146 THE SKELETON. 



borders of the shaft of the bone. In the recent state it is covered with a thin 

 layer of cartilage, lined by a prolongation of the synovial membrane of the 

 shoulder-joint, and receives the tendon of insertion of the Latissimus dorsi muscle. 

 The Shaft of the humerus is almost cylindrical in the upper half of its extent, 

 prismatic and flattened below, and presents three borders and three surfaces for 



examination. 



The anterior border runs from the front of the great tuberosity above to the' 

 coronoid depression below, separating the internal from the external surface. Its 

 upper part is very prominent and rough, and forms the outer lip of the bicipital 

 groove. It is sometimes called the anterior bicipital or pectoral ridge, and serves 

 for the attachment of the tendon of the Pectoralis major. About its centre it 

 forms the anterior boundary of the rough deltoid impression ; below, it is smooth 

 and rounded, affording attachment to the Brachialis anticus. 



The external border runs from the back part of the greater tuberosity to the 

 external condyle, and separates the external from the posterior surface. It is 

 rounded and indistinctly marked in its upper half, serving for the attachment of 

 the lower part of the insertion of the Teres minor, and below this of the external 

 head of the Triceps muscle ; its centre is traversed by a broad but shallow, oblique 

 depression, the musculo-spiral groove ; its lower part is marked by a prominent, 

 rough margin, a little curved from behind forward, the external supracondylar 

 ridge, which presents an anterior lip for the attachment of the Supinator longus 

 above and Extensor carpi radialis longior below, a posterior lip for the Triceps, 

 and an intermediate space for the attachment of the external intermuscular septum. 



The internal border extends from the lesser tuberosity to the internal condyle. Its 

 upper third is marked by a prominent ridge, forming the posterior lip of the bicipital 

 groove, and gives attachment to the tendon of the Teres major. About its centre is 

 an impression for the attachment of the Coraco-brachialis, and just below this is seen 

 the entrance of the nutrient canal, directed downward. Sometimes there is a second 

 canal, situated at the commencement of the musculo-spiral groove, for a nutrient 

 artery derived from the superior profunda branch of the brachial artery. The 

 inferior third of this border is raised into a slight ridge, the internal supracondylar 

 ridge, which becomes very prominent below ; it presents an anterior lip for the attach- 

 ment of the Brachialis anticus, a posterior lip for the internal head of the Triceps, 

 and an intermediate space for the attachment of the internal intermuscular septum. 



The external surface is directed outward above, where it is smooth, rounded, 

 and covered by the Deltoid muscle ; forward and outward below, where it is 

 slightly concave from above downward, and gives origin to part of the Brachialis 

 anticus muscle. About the middle of this surface is seen a rough, triangular 

 impression for the insertion of the Deltoid muscle ; and below it the musculo-spiral 

 groove, directed obliquely from behind, forward and downward, and transmitting 

 the musculo-spiral nerve and superior profunda artery. 



The internal surface, less extensive than the external, is directed inward above, 

 forward and inward below ; at its upper part it is narrow and forms the floor of 

 the bicipital groove : to it is attached the Latissimus dorsi. The middle part of 

 this surface is slightly rough for the attachment of some of the fibres of the tendon 

 of insertion of the Coraco-brachialis ; its lower part is smooth, concave from above 

 downward, and gives attachment to the Brachialis anticus muscle. 1 



1 A small, hook-shaped process of bone, the supracondylar process, varying from T \y to f of an inch 

 in length, is not infrequently found projecting from the inner surface of the shaft of the humerus two 

 inches above the internal condyle. It is curved downward, forward, and inward, and its pointed ex- 

 tremity is connected to the internal border, just above the inner condyle, by a ligament or fibrous 

 band, which gives origin to a portion of the Pronator radii teres ; through the arch completed by this 

 fibrous band the median nerve and brachial artery pass when these structures deviate from "their 

 usual course. Sometimes the nerve alone is transmitted through it, or the nerve may be accompanied 

 by the ulnar artery in cases of high division of the brachial. A well-marked groove is usually found 

 behind the process in which the nerve and artery are lodged. This space is analogous to the" supra- 

 condyloid foramen in many animals, and probably serves in them to protect the nerve and artery 

 from compression during the contraction of the muscles in this region. A detailed account of this 

 process is given by Dr. Strnthers, in his Anatomical and Physiological Observations, p. 202. An acces- 

 sory portion of the Coracobrachialis muscle is frequently connected with this process, according to 

 Mr. J. Wood (Journal of Anat. and P/w/.s.,'No. 1, Nov., 1866, p. 47). 



