Anatomy of the humerus

The humerus is designed to be round so that it is less susceptible to damage. Its upper end is convex and fits into the cavity of the scapula, forming a weak, not very tight joint. Due to the weakness of this joint, dislocations often occur with it, and the benefit of such weakness is due to two factors: necessity and safety. As for necessity, it is the need to move freely in any direction, but as for safety, the fact is that although the humerus needs the ability to make various movements in different directions, these movements are not too numerous and do not occur constantly, so there is nothing to be afraid of ligament rupture. Instead, the humerus most often remains stationary while the rest of the arm moves. Therefore, the other joints of the arm are strengthened to a greater extent than the joint of the shoulder.

Four ligaments surround the shoulder joint. One of them, expanding and shell-like, covers the joint, like the ligaments of other joints. Two other ligaments descend from the perforated process; one of them expands at the end and covers the end of the humerus, and the second, larger and harder than the first, descends along with the fourth ligament, which also descends from the “coracoid” process into the groove prepared for them. These ligaments are quite wide in shape, especially at the point of contact with the humerus; they run along the inside of the shoulder and come into contact with the muscles lining the back of the humerus. The humerus is concave towards the inside and convex towards the outside in order to cover the muscles, nerves and blood vessels located on it, so that a person can hold well what he holds under the arm, and so that one hand can be directed well towards the other.

As for the lower extremity of the humerus, two processes adjacent to each other are attached to it. The one adjacent to the inner side is longer, thinner and has no articulation with anything; it is only protection for muscles and blood vessels. As for the process adjacent to the outer side of the humerus, thanks to it and the fossa in it, the elbow joint is formed in the same way as will be discussed below. Between these processes there is, of necessity, a groove with dimples at both ends; the upper dimple faces forward and the lower dimple faces backward. The inner, upper dimple is even and smooth, without partitions; external dimple - the larger of the two in the part that is adjacent to the internal dimple, not smooth and not roundly deepened, but, on the contrary, like a vertical wall, so that when the process of the forearm moves outward in the dimple, it stops when it reaches this wall . We'll explain why this is necessary shortly. Hippocrates calls these two pits “thresholds.”