Arm

Arm, Anatomy of. The arm is divided into the upper arm, the fore-arm, wrist, and hand. The bone of the upper arm is called the humerus; its head or upper extremity articulates with the scapula, forming the shoulder-joint; the lower extremity of the humerus articulates with the radius and ulna, the two bones which form the framework of the fore-arm. In the neighbourhood of the wrist are the eight carpal bones, then follow the five metacarpal bones, corresponding to the four fingers and thumb; and finally the phalanges complete the series of bones of the upper extremity. To each finger there are three phalangeal bones, but the thumb has only two. Movement at the shoulder-joint is very free, and dislocation of the shoulder is, in correspondence with this fact, one of the most common forms of dislocation. The rounded prominence of the shoulder is mainly formed by the deltoid muscle, the action of which is to raise the arm; the anterior fold of the armpit is formed by the pectoralis major muscle, which draws the arm across the chest, The fore-arm is bent or flexed on the upper arm by means of the biceps. The elbow is a true hinge joint, only permitting of movements of flexion and extension, forming thus a marked contrast to the shoulder. The rotation of the radius upon the ulna permits of the rotation of the hand upon the fore-arm, or of pronation and supination, as it is called; the position of pronation being that in which the palm is downwards, while in supination the back of the hand faces downwards. In addition to this the hand can be flexed or extended by movement at the wrist joint. The main artery of the arm is the axillary or brachial, as it is called, after reaching the lower fold of the axilla (armpit). The brachial divides into the radial and ulnar arteries; the radial artery at the wrist lies quite superficially, and pulsation in it being so readily felt, it is the vessel always examined in observing the arterial pulse. In the days of bleeding, the vein which was commonly operated upon was the medium basilic, which lies just in front of the elbow. The nerves of the arm come from the spinal cord, and are grouped together, forming what is called the brachial plexus before they divide into special trunks. Finally, three great branches, the musculo-spinal. median and ulnar nerves are formed, as well as other smaller ones. The ulnar nerve lies just underneath the skin, behind the lower and inner process or condyle of the humerus, and pressure there causes the well-known tingling in the course of distribution of that nerve. The arm is well supplied with lymphatic vessels, which convey the lymph upwards and finally empty it into the great veins.