CHAPTER XVIII. 



THE THIGH AND LEG. 



The skeleton of the first segment of the limb proper 

 consists of an elongated, more or less cylindrical bone, 

 the femur, which is described as having a shaft and two 

 extremities. 



The dorsal, or (in the ordinary position of the limb) 

 ajiterior surface of the shaft is smooth and rounded 

 from side to side, and generally arched somewhat for- 

 wards from above downwards ; the ventral or posterior 

 surface is more or less compressed, and has a rough longi- 

 tudinal ridge, the linea aspera. 



^ At the proximal extremity is a hemispherical, smooth, 

 articular "head" (Fig. 109, h, p. 298) which fits into the 

 acetabulum of the innominate bone, and is generally more 

 round and more distinctly separated from the rest of the 

 bone by a constriction or " neck " (;/) than is the corre- 

 sponding part of the humerus. The axis of the head does 

 not coincide with that of the shaft of the bone, but crosses 

 it at an angle varying in different animals, being directed 

 towards the preaxial ^ or (in the ordinary position of the 

 limb) internal side, and slightly also towards the anterior 

 aspect. In nearly all Mammals there is a rounded depres- 

 ^ See note to p. 244. 



