The fourth digital nerve passes in front of the second interosseous space, gives a filament to the second lumbrical muscle, and about the middle of this space divides into two branches, which are directed along the op posed sides of the middle and index fingers. The Afth passes downwards and slightly in wards in front of the third metacarpal space, gives a filament to the third lumbricus, com municates by a delicate filament with the ulnar, and at the middle of this space ter minates in two branches for the opposed sides of the middle and ring finger. The termina tion of the divisions of the fourth and fifth digital nerves, and the branches given off from them, are exactly similar in distribution to the third digital nerve, giving off, like it, on the proximal and distal phalanx, a dorsal branch. Each of the digital nerves, although running along the sides of the fingers, and giving off in their course numerous cutaneous filaments, which are directed towards the axes of the fingers, are not observed to anastotnose with each other.
The median nerve in the palm of the hand is situated on a plane anterior to all the flexor tendons, and the trunk before dividing is situated half an inch or more above the level of the superficial palmar arch of arteries which crosses in front of its three internal branches. The accompanying digital arteries are placed somewhat behind, and further from the longitudinal axes of the fingers than the nerves, which, however, in their course send numerous small filaments which wind around them.
The ulnar nerve, somewhat smaller than the median, arises from a trunk common to it, the internal cutaneous and the inner head of the median. The first dorsal and last cer vical are consequently principally engaged in forming it. Almost immediately after its origin it is directed slightly inwards and out wards from the median, and behind the in ternal cutaneous, and at the lower part of the axilla appears deeply seated at the inner aspect of the arm, being directed in front of the triceps extensor muscle. Below the level of the coraco-brachialis it perforates the in ternal intermuscular septum, and becomes surrounded by several fasciculi, derived from the inner head of the triceps, and passes be hind the interinuscular septum to gain the space between the internal condyle and ole cranon, being here situated between the two heads of' the flexor carpi ulnaris. It now in clines downwards and slightly outwards along the inner part of the coronoid process of the ulna, and then takes a vertical course down the fore arm, covered over by the flexor carpi ulnaris, and between it and the flexor digi torum profundus. It gradually inclines to the surface, and at the lower third of the fore arm becomes sub-aponeurctic, and passes from between the flexor carpi ulnaris and inner tendon of the flexor sublimis to the lower part of the anterior surface of .the
annular ligament, passing along it in a distinct sheath with the artery, in close contact with, and external to, the pisiform and unciform bones, and divides into its terminal branches. In the upper part of the arm the ulnar nerve is in relation with the axillary artery, which is placed between it and the median, nearer however the latter. In the upper part of the fore-arm it is about half an inch or more dis tant from the artery, but gradually inclines, so as to come in close relation with, but internal to it, in the two lower thirds of the fore-arm, and in the palm of the hand.
The ulnar gives off no branches in the arm ; and the first that comes off from it, is when the nerve is placed between the two heads of the flexor carpi ulnaris. There are several small articular filaments which enter the inner part of the joint, and three or four which are distributed to the above muscle. In the upper third of the fore-arm some filaments are again given off' to the flexor carpi ulnaris, and others for the supply of the inner half of the flexor digitorum profundus. About the mid dle a small branch is given off, which, after sending satellite filaments to accornpany the ulnar artery, perforates the fascia, and be comes cutaneous to communicate with the internal cutaneous. The largest branch, how ever, given off from the ulnar, comes away about two inches above the wrist-joint, and is named, its dorsal branch (dorsalis carpi ulna ris : internal dorsal nerve). This winds down wards and inwards, and having passed be tween the tendon of the flexor carpi ulnaris and the bone, perforates the fascia at the back of the fore-arm, and becomes cutaneous a little above the styloid process. It runs then along the inner edge of the carpus ; and on the posterior annular ligament terminates in two branches. The inner branch passes alone the inner and back part of the metacarpal bone, and phalanges of the little finger, supply ing the integument as far as its extremity, and sending in its course some small filaments to the abductor minimi digiti. The outer branch crosses obliquely the tendon of the extensor ininimi digiti, and on the fourth interosseous space sub-divides. The inner sub-division at the extremity of the space bifurcates in order to supply the opposed sides of the little and ring finger. The outer sub-division at the lower extremity of the third interosseous space having communicated with the dorsal branch of the radial, similarly bifurcates for the supply of the integument of the opposed sides of the middle and ring finger. The dorsalis carpi ulnaris, independently of the above branches, sends numerous filaments to the inner and back part of the wrist and hand, and communicates above with the external or posterior cutaneous.