172 ANIMAL BIOLOGY. [Parti. 



crest more pronounced in male than female. The radius and 

 ulna coalesce to form a radi-ulna, which shows, however, at the 

 distal end signs of its composite nature. The carpus consists of 

 six bones, two articulating with the radi-ulna, three with the 

 metacarpals (of which the largest articulates with iii., iv., and v.) 

 and one having an intermediate position on the radial side. 

 There are five metacarpals, the first very small ; and four com- 

 plete digits, the pollex (thumb) being present only in rudiment. 

 For the changes of position which the fore-limbs have under- 

 gone see Chapter II., pp. 7, 14, and 19. 



The Pelvic Arch. Three elements enter into the composition 

 of the haunch-bone of either side. And these three meet at the 

 acetabulum or articular cavity for the hind-limb. These three 

 are the ilium (anterior); the ischium, postero- dorsal; and the 

 pubis, postero-ventral. Internal to the pubis there is, however, 

 in some forms a fourth bone, the acetabular, which may in some 

 cases (e.g. rabbit) shut out the pubis from any share in the 

 formation of the acetabular cavity. 



In the frog these bones (Fig. 56, C. D.) are tolerably easily 

 separable, though the elements of the two sides fuse somewhat 

 closely in the mid-line at the symphysis. The part marked pu. 

 is regarded as pubis. The feature to be specially noticed is the 

 great lengthening of the ilia (il.), and their articulation with a 

 single sacral vertebra, through the agency of supra-iliac car- 

 tilages. The swinging back of the point of articulation of the 

 hind-limb, brought about by the lengthening of the ilia, is more 

 marked in the adult than in the young frog. The student 

 should compare the pelvic arch of the frog with that of the 

 newt, where the acetabulum is not far removed from the sacrum. 



In the rabbit (56, B.) the ilium (il.\ ischium (isc.), and pubis 

 (pu.) (together with a small acetabular, which, in the very 

 young rabbit, shuts out the pubis from the acetabulum), com- 

 pletely fuse into a single innominate bone. The two innominates 

 unite by synchondrosis, or cartilaginous union, at the symphysis 

 (sy.). They articulate with the expanded lateral plates (ankylosed 

 ribs) of the first two sacral vertebrae. In B. t. isc. is the tuber 



