366 APPLIED PHYSIOLOGY 



of the joint sockets and in the lengths of the ligaments in different per- 

 sons. In some persons the sockets are shallow and the ligaments long> 

 so that the joints can be bent to a far greater degree than usual. These 

 persons are able to twist and contort themselves into strange posi- 

 tions and shapes, and thus they make good circus actors. 



656. Action of muscles as ligaments. Nearly every 

 joint is crossed by muscles. By their pressure the mus- 

 cles aid in keeping the bones in place. In addition, when 

 one muscle acts, those upon the opposite side of the joint 

 also contract enough to prevent the head of the bone from 

 being drawn out of its socket. 



If all the muscles and cords about a joint are cut, the 

 ligaments stretch and the joint becomes loose and flabby. 

 If the ligaments are cut while the muscles and cords are 

 left, the joint remains snug and firm. 



657. Effects of pressure. After being kept in an un- 

 natural position for some time, joints tend to retain the 

 deformity. In wearing tight shoes, the great toes are 

 bent outward, while the little toes are bent inward. If 

 the joints are kept in this position day after day for years, 

 they remain permanently fixed in the deformed position. 

 The great toe joint may be tender, forming a bunion. 



658. Curvature of the Spine. The spine is naturally straight 

 from side to side. Strong muscles aid in keeping the head erect and 

 the shoulders thrown well back. By weakness of the muscles or by 

 carelessness the shoulders fall forward, increasing the natural curve of 

 the spine so that a person becomes round shouldered. 



If a child habitually sits sidewise at the desk, leaning continually 

 upon one arm, the growing bones and the ligaments of the spine will 

 gradually become fixed in the deformed position, which persists all 

 through life. Any person who, in his occupation, always assumes the 

 same attitude, may finally be unable to remove the curvature from his 

 spine. On the other hand, if one acquires a habit of sitting and walk- 

 ing and working in an erect position, the spine will grow in a natural 

 curve. 



