THE HUMERUS. 265 



THE HUMERUS. 



The bone of the arm consists of a shaft and two enlarged extremities. 



The upper extremity includes a globular articular head and two tuberosities 

 for muscular insertions. The head ' looks upward, inward, and backward. It is 

 not truly a part of a sphere, for the curve in the horizontal plane is bolder than that 

 in the vertical. The vertical diameter between the edges of the articular surface is 

 longer than the transverse. It is surrounded by a slight groove for the attachment 

 of the capsular ligament at what is called the anatomical neck^ The surgical neck'^ is 

 just below the whole upper extremity. The tuberosities are separated in front by a 

 deep furrow, the bicipital groove,^ through which runs the tendon of the long head 

 of the biceps. The greater tuberosity^ is a rough enlargement placed externally. 

 Its highest point is at the front, just by the groove. A superior surface of this 

 tuberosity begins here and, passing downward and backward beside the head, 

 broadens as it goes. It bears three smooth facets for the insertion of the supra- 

 spinatus, the infraspinatus, and the teres minor, in this order ; the first being highest 

 and most in front, the last and lowest most behind. The lesser tuberosity,® much 

 smaller, is on the front of the bone. It bears a prominent angle, sometimes an 

 actual crest running downward and inward for the subscapularis. The upper aspect 

 of the process, which looks also inward, is smooth for a bursa beneath the tendon. 



The shaft is roughly cylindrical above and prismatic below. It is convenient 

 to divide it by three borders into three surfaces. The anterior border starts from 

 the greater tuberosity as the outer lip of the bicipital groove, which, growing 

 shallower, can be traced through the first quarter of the shaft. This outer lip 

 becomes thicker and more prominent for some two inches below the surgical neck 

 to receive the insertion of the pectoralis major. Below this it is joined by the lower 

 end of the deltoid eminence, after which, smooth and rounded, it grows fainter, but 

 may be traced downward to a ridge separating the capitellum from the trochlea, 

 where it ends. The internal border starts at the inner side of the neck, often so 

 near the inner lip of the bicipital groove as to be confounded with it, and runs 

 straight down to the very tip of the internal condyle. It is at best very faint in the 

 first quarter, and often barely visible ; but it is distinct in the middle and prominent 

 in the last third, where it is known as the internal supracondylar ridge. The 

 external border begins at the back of the greater tuberosity and runs to the outer 

 condyle, the lower part being the external supracondylar ridge., which has a forward 

 curve. A great exaggeration of this ridge has been seen in the negro. The inter7ial 

 surface bears the inner lip of the bicipital groove, which, starting from the lesser 

 tuberosity, is often very faint ; it receives the tendon of the teres major. The 

 bicipital groove soon becomes shallow, and is lost after two or three inches. The 

 mdrient fora^nen, running downward into the bone, is rather below the middle of 

 this surface, sometimes being almost in the internal border. The external surface 

 is convex in the upper half and concave in the lower. Its second quarter is occupied 

 by a long, rough elevation, the deltoid eminence,^ slanting downward and forward 

 against the inner border for the insertion of the deltoid muscle. The posterior 

 surface is twisted, facing somewhat inward above and backward below. The upper 

 plane portion gives origin to the outer head of the triceps, and the lower, convex 

 except below, to the inner head. A broad spiral groove beginning on the external 

 surface behind the deltoid eminence", in front of the outer border, twists forward and 

 downward. This is generally improperly called the viusculo-spiral groove.^ The 

 groove truly deserving that name, containing the musculo-spiral nerve and the supe- 

 rior profunda artery, occupies the lower and posterior part of the greater groove, 

 from which it usually is not to be distinguished throughout, though both grooves 

 may be distinct. The musculo-spiral groove is some five millimetres broad, and, 

 when well developed, begins on the posterior surface, separating the areas for the 

 outer and inner heads of the triceps muscle, and interrupts the external border, 

 behind which the broad spiral groove never passes. A second nutrient foramen, 

 also running downward and sometimes the larger of the two, may occur in the 

 groove. The shaft takes a forward bend just at its termination, so that most of the 

 lower end lies in front of the continuation of the axis of the shaft. 



'Caput humeri. " Collum anatomicum. ^C. chirurgicum. ■* Sulcus intertubercularis. ^ Tuberculoiu majus. *Tiib. 

 minus. ' Tuberositas deltoidea. '^ Sulcus radialis. 



