146 COMPARATIVE ZOOLOGY. 
generally consolidated into one complex bone, called sa< 
crura . 
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 
