856 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

 touch, although in some joints 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 and 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 bands, 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 bones is effected chiefly by muscular contractions, 

 but the direction and extent of the movements is for the most part determined 

 by the form of the joint-surfaces and the limitations to movement which result 

 from the method of attachment of the ligaments. In the case of sliding joints, 

 in which the articular surfaces are nearly flat, a sliding movement may occur in 

 various directions, but the extent of the movement is slight, being limited by the 

 capsule and the ligaments. Hinge joints have but a single axis of rotation, 

 because the convex and somewhat cylindrical surface of one bone fits quite 

 closely the concave surface of the other, and because of tense lateral ligaments 

 which permit of movements only in a single plane. The joint between the 

 humerus and the uluar at the elbow is an example. The knee-joint 2 is a less 

 simple form of hinge joint. The presence of the semilunar cartilages and 



1 Quain's Anatomy, vol. ii. pt. 1. 



2 W. Braunne and Fischer have studied with mathematical accuracy the construction and 

 movements of many of the joints of the human body. Their articles are published in the 

 Abfuindlungen der maih.-phys. Classe der konigl Sdchsischer Gesellschaft der Wissenschaften, Bd. 

 xvii., and others. 



