Its relations in the forearm. —In this part of its course its direction is nearly straight, but with a slight convexity inwards, and it corresponds to the ulna in its whole length. At first lying on the brachialis anticus, by passing downwards and rather inwards, it next comes into contact with the flexor pro fundus digitorum which covers the bone ; and it continbes to lie on it to near the annular ligament. Superficial to the vessel are the skin and fascia of the forearm, together with the first layer of the muscles which oc cupy this situation, or the flexors which pro ceed from the inner condyle ; viz., the pronator radii teres, flexor carpi radialis, palmaris lon gus, and flexor carpi ulnaris, successively. It is overlapped by the outer head and border of the latter muscle during about two-thirds of the forearm, being only uncovered where it becomes tendinous; in this lower part the artery lies external to this tendon, situated between it and the two inner tendons of the flexor sublimis ; structures which vvould form a very easy guide to its locality during life. Although thus uncovered, the artery is by no means so superficial as was the case with the radial in the same stage ; having in front of it fascim of great strength, and being placed in a deep depression, from the coining forward of the tendon of the ulnar flexor to its in sertion in the pisiform bone.
The median nerve, which lay to its inner side on the brachialis anticus, crosses the ulnar artery very soon after the origin of the latter, the point of decussation exactly cor responding to the coronoid origin of the pro nator radii teres, which slip of muscle lying over the vessel, separates the two structures. The ulnar nerve at its inner side above, where it enters the forearm between the condyle and olecranon, is in close contact with it in the lower half of this region, placed somewhat superficially and to its inner side. The or dinary venw comites accompany the vessel.
In the hand.—In this latter part of its course the artery passes over the annular ligament of the wrist, internal both to the pisiform bone and the muscles of the hypothenar eminence ; and next, as the superficial palmar arch, it passes transversely through this part of the hand, crossing superficially to the flexor ten dons and the branches of the median nerve, until it arrives at the point to which we conducted the superficialis volx, and the communicating branch sent upwards from the radialis indicis ; a point nearly at the inner border of the prominent ball of the thumb. Though placed above the tendons and nerves the artery', however, is far from subcutaneous in any part of its progress ; for while on the annular ligament, fibres front the insertion of the flexor carpi ulnaris into the pisiform bone pass outwards over its surface to join that structure external to the vessel lying on it ; a little further downwards, the palmaris brevis, where present, is also directed inwards in front of it ; and during the remainder of its length, the strong palmar fascia effectually shields it from immediate pressure. The ver tical part of the vessel is accompanied by the ulnar nerve, which maintains the relation it had in the lower part of the forearm, and, inferiorly, divides into its digital branches. The arch, like that of the radial artery, pre viously described, is convex downwards, con cave upwards; but it is obvious that its situation is considerably inferior to it, as well as much more superficial. A transverse line
across either the middle of the hand or the centre of the metacarpus would tolerably indicate its position, or one continued across the palm from the forcibly extended thumb might be taken as a more accurate guide to this part of the vessel.
Branches of the ulnar artery. —The first branches of the vessel are two, which usually come off by a common trunk, but are nearly as often separate at their origin. They are called the arterke recurrentes ulnares anterior et posterior, being so named from their taking a recurved course upwards into the arm ; the former in the front of the internal condyle of the humerus, and the latter between it and the olecranon process of the ulna. The an terior recurrent passes upwards from beneath the flexor muscles which cover the artery where it rises, lying on the brachialis anticus, and corresponding to the elbow joint which it partially supplies ; its superior termination inosculates with the lowest or anastomotie branch of the brachial. The posterior re current, having at first sitnilar relations, passes more inwards so as to reach the above in terval, being situated beneath the flexor carpi ulnaris, and meeting the ulnar nerve de scending from the arm between the two heads of this muscle. Here it breaks up, anasto mosing freely with the inferior profunda which has hitherto accompanied the nerve, uniting also by small branches with twigs sent down wards from the superior profunda in the substance of the triceps, and giving many branches to the articulation and the neigh bouring muscles.
The next considerable branch is the arteria interossea, which diverges from the trunk of the vessel a little below the coronoid pro cess, and whilst it is covered by the flexor muscles. Directed downwards from its origin, after a course of about an inch in length, it reaches the interosseous mem brane in the upper part of the interval between the flexor longus pollicis and the flexor profundus digitorum, and here it bi furcates into two branches. One of these, the anterior interosseous, continues on the front of this membrane, lying deeply in the interval between the two muscles and con cealed by them, until, arriving at the pro nator quadratus which lies transversely across the lower extremities of the radius and ulna, it passes under this muscle. At its inferior border it reappears, though much diminished in size, and now situated on the anterior ligament of the wrist, it divides into many small branches, which supply the articulation and anastomose with the anterior carpal twigs from the radial and ulnar vessels. In this course, the branch now described supplies the muscle on each side of it, and usually it gives off' one or two small branches which perforate the iuterosseous membrane beneath it in their passage backwards to the posterior region of the forearm. One of these, by far the largest and the most constant, is fre quently named as " the posterior branch of the anterior interosseous ;" and it escapes to the back of the forearm, through an aper ture which exists in the interosseous mem brane, near its inferior border, and about an inch and a half above the radio-ulnar articu lation.