difficult to get at it.

’Fig. 62 shows the exact size and distribution of the nerves.  After the separation of the accessory branch, sensation was taken from the foot, and the horse went perfectly sound.’