The Posterior Limb Skeleton. 103 



THE METACARPUS AND PHALANGES. ■ 



The metacarpus (Fig. 39) comprises five stout elements, the meta- 

 carpal, bones (ossa metacarpalia), which form the basal supports of the 

 digits. Each consists, in addition to a main portion or body, of a 

 flattened proximal end, or base, and a rounded distal extremity, or head. 

 The four lateral bones are normally developed, while the first, which 

 belongs to a reduced digit, is of very small size. 



The phalanges or bones of the digits are distributed according to the 

 formula 2, 3, 3, 3, 3. They are similar- in form to the metacarpals, with 

 the exception, however, of the terminal, ungual phalanges, which are 

 laterally compressed, pointed, and cleft at their tips for the attachment 

 of the claws. 



SESAMOID BONES. 



Accessory elements, sesamoid bones (ossa sesamoidea), developed in 

 connection with the ligaments of muscles, are found on the volar surface 

 of the foot in association with certain of the joints. They occur in trans- 

 verse pairs at the metacarpophalangeal articulations and in linear pairs 

 at the articulations of the second with the third phalanges. The pisi- 

 form bone of the carpus is also a sesamoid, being formed in the insertion 

 tendon of the flexor carpi ulnaris muscle. 



THE SKELETON OF THE POSTERIOR LIMB. 



In the posterior limb the proximal or girdle portion comprises the 

 paired coxal bones, which are united ventrally at the pelvic symphysis, 

 thus forming the pelvic girdle. The distal portion — comprising, as in 

 the anterior limb, the supports of the free extremity — is divisible into 

 proximal, middle and distal segments. The proximal segment contains 

 a single element, the femur; the middle segment two elements, the 

 tibia and fibula, which, however, are extensively coalesced; and the 

 distal segment twenty-three elements, of which six form the tarsus, five 

 the metatarsus, and twelve the phalanges. 



THE COXAL BONE. 



The coxal bone (os coxae) (Fig. 40) is a somewhat triradiate struc- 

 ture, the posterior limbs of which are united, so that they enclose a large 

 aperture, the obturator foramen (foramen obturatum). It is firmly 

 articulated with the sacrum, and is united with its fellow of the opposite 

 side by a thin strip of cartilage containing a small amount of fibrous 

 material. The latter connection is the pubic symphysis (symphysis pubis), 

 better termed in the rabbit the pelvic symphysis, since it is somewhat 

 more extensive than the corresponding articulation of the human pelvis. 



In the young animal each half of the pelvis consists of three elements, 

 namely, the ilium, ischium, and pubis. They form the three rays 

 of the coxal bone, and are united with one another in the region of 

 the acetabulum, which is the basin-like depression for the articulation of 

 the pelvis with the femur. Only two of the original elements, however, 

 actually take part in the formation of the acetabulum, the pubis being 



