Aponeurosis. — Beneath the subcutaneous layer of cellular tissue lies the aponeurosis or fascia, which invests the muscles and the deep seated vessels and nerves of the arm : this fascia commences above at the superior attach ment of the deltoid muscle ; externally and internally it is continuous with the fascia, which extends over the axillary space; descend ing along the arm it is strengthened by ex pansions which it receives from the tendons of the deltoid, pectoralis major, and coraco brachialis in front ; and behind it derives an accession of strength from the aponeurosis which covers the infra-spinatus and teres minor, and from the tendons of the latissimus dorsi and teres major. At the lower part of the arm the fascia is attached to the condyles of the humerus ; laterally and posteriorly it is at tached to the olecranon, on either side of which it is continuous with the fascia on the posterior of the fore-arm. In front of the elbow this fascia receives a fasciculus of fibres from the tendon of the biceps, and becomes continuous with the fascia covering the anterior surface of the fore-arm. The fascia of the arm varies in strength in different parts ; it is very indistinct over the deltoid, thin but very fibrous on the posterior surface of the arm where it covers the triceps ; it is much stronger over the biceps, and the thickest portion of it is found along the inner edge of the arm, where it covers the brachial artery and its accompanying veins and nerves. A strong aponeurotic septum passes in from the fascia of the arm to each of the lateral ridges of the humerus ; these septa are called intermuscular ligaments, and, together with the humerus, divide the space included within the general fascia into an anterior and a posterior sheath ; the external intermuscular ligament extends from the insertion of the deltoid to the external condyle ; the internal extends from the inser tion of the coraco-brachialis to the internal condyle. Both intermuscular ligaments are narrowest above, and grow broader as they approach the condyles : their surfaces give at tachment to fibres of the triceps posteriorly and to the brachizeus anticus, supinator radii longus, and extensor carpi radialis anteriorly. The posterior sheath;formed as above described, is chiefly occupied by the triceps muscle, be neath which, in the spiral groove on the pos terior surface of the humerus, lie the musculo spiral or radial nerve and the superior pro funda artery : this nerve and the anterior or musculo-spiral branch of the superior pro funda artery perforate the external inter muscular ligament and enter the anterior sheath of the arm to get between the brachizeus anticus and supinator radii longus, while the posterior branch of the profunda descends within the posterior sheath to the back part of the external condyle ; the ulnar nerve and the inferior protunda artery enter this posterior sheath together at its internal side, about the middle of the arm, and descend within it to the back of the internal condyle. A considerable branch of the brachial artery, the ramus alias tomoticus, perforates the internal intermuscular ligament above the internal condyle, and enters the lower part of the posterior brachial sheath. The anterior sheath of the arm contains the biceps, coraco-brach ial is, brach jeans anticus, and the origins of the long supinator and long radial extensor muscles; the external cutane ous nerve traverses this sheath, perforating the coraco-brachialis above, and descending ob liquely outwards between the brachiwus anticus and the biceps it gets to the outer side of the latter, between the tendon of which and the supinator radii longus it pursues its course to the fore-arm ; the radial nerve and the branch of the superior profunda artery accompanying it are to be found in the lower and external part of the anterior sheath, which they enter as above described : these lie deep between the brachizeus anticus and supinator longus. Along the internal side of this anterior sheath, through its whole extent, run the brachial artery, and its two venw comites included in a sheath proper to them, and accompanied by the median nerve, which has very important relations to these vessels : this nerve is external to the artery above, crosses it in the middle of the arm, and lies internal to it below. Superiorly the ulnar nerve lies to the inner side of the brachial artery, from which it se parates to enter the posterior sheath, as already noticed ; the internal cutaneous nerve, the cutaneous twig of the ulnar nerve, and the basilic vein for a short part of its course before it enters the brachial vein, also lie within this sheath ; and deeply situated in its lower part is the ramus anastomoticus magnus of the brachial artery.
In the progressive deve lopement of the upper extremity in the foetus, the arm is formed subsequently to the hand and fore-arm, and at an earlier period than the shoulder. In men the deltoid is fuller, and the biceps in front and the triceps behind are more prominent than in women : the greater fulness of these two latter muscles, with the smaller quantity of subcutaneous fat, give to the male arm a greater diameter from before backwards than in the transverse direction ; while the more slender character of the muscles and the greater abundance of subcutaneous fat laterally cause the arm of the female to assume a more rounded form. In the course of the brachial artery two trunks are often found to exist, in consequence of a high branching of that vessel, which sometimes occurs even at the lower border of the axilla.: the supernumerary branch in such cases is most frequently the radial : in some instances it is the ulnar and less fre quently the interosseous or median artery of the fore-arm. When this irregularity occurs, the brachial artery usually preserves its ordinary relations to the surrounding parts, while the supernumerary trunk lies to its internal side and takes a more superficial course, some times getting above the fascia of the arm, as we witnessed in a few rare cases. It occasionally happens that the brachial artery divides at its commencement into two trunks, which again unite at its lower part. It is ob vious that the surgeon, in performing operations on this artery, should constantly bear in mind that it is subject to the above-mentioned irre gularities, and that he should cautiously guard against committing the error of including the wrong vessel in his ligature.
The internal side of the arm in the middle of its length is the most eligible place for making compression on the brachial artery ; here this vessel is superficial, so that its pul sation can be felt at once, whilst it has nothing interposed between it and the bone but the tendinous insertion of the coraco-brachialis muscle. It happens, however, that the median nerve lies immediately over the artery in this situation, a circumstance which causes com pression of the latter to be attended with con siderable pain, and productive of injury to the nerve if maintained for too great a length of time.
As the trunk of the brachial artery and several large nerves traverse this part of the arm, it is obvious that wounds in this region are liable to be attended with more serious consequences than those of any other part of the arm. A wound in the posterior region of the arm may be attended with considerable haemorrhage, if it should happen to penetrate so deep as to divide the profunda artery, or it may cause paralysis of the extensor muscles of the hand and fingers by dividing the radial nerve.
When the humerus is fractured, the con sequent derangement of the fragments varies according to the part at which the bone hap pens to be broken ; when fracture occurs im mediately above the insertions of the pectoralis major and latissimus dorsi, the lower fragment is brought inward towards the axilla by the action of these muscles, and drawn upwards by the action of the deltoid, biceps, coraco brachialis, and long head of the triceps, whilst the extremity of the upper fragment is rather turned outwards by the supra-spinatus. In cases where the humerus is fractured imme diately above the insertion of the deltoid and below the attachments of the latissimus dorsi and pectoralis major, the deltoid will draw the lower fragment upwards and outwards, whilst the upper fragment will be drawn inwards towards the axilla by the pectoralis major and latissimus dorsi. If the bone be broken im mediately below the insertion of the deltoid, little or no displacement of the fragments may ensue, as the opposing forces exercised on the superior fragment by the deltoid on the ex ternal side, and the pectoralis major and latis simus dorsi on the internal, pretty nearly counterbalance each other; it more generally happens, however, that the upper fragment is turned outwards by the preponderating action of the deltoid upon it, whilst the lower frag ment is drawn upwards by the action of the biceps, coraco-brachialis, and triceps. Frac tures of that portion of the humerus which is covered by the brachheus anticus in front and the triceps behind, are often unattended by any very obvious displacement, in consequence of these muscles being inserted into both frag ments ; fractures near the elbow are occa sionally followed by deformities presenting some of the characters of dislocations of the elbow, of which more notice will be taken in the article ELBOW.