THE SKELETAL SYSTEM 45 



other soft organs, and lies posterior to the abdominal cavity. In 

 front the pelvic cavity communicates with the abdominal cavity 

 by a large free opening, the inlet. The inlet is a bony circle cir- 

 cumscribed by the sacrum above, the pubic bones below, and the 

 bodies of the ilia on each side. In the mare the pelvic floor is 

 almost level, and is more roomy than that of the male on account 

 of the function it has to perform. The thick, sacrosciatic liga- 

 ment stretches across each side of the pelvic cavity. This liga- 

 ment is attached above at the edge of the sacrum and extends to 

 the external borders of the ilium and ischium below. It consti- 

 tutes much of the lateral wall of the cavity and is strong, yet yield- 

 ing enough so that under pressure during parturition it assists in 

 making the diameters of the pelvis more even and allows the fetus 

 to pass more easily. The cavity of the pelvis is the space between 

 the inlet and the outlet. During the act of parturition it gives 

 passage to the fetus. 



The femur, or thigh, is the most massive bone of the body. It 

 is situated between the pelvis above and the tibia below, and slopes 

 downward and forward. It is a long bone presenting a shaft and 

 two extremities. The shaft has four surfaces and two borders. 

 The anterior, internal, and external surfaces are convex, continu- 

 ous, and smooth. The posterior surface is flattened and triangular 

 in shape. The internal border carries in its upper third the small 

 or internal trochanter, and a little below the middle the medullary 

 foramen. The external border presents the large or external tro- 

 chanter, below which is the deep supracondyloid fossa. The 

 upper extremity is composed of two parts, the articular head and 

 the great trochanter. The head is a large hemispheric eminence 

 adapted to the acetabular cavity. The great trochanter lies to 

 the outer side of the head and from it the prominent trochanteric 

 ridge descends. The distal extremity is very large and presents 

 a double articular surface, consisting of the two condyles for articu- 

 lation with the tibia, separated by the intercondyloid fossa, and the 

 trochlea, situated anteriorly, for articulation with the patella. 



The patella, or knee-cap, has an articular surface to correspond 

 with the trochlea of the femur. This is the bone that is displaced 

 in the condition known as "stifled" in the horse. It is pyramidal 

 in shape with the base upward. 



The tibia is a long bone situated between the femur and the 

 hock in a direction obliquely downward and backward. It has a 



