40 THE INDIVIDUAL ORGANISM 



and between femur and tibia ; and the sliding joint, as found between the 

 articular processes of the vertebrae and between the bones in the arch 

 of the foot. Other joints may be of one of these types, or may show com- 

 binations of hinge and sliding movements. Examining Figs. 3.6 and 3.8, 

 one may note the peculiar arrangement of the radius and ulna at the 

 elbow, with a strong hinge joint between ulna and humerus, and a pivot- 

 and-hinge combination in the joint between radius and humerus. All 

 joints that involve much motion are particularly adapted to reduce fric- 

 tion. The cartilaginous cap of each bearing surface is comparatively thick 

 and extremely smooth; and the whole joint is bathed in a lubricating 

 fluid held by a surrounding capsule of flexible but fluid-tight ligaments. 



The Skeletal Muscles 



The active, energy-using, heat-producing, and waste-forming part of 

 the locomotor system is the great complex of voluntary or skeletal muscles. 

 These muscles are attached to the skeleton and provide the power for its 

 movements. (Later we shall encounter other types of contractile tissue — 

 heart muscle, and involuntary or visceral or smooth muscle — as parts of 

 the circulatory, digestive, reproductive, and other organ systems.) 



We have already noted that the skeletal muscles account for the greater 

 part of the weight and bulk of the human body. They are made up largely 

 of a type of contractile tissue known as voluntary or striated muscle, which 

 is the most highly adapted of all contractile tissues for rapid and powerful 

 movement. The familiar "meat" of our diet consists to a great extent of 

 the skeletal muscles of other animals. A slice of round steak or ham is a 

 section of the large leg muscles of a cow or pig, together with portions of 

 bone and connective tissue. 



Nearly all skeletal muscles are attached to bones in such a way as to 

 cause movement of parts of the skeleton when the muscles contract. 

 Frequently the bone serves as a lever. Thus, in the instance of the fore- 

 arm, the elbow is the fulcrum, the force is applied beyond the elbow at 

 the point of muscle insertion, and the action is that of a lever of the third 

 class. The muscle here works at a mechanical disadvantage and must 

 exert a force of many pounds to lift a weight of a few pounds held in the 

 hand, but the muscles are able to move the forearm rapidly and to carry 

 the hand through a wide arc while they themselves shorten by only a few 

 inches. 



The structure and functioning of the biceps. As an example of a 

 typical skeletal muscle let us consider the biceps — one of the larger muscles 

 of the upper arm. The biceps is attached at its upper end by two tendons, 

 one to the shoulder blade and the other to the end of the humerus just 

 below its joint with the scapula. The two "heads," each with its own 



