EXTREMITY. 



163 



sjgmoid cavity ; like the similar surface on the 

 olecranon, it is divided by an obtuse ridge 

 directed from before backwards, into two une- 

 -qual portions ; these portions correspond in 

 shape and size with those already noticed on 

 ,the olecranon. 



The shaft of the ulna gradually tapers from 

 .above downwards ; it is triangular in its entire 

 extent, excepting for about an inch above the 

 inferior extremity, where the bone is distinctly 

 cylindrical. On the shaft anatomists commonly 

 describe three surfaces. The anterior surface 

 is broader in the middle than at its extremities; 

 it is slightly concave in the transverse direction 

 in its middle third ; on this surface, at its upper 

 part, we notice the orifice of the nutritious canal, 

 which is directed upwards towards the coro- 

 noid and olecranon. By its three superior 

 fourths this surface affords attachments to the 

 flexor digitorum profundus, and by its inferior 

 fourth to the pronator quadratus ; the place of 

 attachment of this latter muscle is limited above 

 by an oblique line which passes from without 

 inwards and from above downwards. The in- 

 ternal surface is smooth, and convex in its en- 

 tire extent; widest above, it gradually tapers to 

 the inferior extremity. In its inferior fourth it 

 is subcutaneous, and to its three superior 

 fourths is attached the deep flexor muscle of 

 the fingers ; the aspect of this surface is back- 

 wards as well as inwards. 



The third surface is posterior. The two in- 

 ferior thirds of this surface are smooth, the mid- 

 dle being flat and the lowest rounded ; here 

 are attached the extensor muscles of the thumb 

 and that of the index finger. In the superior 

 third we distinctly notice two surfaces, easily 

 distinguishable by the difference of aspect; the 

 internal one, which is continued up on the 

 olecranon process, looks backwards and slightly 

 outwards; to it the anconaeus muscle is attached 

 superiorly, and inferiorly the extensor carpi 

 ulnaris. The external of these two surfaces 

 looks directly outwards, and is separated from 

 that last described by a line which passes ob- 

 liquely downwards and inwards ; to this sur- 

 face, which commences just below the lesser 

 sigmoid cavity, the supinator brevis is attached, 

 and below it, commences the line of attachment 

 of the extensor muscles already alluded to. 



Three edges separate the surfaces above de- 

 scribed ; of these the external is at once distin- 

 guished by its greater prominence ; it is sharp 

 in nearly its two inferior thirds, and superiorly 

 is lost on the surface to which the supinator 

 brevis is attached ; all that part of this edge 

 which is prominent and sharp gives insertion 

 to the interosseous ligament. The anterior 

 edge commences just below the coronoid pro- 

 cess, and terminates, inclining a little back- 

 wards, in front of the styloid process of the 

 ulna : it is rounded and smooth in its entire 

 extent, and has the deep flexor of the fingers 

 and the pronator quadratus inserted into it. 

 The posterior edge commences at the apex of 

 the posterior surface of the olecranon, and ter- 

 minates insensibly towards the inferior fourth 

 of the bone. 



The inferior or carpal extremity of the ulna 



is very small; it forms a slightly rounded 

 head ; on its posterior and internal part is a 

 small process, projecting vertically downwards 

 and ending in a point, to which the internal 

 lateral ligament of the wrist-joint is attached : 

 this process is the styioid process ; external to this 

 is a depression or pit, into which is inserted 

 the triangular cartilage of the wrist-joint, and 

 external to this depression is the rounded head, 

 which is smooth on its inferior surface, covered 

 with cartilage in the recent state ; the triangular 

 cartilage glides upon this surface. On the 

 outer side of the head is an articular convexity 

 which articulates with a concave surface on the 

 inner side of the carpal extremity of the radius. 

 On the posterior surface of the head, imme- 

 diately external to the styloid process, there is 

 a slight channel, in which is lodged the tendon 

 of the extensor carpi ulriaris. 



Structure. The olecranon and coronoid 

 processes are completely cellular in structure, 

 excepting the external cortex of compact tissue. 

 The inferior extremity of the ulna is likewise 

 cellular, but the shaft is mainly composed of 

 compact tissue, hollowed by a medullary canal, 

 which commences a little below the coronoid 

 process, and terminates just above the inferior 

 extremity. 



Radius, (Germ, die Speiche,) so called from 

 its being compared to the spoke of a wheel ; it 

 is the shorter of the two bones of the forearm ; 

 its proportion to the ulna being as 11 to 12. 



The superior extremity or head of the radius 

 is a cylindrical head excavated on its superior 

 surface so as to form a superficial cavity, cavitas 

 glenoidea, which is articulated with the external 

 condyle of the humerus. The circumference of 

 this head consists of a deep lip of bone present- 

 ing a smooth surface covered by cartilage in the 

 recent state, the depth of which, measured verti- 

 cally, is greatest on the inner side, so as there to 

 form an oval convex articular facet which is 

 adapted to the lesser sigmoid cavity of the ulna ; 

 the remainder of the circumference is embraced 

 by the annular ligament of the radius. The head 

 of the radius is connected to the shaft by a short 

 and cylindrical neck, which passes obliquely 

 downwards and inwards; the neck of the radius 

 is limited inferiorly and on the ulnar side by a 

 rounded tubercular process, into the internal 

 posterior and rough partof which the biceps mus- 

 cle is inserted, the bicipital tuberosity or tubercle 

 of the radius; the anterior part of this tubercle, 

 over which the tendon of the biceps glides, is 

 smooth. For about an inch below this process 

 the bone retains the cylindrical form, being 

 here embraced by the inferior fibres of the su- 

 pinator brevis muscle ; but below this the bone 

 becomes distinctly prismatic in its form, and 

 begins to expand to its inferior or carpal extre- 

 mity. We here describe three surfaces as in 

 the ulna : the anterior is inclined inwards, its 

 aspect is forwards and inwards ; about its 

 middle this surface is slightly hollowed from 

 above downwards; at the junction of its middle 

 and inferior third it is convex, and in its inferior 

 third, where it attains its greatest lateral expan- 

 sion, it is concave again. At the superior third 

 of the bone we notice on this surface the nutri- 



