268 



HUMAN ANATOMY. 



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The lower extremity is broad from side to side, with an articular surface 



below, and two lateral projections, the condyles. The inner condyle ^^ much the 



larger, is sharp and prominent, giving rise with a part 

 Fig. 2.8.^. of the supracondylar ridge to the flexor pronator mus- 



S#W^tli^^th^ ^^^^- ^^ ^^ faintly grooved behind by the ulnar nerve, 



and the lower part of the front often presents a smooth 

 surface. The outer condyle ^ is a slightly raised knob. 

 The articular surface, most of which is at a lower level 

 than the condyles, consists of two parts, — an inner 

 pulley-like surface, the trochlea, for the ulna, and an 

 outer convexity, the capitelluni, for the radius. 



The trochlea ■'■ is bounded internally by a sharp 

 border, forming about three-quarters of a circle, and 

 projecting below the rest of the bone as well as before 

 and behind it. It is bounded externally by a ridge, 

 which is prominent behind where the trochlea forms 

 the whole of the articular surface, but is faint in front 

 where it separates the trochlea from the capitellum. 

 ^'^M Above the joint this ridge is continuous with the an- 



terior border of the shaft. 



The trochlea is convex from before backward. A 

 section through the middle forms almost a complete 

 circle, being broken only above, where a thin plate con- 

 nects it with the shaft. It is concavo-convex from side 

 to side, the convexity being greatest at the inner bor- 

 der. There is a depression above the trochlea both 

 before and behind ; the former, the coronoid fossa, is 

 small and receives the coronoid process of the ulna in 

 flexion ; the posterior depression, triangular and much 

 the larger, is the. olec)'ano7i fossa, receiving that process 

 in extension. The bone separating these fossae — the 

 plate just alluded to — is so thin as to be translucent. 

 It may be perforated by the siipratrochlear foj'amen, 

 most frequently found in savage tribes. The joint be- 

 tween the humerus and ulna is commonly called a 

 hinge-joint, but there are serious modifications. First, 

 the axis of the trochlea is not at right angles to that of 

 the shaft, but slants downward and inward ; next, the 

 borders of the trochlea are not at right angles to its 

 axis, but are so placed as to transform it into a spiral 

 or screw -joint ; finally, these borders are not parallel to 

 each other, but the inner slants downward and inward 

 so that the transverse diameter of the joint is greater 

 below than at the top, either before or behind. 



The capitellum,* on which the concave head of 

 the radius plays, is situated on the front of the outer 

 i¥-Vi>AK P^^t of the lower end. It is not far from being a por- 

 tion of a sphere, since it is convex and nearly equally 

 so in all directions, but the arc from above downward 

 is the longest. A groove runs between it and the outer 

 ridge of the trochlea ; the outer border is straight ; the 

 posterior runs from it obliquely backward and inward. 

 The capitellum is placed so much to the front as to be 

 nearly or quite invisible from behind ; hence the articu- 

 lar surface is much more extensive on the front than 



the back. The radial fossa, a small depression above the capitellum, receives the 



edge of the head of the radius in extreme flexion. 



The supracondylar process is a small bony spur occurring in probably two 



or three per cent., which arises from the front of the bone a little anterior to the 



" Trochlea. ^ Capitulum. 



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Lotiiijitudinal section of humerus, 

 showing relation of compact and 

 spongy bone. 



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' Epicondjius medialis. 



Epicondylus lateralis. 



