MAMMALIA. 281 



joint. The radius and ulna, which support the antebrachium, 

 are immovably articulated, but not fused together in the position 



of pronation. 



Where, as in Man, the radius and ulna are movable, they may assume 

 two main positions: () Supijiation, when they are parallel, the palm of 

 the hand is upwards, and the thumb outside (like the radius), (b) Prona- 

 tion, when the radius crosses over the ulna towards the inside, the palm of 

 the hand is downwards, and the thumb inside (like the distal end of the 

 radius). 



The preaxial radius possesses a proximal head for articulation 

 with the trochlea, and two distal concavities for the carpal bones. 

 The postaxial ulna has a proximal sigmoid cavity for articulation 

 with the trochlea, and a convex distal end. Proximally the ulna 

 is produced into the olecranon, a process which, in the extended 

 limb, fits into the olecranon fossa, a pit situated above the trochlea. 



The carpus consists of a proximal and a distal row of small 

 bones, and a diminutive centrale. The proximo! row is made up 

 of four bones, which are (beginning on the inner (preaxial) side) 

 (a) radiale (scaphoid), and (/3) intermedium (lunar), articulating 

 with the concavities on the radius, (y) ulnare (cuneiform), and 

 (S) pisiform (not shown), articulating with the convexity on the 

 ulna. The distal row is made up of four carpalia, which are, 

 beginning as before, carpale 1 (trapezium), carpale 2 (trapezoid), 

 carpale 3 (magnum), carpale 4 -f- carpale 5 (unciform). Five 

 digits are present supported by five metacarpals, articulating 

 with the corresponding carpalia, and completed by phalanges, 

 of which the short pollex (1st digit) possesses 2, the others 

 3 each. The terminal phalanges are conical, and grooved to 

 support the claws. 



(b) Hind-limb. The hip-girdle is formed by an innominate bone 

 on each side, made up of four fused bones, ilium, ischium, pubis, 

 and cotyloid. Upon the outer side of each innominate is a deep 

 cup, the acetabulum, the floor of which is complete, and in the 

 young rabbit, marked by the three-rayed junction of the ilium, 

 ischium, and cotyloid. 



The ilium is placed above and in front. It is laterally flat- 

 tened, and the sacrum articulates with its inner surface. The 

 ischium is placed above and behind. It is separated from the 

 pubis by the oval obturator foramen, and presents posteriorly a 

 rough and thickened tuberosity. A plate-like expansion runs 



