32 COMPARATIVE ANATOMY 



beginning on the radial or thumb side, two small bones cor- 

 responding with the rudimentary thumb and the second digit, 

 these we call distals i and 2 ; and there is a much larger cres- 

 centic bone which represents distals 3, 4, and 5 fused together. 

 The hand consists of a set of five proximal bones joined on to 

 the wrist ; these are the metacarpals, and the first is very small. 

 To the metacarpals succeed the phalanges, forming the fingers. 

 The two longest fingers, IV and V, have each three phalanges; 

 the third and second have only two. The thumb is only 

 represented by the metacarpal. 



The hind-limb, like the front, is swung on the body by 

 means of a girdle, the pelvic girdle. There is this important 

 difference between the pectoral and pelvic girdles, that where- 

 as the former is not directly attached to the vertebral column, 

 and is only kept in its place by muscles, the latter is arti- 

 culated to the transverse processes of one or more vertebrae, 

 known as the sacral vertebrae. In the frog there is one sacral 

 vertebra, the ninth, and to its broad and long transverse 

 processes a pair of long and rather slender bones, the ilia, 

 are articulated. Each ilium curves downward and inward 

 so as to approach its fellow of the opposite side, and, ex- 

 panding into a broad plate posteriorly, it is firmly bound to 

 the similar expansion of its fellow by ligament. Each ilium 

 has a crest on its upper edge, and the external surface of its 

 posterior expansion is occupied by the half of a deep articular 

 concavity, the acetabulum, into which the head of the thigh- 

 bone fits. The acetabulum is completed by an irregularly- 

 shaped bone, the ischium, above and behind, and a wedge- 

 shaped piece of cartilage, the pubis, in front and below. 

 Both ischium and pubis are placed back to back with their 

 fellows of the opposite side ; and are firmly united to them by 

 ligament. The whole pelvis of the frog has thus somewhat 

 the shape of the " merry-thought " of a bird (but the latter has 

 nothing to do with the pelvis) and is abnormal in structure. 

 Each half of a typical pelvic girdle consists of an ilium above 

 united to one or more sacral vertebrae. The ilium consti- 

 tutes the dorsal moiety of the girdle, and is analogous with the 

 scapula in the pectoral girdle. The ventral moiety consists of an 

 ischium, which inclines backward, and a pubis, which inclines 

 forward. The ischia and pubes of opposite sides commonly 

 meet, and are joined together in the mid-ventral line; the points 



