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 vertebre. 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 “fabellz.” 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 
