146 COMPARATIVE ZOOLOGY. 



generally consolidated into one complex bone, called sa- 

 crum. 



The caudal vertebrae are placed behind the sacrum, and 

 form the tail. They diminish in size, losing processes and 

 neural arch, till finally nothing is left but the centrum. 

 They number from 3 or 4 in Man to 270 in the Shark. 



Besides the lower jaw, hyoid, and ribs, Vertebrates 

 have other appendages to the spinal column two pairs 

 of limbs. The fore limb is divided into the pectoral 

 arch (or shoulder girdle), the arm, and the hand. The 

 arch is fastened to the ribs and vertebrae by powerful 

 muscles, and consists of three bones, the scapula, or shoul- 

 der-blade, the coracoid, and the clavicle, or collar-bone. 

 The scapula and coracoid are generally united in Mam- 

 mals, the latter forming a process of the former ; and the 

 clavicles are frequently wanting, as in the hoofed animals. 

 The humerus, radius, and ulna are the bones of the arm, 

 the first articulating by ball-and-socket joint with the 

 scapula, and by a hinge-joint with the radius and ulna. 

 The humerus and radius are always present, but the ulna 

 may be absent. The bones of the hand are divided into 

 those of the carpus, or wrist; the metacarpus, or palm; 

 and the phalanges, or fingers. The fingers, or " digits," 

 range in number from 1 to 5. 



The hind limb is composed of the pelvic arch (or hip- 

 bones), the leg, and the foot. These parts correspond 

 closely with the skeleton of the fore limb. Like the 

 shoulder, the pelvic arch, or os innominatum, consists of 

 three bones ilium, ischium, and pubis. The three are 

 distinct in Amphibians, Reptiles, and in the young of 

 higher animals; but in adult Birds and Mammals they 

 become united together, and are also (except in Whales) 

 solidly attached to the sacrum. The two pelvic arches 

 and the sacrum thus soldered into one make the pelvis. 

 The leg-bones consist of the femur, or thigh; the tibia, or 



