THE ACTION OF LOCOMOTOR MECHANISMS. 415 



play, the extent of the movement varying with the character of the surfaces 

 and the length of the ligaments, 



4. Union by Joints ( Diarthrosis). — The adjacent surfaces of most of the 

 bones are so formed as to permit of close contact and freedom of movement in 

 special directions. The parts of the bones entering into the joint are clothed with 

 very smooth cartilage, and the joint-surfaces are lubricated by synovial fluid, 

 a viscid liquid secreted by a delicate membrane which lines the fibrous capsule 

 by which the joint is surrounded. The joint-capsule is firmly attached to the 

 bones at the margin of the articular cartilages, and, at the same time that it 

 completely surrounds and isolates the joint-cavity, it helps to bind the bones 

 together. The bones are further united by strong ligaments, in some cases 

 within aud in other cases without the capsule. These ligaments are so placed 

 that they are relaxed in certain positions of the joints and tightened in others ; 

 they guide and limit the movements of the joints. The joint-surfaces always 

 touch, although usually the parts in contact change with the position of 

 the joint. If continuous contact of the joint-surfaces is to be maintained 

 and free movement is to take place in special directions, it is evident that the 

 opposing surfaces must not only be so constructed that they shall fit each 

 other with great accuracy, but also have forms especially adapted to the move- 

 ments peculiar to each of the joints. 



The different joints exhibit a great variety of movements aud may be clas- 

 sified as follows : gliding joints, hinge joints, condyloid joints, saddle joints, 

 ball-and-socket joints, pivot joints. For a description of the structure and 

 the peculiarities of these joints the student is referred to works on anatomy. 1 

 The contact of the surfaces of the joint is secured in part by the fibrous capsule, 

 in part by the joint ligaments, and in part by the tension of the muscles. The 

 elastic muscles are attached under slight tension, and, moreover, during wak- 

 ing hours are kept slightly contracted by tonus impulses of reflex origin. 

 Another less evident but no less important condition is the atmospheric pres- 

 sure. The capsule fits the joint closely and all the space within not occupied 

 by the bones is filled by cartilages, fibrous bauds, fatty tissues and synovial 

 fluid. The joint is air-tight, and, as was first demonstrated by the Weber 

 brothers, the atmospheric pressure keeps all parts in close apposition. This 

 force is sufficiently great in the case of the hip-joint to support the whole 

 weight of the leg even after all the surrounding soft parts have been cut 

 through. The proof that the air-pressure gives this support is found in the 

 fact that the head of the femur maintains its place in the acetabulum after 

 all the soft parts which surround the joint have been divided, but falls out 

 of its socket if a hole be bored in the acetabulum and air be permitted to 

 enter the cavity of the joint. Though the air-pressure keeps the bones in 

 constant contact it offers no resistance to the movements peculiar to the joints. 



The movements of the hones arc effected chiefly by muscular contractions, 

 but the direction and extent of the movements arc for the most part deter- 

 mined by the form of the joint-surfaces and the limitations to movement 

 1 Quoin's Anatomy, vol. ii. pt. 1 ; Grm/s Anatomy; Horrids Anatomy. 



