1 32 LOCOMOTION. [CHAP. vi. 



A muscular capsule is thus provided for the joint, by which the 

 bones are maintained much more firmly and powerfully in appo- 

 sition than they would be if kept together by an uncontractile 

 ligamentous capsule alone ; hence the elongation of the arm that 

 occurs as a consequence of paralysis, and hence also the greater 

 liability to luxation which exists in a debilitated state of the system. 

 Articular or capsular muscles thus placed, have also the effect, as it 

 is said, of preventing the pinching of the capsule or synovial mem- 

 brane between the articular extremities of the bones in the different 

 motions of the joint. 



Atmospheric pressure, exerting as it does a force of nearly fifteen 

 pounds on the square inch, is a powerful agent in maintaining the 

 contact of articular surfaces. This is well illustrated by the diffi- 

 culty of separating the surfaces composing the hip and shoulder 

 joints, when the surrounding ligaments are air-tight; while, on the 

 other hand, these surfaces may be separated by the mere weight of 

 either bone, if one of these joints be suspended in the exhausted 

 receiver of an air-pump. In exhibiting this experiment, we find 

 the shoulder joint shews the effects of atmospheric pressure more 

 strikingly than the hip, for its capsule being loose, and the osseous 

 cavity for the reception of the head of the humerus small, the pres- 

 sure of the atmosphere pushes in the capsule, so that it fits closely 

 to the head of the bone. When this pressure has been removed by 

 exhausting the receiver, the head of the humerus falls rapidly from 

 the socket, and the ligament becomes stretched by the weight of 

 the bone. 



The joints are supplied copiously with blood, and are remark- 

 able for the arterial anastomoses which take place about them. 

 The best examples of these inosculations are met with around the 

 large joints of the extremities. The parts supplied with blood are 

 the synovial membranes, the ligaments, the fat, and the extremities 

 of the bone; but the cartilages certainly do not contain blood- 

 vessels. 



Of the Forms and Classification of the Joints. It is not difficult, 

 by passing in review the various motions which take place between 

 any two segments of a limb, to form an idea, a priori, as to the 

 kinds and shapes of the articulations by which these segments will 

 be united ; it is only necessary not to lose sight of the fact, that in 

 the construction of a joint, regard is had not to its mobility alone, 

 but to its security, its durability, and the safety of the neigh- 

 bouring parts. We may expect to find joints varying in the de 

 gree of motion, from the slightest perceptible, to the freest that i 



