216 ELEMENTARY BIOLOGY. [XIII. 



j. A small triangular piece (os pubis), (cartilagin- 

 ous except in old frogs), wedged in between 

 ilium and ischium, and meeting its fellow in 

 the median ventral line, thus forming the sym- 

 physis pubis. 



h. The bones of the lower limb. 



a. The thigh-lone (os femoris) ; its long cylindrical 

 shaft and expanded articular extremities. 



&. The leg-bone (&s cruris). 



a. A very long cylindrical bone, expanded at each 

 end. 



ft. The grooves on it; one running along the 

 whole ventral surface, but most marked near 

 the ends ; other grooves on the dorsal surface, 

 one at the upper, another at the lower extrem- 

 ity : these indicate that the os cruris is really 

 made up of two united bones, the fibula and 

 the tibia. When the limb is stretched out at 

 right angles to the body, the tibia is anterior, 

 corresponding with the radius ; and the fibula, 

 posterior, corresponding with the ulna. 



<c. The tarsus. 



a. Two elongated bones (separate in the middle 

 but united by confluence of their cartilaginous 

 extremities) articulate with the ankylosed tibia 

 and fibula; the anterior, or tibial, of these is the 

 astragalus; the posterior, or fibular, the cal- 

 caneum. 



0. With the distal ends of these, two partially 

 ossified cartilages, one on the calcaneal and 



