206 ANATOMY OF THE RABBIT 



Sesamoid Bones 



Accessory elements, sesamoid bones (ossa sesamoidea), de- 

 veloped in the ligaments or tendons of muscles, are found on the 

 volar surface of the foot in association with certain of the joints. 

 They occur in transverse pairs at the metacarpophalangeal articu- 

 lations and in linear pairs at the articulations of the second with 

 the third phalanges. The pisiform 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 LLMB 



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. Along with the sacrum, 

 which is interposed between them dorsally, they constitute the 

 pelvis. This rigid framework, involving part of the vertebral 

 column, provides a strong basis for the powerful thrust of the 

 hind limbs in locomotion and contrasts with the elastic attachment 

 of the fore limbs to the trunk. The distal portion of the posterior 

 limb, like that of the anterior, comprises the supports of the free ex- 

 tremity and 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. 95) is a somewhat triradiate 

 structure with one anterior ray and two posterior ones, the latter 

 united so that they enclose a large aperture the obturator foramen 

 (foramen obturatum). The bone is firmly articulated with the 

 sacrum dorsally and is united ventrally 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 ternied in the rabbit the pelvic 

 symphysis, since it is somewhat more extensive than the correspond- 



