The bicipital groove commences above be tween the two tuberosities, and passes down wards and slightly inwards, bounded before and behind by the spines which proceed from those tubercles. This groove, very distinct at its commencement, ceases to be so a little above the termination of the superior third ; in the recent state it is lined by the tendinous expansion of the latissimus dorsi and teres major muscles, and lodges the tendon of the biceps muscle, whence its name.
From the anatomical neck the bone gra dually tapers down and becomes more cylin drical in its form ; this upper portion is, for the convenience of description, distinguished by the name of surgical neck of the humerus. The middle third of the shaft of the bone is prismatic in form ; the external spine which commences at the greater tuberosity is continued down, forming a prominent ridge all down the front of the bone to the termination of its flattened inferior third. The outer part of the middle third of the humerus is remarkable for the rough surface into which the deltoid muscle is inserted, the deltoid ridge, situated nearer the upper than the lower part of this portion, and directed downwards and very slightly forwards. The inner part of the middle third presents a smooth, flattened, and inclined surface, which is continued down in this form to within a very short distance of the inferior extremity of the bone. The posterior surface is rounded and very smooth.
At the junction of the middle and inferior thirds we notice a very slight and superficial groove passing downwards and inwards, and very much resembling what one would ima gine might be produced by an attempt to twist the bone while yet in a yielding condition, the inferior third having been twisted inwards and the two superior thirds outwards. This groove indicates the spiral course from above down wards and from without inwards of the musculo spiral or radial nerve. Below this groove is the inferior third of the humerus, the anatomical characters of which are very distinct from those of the remaining parts of the bone. A pro minent and rounded ridge, continuous with that already noticed in connexion with the greater tuberosity, passes vertically down in front of it; from each side of this ridge a smooth surface inclines backwards, forming an inclined plane on each side of it, the ex ternal being larger and more distinct than the internal.
The posterior surface of the upper part of this portion is flat and very smooth. As the bone descends it expands considerably late rally, so as to present in front a broad surface slightly convex from side to side, bounded on either side by prominent edges, continued from the edges of the inclined planes above de scribed. Each edge terminates in a pro minence, the inner one being the largest ; the inner edge itself being thicker, more pro minent, and describing a slight curve as it descends. The posterior surface is limited below by a deep depression, to be further de scribed hereafter. Thus, by its gradual expan sion laterally, the inferior portion of the hu merus, being about one fifth of the entire length of the bone, has a triangular figure, the base being formed by the inferior articular ex tremity of the bone.
The whole shaft of the humerus is com pletely clothed with muscle. We have already indicated the place of insertion of the deltoid muscle on the outer surface of the bone; all that portion of the outer and anterior surface below the deltnid ridge, and for a little way on each side of its inferior extremity, is co vered by the brachimus anticus muscle. In ternal to the bicipital groove, on the inner surface of the humerus, about its middle, the coraco-brachialis muscle is inserted. The ex ternal edge below the spiral groove affords attachment to the brachiaus anticus, supinator longus, extensor carpi radialis longior, and the triceps muscles.
The internal edge below the insertion of the coraco-brachialis has the brachizeus anticus and triceps muscles inserted into it, and both edges afford insertion to intermuscular apo neuroses, which separate the muscles con nected with the anterior from those on the posterior part of the bone. The posterior sur face is completely covered by the triceps mus cle, excepting in the line which corresponds to the groove already referred to, in which the radial nerve and musculo-spiral artery pass.
The foramen for the nutritious artery is found upon the internal surface at the inferior ex tremity of its middle third ; the direction of the canal is downwards; sometimes this fora men exists upon the external, or upon the in ternal surface.