THE SKIN AND SKELETON 355 



The neural spines of the dorsal series are generally long, 

 pointing backward. 



The lumbar vertebra are the massive vertebrae lying 

 in the loins between the dorsals and the hip bones. 



The sacral vertebra lie between the hip bones, and are 

 generally consolidated into one complex bone, called 

 sacrum. 



The caudal vertebra are placed behind the sacrum, 

 and form the tail. They diminish in size, losing pro- 

 cesses 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 

 shoulder blade, the coracoid, and the clavicle, or collar 

 bone. The scapula and coracoid are generally united 

 in mammals, the latter being a process of the former ; 

 and the clavicles are frequently wanting, as in the hoofed 

 animals. The hjimerus, 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 meta- 

 carpus, or palm ; and the phalanges, or fingers. The 

 fingers, or " digits," range in number from i 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 ptibis. The three are 



