112 HUMAN ANATOMY. 



individuals to uncommon pressure or frictiion, and, under these circumstances, the 

 ones usually present may be enormously enlarged. 



Modes of Fixation in Joints. — Ligaments, muscles, atmospheric pressure, 

 and cohesion are the agents for fixation. 



Ligaments. — A capsular ligament, pure and simple, has little retaining 

 strength. The accessory ligaments, on the contrary, have great influence. Their 

 arrangement differs with the nature of the joint. Thus, a ball-and-socket joint has 

 thickenings at such parts of the capsule as the particular needs of that joint require. 

 A hinge-joint implies strong lateral ligaments ; a rotary joint, some kind of a retain- 

 ing-band that shall not arrest motion. Sometimes certain ligaments are tense, or 

 nearly so, in every position of the joint, as the lateral ligaments of a hinge-joint. 

 Often a ligament is tense only when a joint is in a particular position, as the ilio- 

 femoral ligament of the hip when the thigh is extended. A strong ligament like 

 the one just mentioned is, when tense, the greatest protection against displace- 

 ment. 



Muscles. — The action of the muscles is of great importance in maintaining 

 the joints in position, in certain instances being the most efficient agency. The con- 

 stant pull of the muscles keeps the more movable bone closely applied to the more 

 fixed in all positions. Muscles which are nowhere in contact with the joint may 

 exercise this function. The tendons of muscles sometimes act as ligaments, which 

 differ from the ordinary ligamentous bands in that they may be made tense or 

 relaxed by muscular action. Sometimes they are intimately connected with the 

 capsule, at other times distinct from it. Some muscles, whose tendons cross several 

 joints, exercise, by their tonicity, an influence on them all. Thus, the peroneus 

 longus is essential to the maintenance of the transverse arch of the foot. Certain 

 muscles passing over more than one joint exert a ligamentous action on one joint 

 determined by the position of the other. This, however, is more properly dis- 

 cussed in connection with the action of muscles. 



Atmospheric Pressure. — Much has been written about the action of this 

 agency in holding joints in place. The atmosphere exerts a certain pressure on all 

 bodies, animate or inanimate, and thus tends to compress them. The joints, as 

 parts of the body, are subject to this general influence. It is by no means very effi- 

 cacious. The shoulder-joint has a capsule long enough to allow very free motion, 

 and consequently too long to hold the humerus in place. This is done chiefly by 

 the muscles. When these are paralyzed the arm falls out of place, atmospheric 

 pressure being inadequate to resist the weight. The most important action of 

 atmospheric pressure is to keep the soft parts closely applied to the bones. 



Cohesion is the action of the viscid synovial fluid which tends to hold the 

 surfaces together. It is very feeble, but probably has an appreciable influence in 

 the smaller joints. 



Limitation of Motion. — The shape of the joint determines the nature of the 

 movement ; its range depends in part on other factors, such as the tension of liga- 

 ments or of the tendons of muscles and the resistance of the soft parts. 



Motion in True Joints. — It is easy to conceive that an upright rod on the 



highest point of rather less than half a sphere may 

 Fig. 138. slide to the periphery along an indefinite number of 



ROTATION^ amq lines. This is angidar motion. The rod on reach- 



■4^^ ing the periphery, or at any point on the way, may 



■•-.%^ travel round in a circle describing the surface of a 

 iJCTioA/  \ cone. This is circu^ndudion. Finally, without any 

 ^\/? ~^* change of position, the rod may revolve on its own 



C;;;;^^-..,N^ : \ ^.-^--'^ axis. This is rotation. 



i^:^^^-^ \.(<^ J^S^^^-^^"^ Changes of Position of Parts of the Body. 



'.^ , . o r — Assuming that the palms are looking forward, an- 



Diagram illustrating different kinds of , .'^ . .. F - P ^ 1 



motion. gular motion of a limb, or of a part or one, towards 



the median plane of the body is called adduction ; the 

 opposite movement, abduction. A motion bringing the distal end of a limb bone 

 nearer to the head is called flexion ; the opposite movement, extension. The move- 

 ments of the ankle and the foot, however, present a difficulty, although the above 



