THE SKELETON 51 



limb, of a flattened rod of bone crossing the long axis of the trunk, 

 having an upper or dorsal and a lower or ventral end. The upper 

 end diverges from that of the opposite side, but the lower end 

 approaches, and, in most cases, meets it, forming a symphysis, 

 without the intervention of any bone corresponding to the sternum. 

 The pelvic girdle differs from the shoulder girdle in being firmly 

 articulated to the vertebral column, thus giving greater power to 

 the hinder limb in its function of supporting and propelling the 

 body. Like the shoulder girdle, it bears on its outer side, near 

 the middle, a cup-shaped articular cavity (" acetabulum "), into 

 which the proximal end of the first bone of the limb proper is 

 received. Each lateral half of the girdle is called the "os 

 innominatum," or innominate bone, and consists originally of three 

 bones which unite at the acetabulum. The "ilium" or upper bone 

 is that which articulates with the sacral vertebrae. Of the two 

 lower bones the anterior or " pubis " unites with its fellow of 

 the other side at the symphysis; the posterior is the "ischium." 

 These lower elements form two bars of bone, united above and 

 below, but leaving a space between them in the middle, filled only 

 by membrane, and called the " thyroid " or " obturator " foramen. 

 The whole circle of bone formed by the two innominate bones 

 and the sacrum is called the pelvis. In the Monotremata 

 and Marsupialia, a pair of thin, flat, elongated ossifications 

 called epipubic or marsupial bones are attached to the fore part 

 of the pubis, and project forward into the muscular wall of the 

 abdomen. 



Thigh and Leg. — The first segment of the limb proper has one 

 bone, the femur, corresponding with the humerus of the anterior 

 limb. The second segment has two bones, the tibia and fibula, corre- 

 sponding with the radius and ulna. These bones always lie in their 

 primitive unmodified position, parallel to each other, the tibia on 

 the preaxial and the fibula on the postaxial side, and are never 

 either permanently crossed or capable of any considerable amount 

 of rotation, as in the corresponding bones of the fore limb. In the 

 ordinary walking position the tibia is internal, and the fibula ex- 

 ternal. In many mammals the fibula is in a more or less rudi- 

 mentary condition, and it often ankyloses with the tibia at one or 

 both extremities. The patella or " knee-cap," which is found in an 

 ossified condition in all mammals, with the exception of some of 

 the Marsupialia, is a large sesamoid bone developed in the tendon 

 of the extensor muscles of the thigh, where the tendon passes over 

 the front of the knee-joint, to which it serves as a protection. 

 There are frequently smaller ossicles, one or two in number, situated 

 behind the femoral condyles, called "fabelke." The processes for 

 the attachment of muscles near the upper end of the femur are 

 termed trochanters ; and the third trochanter, found on the hinder 



