286 AN INTRODUCTION TO ZOOLOGY 



three elements, the ilium, the ischium and the pubis, closely fused 

 together and all taking part in the formation of the acetabulum, a 

 deep cavity in which the head of the femur articulates. The ilium 

 is the anterior dorsal bone of the girdle on each side and is flattened 

 in the vertical plane. Its inner, sacral surface, is roughened for 

 junction with the sacrum. The outer surface is marked by a 

 longitudinal ridge, leaving a shallow dorsal depression, the gluteal 

 fossa, and a similar ventral one, the iliac fossa. It forms about one- 

 half of the acetabulum. The ischium is the second largest bone in 

 the girdle passing backwards from the acetabular cavity, of which 

 it forms about one- third, in the same line as the ilium. About 

 half-way back it gives off dorsally a- blunt ischial spine and it 

 terminates in an expanded roughened enlargement, the ischial 

 tuberosity. The smallest bone is the pubis, whose contribution to 

 the acetabulum, less than one-third, usually ossifies separately and 

 is termed the cotyloid bone. It passes ventrally as a thin strip of 

 bone to meet its fellow in the middle line in the pubic symphysis, 

 continuing this union backwards until it joins the inner portion of 

 the expanded end of the ischium. Save for this posterior junction, 

 only visible in young animals, the pubis and ischium are separated 

 by a large hole, the obturator foramen. 



Fore Limb. 



The fore limb of the rabbit is composed of the same main 

 divisions as in the frog, but it is more highly specialised, and it 

 differs considerably in the details of its structure. 



The humerus is a long bone with a well-developed rounded head, 

 articulating with the glenoid facet of the scapula, and an enlarge- 

 ment, the trochlea, at its distal end articulating with the radius and 

 ulna. Near the head are two roughened projections, an outer or 

 greater tuberosity, and an inner or lesser tuberosity, and they are 

 separated by a bicipital groove for the tendrons of the biceps muscle. 

 The trochlea is a ridged pulley-like surface just above which on each 

 side is a deep supra trochlear fossa, and these two fossae communicate 

 with one another by means of a supra trochlear foramen. The 

 posterior of the two is considerably the larger and often termed the 

 olecranon fossa, since it lodges part of the olecranon process when the 

 arm is fully extended. The radius and ulna are almost of equal 

 size, and although not actually fused are immovably articulated. 

 The radius is the inner bone and articulates proximally with the 

 trochlear notches, while distally, where it is somewhat enlarged, it 

 touches the carpal bones. The ulna has a small distal end, but its 

 proximal extremity is enlarged and marked by a deep groove, the 

 sigmoid notch, in which lies part of the trochlea, while it is continued 



