a 
whilst the tendons of the extensor ossis metacarpi pollicis and the extensor brevis 
pollicis muscles lie within the groove. To the tip of the styloid process is attached 
the external lateral ligament of the wrist. On the inner side of the lower extremity 
is placed the sigmoid cavity (incisura ulnaris) for the reception of the head of the 
ulna. Concave from before backwards, and plane from above downwards, it forms 
by its inferior margin a rectangular edge which separates 1 from the inferior 
carpal surface. To this edge the base of the triangular fibro-cartilage is attached, 
a structure which serves to separate the inferior articular surface of the head of 
the ulna from the carpus. The anterior and posterior edges of the sigmoid cavity, 
more or less prominent, serve for the attachment of ligaments. 
The proportionate length of the radius to the body height is as 1 is to 
Gr0= 7-11. 
Nutrient Foramina.—The openings of several small nutrient canals may be seen in the 
region of the neck. That for the shaft, which has an upward direction, is usually placed on the 
front of the bone, internal to the anterior oblique line, and from an inch and a half to two inches 
below the bicipital tubercle. The back of the lower extremity of the bone is pierced by many 
196 OSTEOLOGY. 
small foramina. 
Connexions. — The radius articulates with the capitellum of the humerus in the flexed 
position of the elbow, with the ulna to its inner side by the superior and inferior radio-ulnar 
joints, and with the scaphoid and semilunar bones of the carpus below. Above, the head of the 
‘hone ean be felt in the intermuscular depression on the outer side of the back of the elbow ; here 
the bone is only covered by the skin, superficial fascia, and the thin common tendinous origin of 
the extensor inuscles, as well as the ligaments which support it. Its position can best be ascer- 
tained by pronating and supinating the bones of the forearm, when the head will be felt rotating 
beneath the finger. The lower end of the bone is overlain in front and behind by the flexor and 
extensor tendons, but its general form can be readily made out. The styloid process lying to the 
outer side of the wrist in line with the extended thumb, can easily be recognised: note that it 
reaches a lower level than the corresponding process of the ulna. The outer border of the lower 
third of the shaft can be distinctly felt, as here the bone is only overlain by tendons. 
Architecture.—The neck is the narrowest part of the bone ; here fracture may occur, though 
not commonly. The point at which the bone is usually broken is about one inch above the 
lower extremity. This is accounted for by the fact that the radius supports the hand at the 
radio-carpal articulation, and the shocks to which the latter is subjected, as i endeavouring to 
save oneself from falling, are naturally transmitted to the radius. On section, the medullary 
canal is seen to extend as high as the neck ; below, it reaches to the level of the inferior fifth of 
the bone. Its walls are thick as compared with the diameters of the bone, particularly along the 
interosseous border, thus imparting rigidity to 
the curve of the shaft; these walls thin out 
Appears Fuses with shaft 18-20 years above and below. Superiorly, the surface of 
phones o the bicipital tubercle is formed of a thin 
shell of bone, which, however, thickens again 
where it passes on to the neck. The upper 
extremity is formed of spongy bone arranged 
in the form of arcades, reaching below the level 
of the bicipital tubercle internally, but not 
extending below the level of the neck ex- 
ternally. Beneath the capitellar articular 
surface there is a dense layer, thickest in the 
centre, and thinning towards the cirecum- 
ference ; this is overlain by a very thin layer 
of less compact bone. 
The inferior fifth of the shaft and lower 
extremity are formed of loose spongy bone 
arranged more or less longitudinally. Iim- 
mediately subjacent to the carpal articular 
surface the tissue is more compact, and dis- 
| 
years 
bout 14-15 
; fuses soon 
Appears early in 2nd month of fcetal life 
plays a striation parallel to the articular plane. 
The nutrient canal of the shaft pierces the 
anterior wall of the upper part of the medul- 
lary cavity obliquely from below upwards for 
the space of half an inch. 
Variations.—Cases of congenital absence of 
the radius are recorded; in these the thumb | 
is not infrequently wanting as well. | 
Appears about 
2-3 years 
Unites with shaft 
20-25 years 
At Birth. About 12 years. About 16 years. 
Fie. 138.—THE OSSIFICATION OF THE RaDpIUs. Ossification.—The centre for the shaft 
makes its appearance early in the second 
month of intrauterine life. At birth the shaft is well formed; its upper and lower — 
extremities are capped with cartilage, and the bicipital tubercle is beginning to appear. — 
