SKELETAL SYSTEMS AND MOVEMENT 



503 



Each side of the pelvic girdle is made up 

 of 3 bones, the ilium, ischium and pubis, 

 united into a single innominate (os coxae) 

 bone. The bones in each leg are the femur, 

 patella, tibia, fibula, tarsals (7), metatarsals 

 (5), and phalanges (14). The total num- 

 ber of bones in the adult human skeleton is 

 206, consisting of the cranium (8), face 

 (14), ears (6), neck (1), vertebrae (26), 

 ribs (24), sternum (1), shoulder girdle (4), 

 pelvic girdle (2), upper limbs (60), and 

 lower limbs (60). 



Primitive vertebrates tend to have a larger 

 number of skull bones than the more recent 

 forms. Some fishes have 180 skull bones; 

 amphibians and reptiles, 50 to 95; and mam- 

 mals, 35 or fewer, with man having only 29. 



Protection and movement 



The vertebrate skeleton provides protec- 

 tion, support, surfaces for the attachment of 

 muscles, and leverage for locomotion. The 



cranium protects the brain; the vertebral 

 column protects the spinal cord; the sternum 

 and ribs protect the organs in the thorax; 

 and sockets protect the eyes and ears. Bend- 

 ing movements of the body and of the neck 

 are made possible by the movable parts of 

 the vertebral column. Breathing movements 



involve enlargement of the thoracic cavity 

 by contraction of the diaphragm and a num' 

 ber of other muscles of inspiration, which 

 force the sternum and ribs upward and out 

 ward, and a subsequent decrease in the size 

 of the thoracic cavity by the elastic recoil 

 of the parts previously stretched. The bones 

 of the skeleton serve for the attachment of 

 muscles that move the limbs and other parts 

 of the body such as the lips, nose, eyelids, 

 etc. Many bone surfaces are large for the 

 attachment of important muscles; in man 

 are the hipbone (innominate Fig. 369) for 

 the muscles that help maintain an upright 

 position, and the shoulder blade (scapula) 

 for the muscles that move the forelimbs; 

 and in birds, the enormous sternum for the 

 muscles of flight. 



Joints 



Locomotion in vertebrates require mov- 

 able joints (Fig. 372). These are of several 

 types: (1) gliding joints, such as those be- 

 tween the carpal bones, which allow only 

 gliding movements; (2) hinge joints, such 

 as the elbow, which permit motion in one 

 plane only; (3) pivotal joints with a rotary 

 movement in one direction, such as between 



Ligament 



Bone 



Joint cavity 

 containing 

 synovial fluid 



Articular cartilage 



Synovial membrane 

 of joint capsule 



OJI 



Figure 372. A diagram of a joint surface (longitudinal section) showing some of its charac- 

 teristics. This is a freely movable joint. 



