HTJMERUS. 209 



The Shaft of the humerus is almost cylindrical in the upper half of its ex- 

 tent, 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, forms the outer lip of the bicipital 

 groove, and serves for the attachment of the tendon of the Pectoralis Major. 

 About its centre is seen 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 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 forwards, 

 which presents an anterior lip for the attachment of the Supinator Longus 

 above and Extensor Carpi Eadialis Longior below, a posterior lip for the 

 Triceps, and an interstice for the attachment of the external intermuscular 

 aponeurosis. 



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

 Its upper third is marked by a prominent ridge, forming the inner lip of the 

 bicipital groove, and gives attachment from above downwards to the tendons of 

 the Latissimus Dorsi, Teres Major, and part of the origin of the inner head of 

 the Triceps. About its centre is a rough ridge for the attachment of the Coraco- 

 brachialis, and just below this is seen the entrance of the nutrient canal directed 

 downwards. Sometimes there is a second canal higher up, which takes a simi- 

 lar direction. The inferior third of this border is raised into a slight ridge, 

 which becomes very prominent below; it presents an anterior lip for the 

 attachment of the Brachialis Anticus, a posterior lip for the internal head of the 

 Triceps, and an intermediate space for the internal intermuscular aponeurosis. 



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

 and covered by the Deltoid muscle ; forwards below, where it is slightly con- 

 cave from above downwards, and gives origin to part of the Brachialis Anticus 

 muscle. About the middle of this surface is seen a rough triangular impres- 

 sion for the insertion of the Deltoid muscle, and below it the musculo-spiral 

 groove, directed obliquely from behind, forwards, and downwards, and trans- 

 mitting the musculo-spiral nerve and superior profunda artery. 



The internal surface, less extensive than the external, is directed forwards 

 above, forwards and inwards below ; at its upper part it is narrow, and forms 

 the bicipital groove. The middle part of this surface is slightly rough for the 

 attachment of the Coraco-brachialis ; its lower part is smooth, concave, and 

 gives attachment to the Brachialis Anticus muscle. 1 



The posterior surface (Fig. 155) appears somewhat twisted, so that its upper 

 part is directed a little inwards, its lower part backwards and a little outwards. 

 Nearly the whole of this surface is covered by the external and internal heads 

 of the Triceps, the former being attached to its upper and outer part, the latter 



1 A small hook-shaped process of bone, varying from -^ to f of an inch in length, is not 

 unfrequently found projecting from the inner surface of the shaft of the humerus two inches 

 above the internal condyle. It is curved downwards, forwards, and inwards, and its pointed 

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

 fibrous band ; completing an arch, through which 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-interosseous 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 supracondyloid 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. Struthers, in his " Anatomical and Physiological Observations," p. 202. 

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