THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 95 



13. The sternum occurs first in the Amphibia and is found in most vertebrates above 

 A mphibia. It is probably the median ventral portion of the pectoral girdle. In many reptiles, 

 and in birds and mammals, the ribs articulate with the sternum. 



14. The bones of the limbs are probably derived from the cartilaginous fin rays of the lower 

 fishes. They are very similar in all land vertebrates, the distal portions being subject to the 

 most modifications. 



1 5. The limbs are divided into three segments. The bones of these segments are as follows, 

 those of the hind limb being named first: proximal segment, one bone, femur or humerus; 

 middle segment, two parallel bones, tibia and fibula, radius and ulna; distal segment, com- 

 posed of three parts, ankle or wrist, sole or palm, and digits. Ankle or wrist consists primitively 

 of nine or ten bones in three rows: a proximal row of three, named tibiale or radiale, inter- 

 medium, and fibulare or ulnare; a middle row of one or two centrales; and a distal row of 

 five tarsales or carpales. Loss or fusion of any of these elements is quite common. The 

 palm or sole is composed of five parallel bones, the metatarsals or metacarpals. The digits 

 consist of phalanges of which the primitive numbers are, proceeding from the first to the fifth 

 digit, 2, 3, 3, 3, 3. Loss of digits is also common; nearly always the inner and outer digits 

 disappear with retention of middle ones; very rarely are middle digits lost. Extra digits 

 may also occur, usually on aquatic forms. 



