THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 81 



dorsally and is firmly articulated to the end of the sacral rib. This bone is the 

 ilium and represents an ossification in the iliac process of elasmobranch girdles. 

 At the point of junction of pubic cartilage, ischium, and ilium, there is a depres- 

 sion, the acetabulum, into which the proximal end of the hind limb is inserted by 

 a ball-and-socket joint. 



The hind limbs of urodeles like Necturus are exceedingly primitive. This 

 matter was already discussed in Section III of this manual. The limb is divided 

 into a proximal segment the thigh, consisting of a single bone, the femur, a 

 middle segment the shank, consisting of two parallel bones, a preaxial bone, 

 the tibia, and a postaxial bone, the fibula, and a distal segment the pes, consist- 

 ing of ankle and foot. The ankle consists of several small bones, the tar sals, which 

 are, unfortunately, impossible to make out on most skeletons. The foot has four 

 elongated bones, the metatarsals, which bear the toes. Each toe consists of two 

 or three small bones, the phalanges, arranged in a row. The first toe is missing. 



Draw the pelvic girdle and hind limb from above. 



4. The pelvic girdle and hind limb of the turtle. In the reptiles the ossi- 

 fication of the pelvic girdle is complete; very little cartilage remains, and the full 

 number of bones characteristic of the pelvic girdle is present. The pelvic girdle 

 of the turtle is a very generalized and representative girdle. Isolated girdles may 

 be studied, but the position and attachment of the girdle should be noted on 

 the entire mounted skeleton. The girdle consists of three pairs of stout bones, 

 two pairs ventral in position, one pair lateral and dorsal. The ventral bones 

 consist of an anterior pair the pubes, which meet in the median ventral line form- 

 ing the pubic symphysis, and a posterior pair the ischia, united similiarly to 

 form the ischial sympJiysis. These symphyses are composed of cartilage, generally 

 missing from dried skeletons. The lateral and dorsal bones of the girdle are 

 the ilia, which are articulated at their dorsal ends to the ends of the two sacral 

 ribs. Note on the mounted skeleton the inverted arch or U formed by the 

 pelvic girdle. The arch is completed dorsally by the sacral vertebrae and sacral 

 ribs. Through the arch of the pelvic girdle pass the terminal portions of diges- 

 tive and urogenital systems. Between the pubis and ischium of each side is a 

 large opening, the obturator foramen, through which in the fresh condition nerves 

 and blood vessels pass. The two foramina are completely separated in life by 

 a cartilage which bridges the space between the pubic and ischial symphyses 

 and continues as the cartilage of these symphyses. Attached to the anterior 

 extremity of the pubic symphysis is a cartilage, the epi pubis. Each pubis has 

 a prominent lateral process, the pectineal process, projecting forward. Pubis, 

 ischium, and ilium meet at the place where the hind limb articulates, and share 

 equally in the formation of a concave depression, the acetabulum, into which the 

 convex head of the femur is inserted. Draw the pelvic girdle (one side is suffi- 

 cient). 



The parts of the hind limb are the same as in the urodeles, but the limb no 

 longer retains its Drimitive orientation with regard to the body. The middle 



