THE SKELETAL SYSTEM 47 



The ischium is the most posterior of the bones of the pelvis. 

 It is a flat bone having two surfaces and four borders. Both 

 surfaces are nearly flat. The internal border is thick and rough 

 and is united to its fellow of the opposite side by cartilage. The 

 external border is concave, as is also the anterior, which forms the 

 posterior margin of the obturator foramen. The posterior border 

 is thick and rough. The large, rough tuberosity which projects 

 backward and outward is the " pin bone." 



The pubis is the smallest of the three and is placed at the ante- 

 rior part of the floor of the pelvic cavity. Its dorsal surface is 

 smooth, slightly concave, and supports the bladder. The ventral 

 surface is convex and rough. The external angle assists in form- 

 ing the acetabulum. The posterior border forms the anterior 

 boundary of the obturator foramen. 



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

 (Fig. 7). It is situated between the pelvis above and the tibia 

 below, and slopes downward and forward. It is a long bone pre- 

 senting a shaft and two extremities. The shaft has four sur- 

 faces and two borders. The anterior, internal, and external 

 surfaces are convex, continuous, 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 trochanter, below which is the 

 deep supracondyloid fossa. The upper extremity is composed 

 of the articular head adapted to the acetabular cavity, and the 

 great trochanter at the outer side of the head. The distal ex- 

 tremity is very large and presents a double articular surface, 

 consisting of the two condyles for articulation with the tibia, 

 separated by the intercondyloid fossa, and the trochlea, situated 

 anteriorly, for articulation with the patella. 



The patella, or stifle, has an articular surface to fit the trochlea 

 of the femur. It is pyramidal in shape with the base upward and 

 corresponds to the knee-cap of man. This is the bone that is 

 displaced in the condition known as " stifled." 



The tibia, or leg, is a long bone situated between the femur 

 and the hock in a direction obliquely downward and backward. 

 It has a shaft and two extremities. The internal surface of the 

 shaft is straight, not covered with muscle, and corresponds to 

 the shin of man; the external surface, curves to the front of the 

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