ii MECHANICS OF LOCOMOTOE APPAKATUS 101 



were divided, and the capsular membrane and accessory ligaments 

 of the articulation were then cut off, the head of the femur remained 

 in the acetabulum, and was not apparently displaced. In this 

 case the entire weight of the lower limb is effectively supported 

 by atmospheric pressure, which is equivalent to admitting that 

 a column of air the height of the atmosphere and section equal 

 to that of the acetabular cavity, would be heavier than the lower 

 limb, which weighs about 22 kgrms. Weber tried a control ex- 

 periment. On the same subject he made a small opening from 

 the internal surface of the pelvis to the acetabulum, and allowed 

 the air to enter into the joint ; the head of the femur no longer 

 remained in the cavity, and the limb fell directly the air was 

 admitted, because the contact of the articular surfaces at the 

 point of perforation was not so intimate as at the edges of the 

 acetabulum, where there is a cartilaginous ring which exactly fits 

 the head of the femur, and consequently the air rapidly penetrated 

 between the surfaces. 



It is, however, known that the results 'of these experiments are 

 not applicable to other articulations : if the fingers are stretched 

 by a traction of not more than 500 grms., the articular surfaces 

 of the metacarpal-phalangeal joints come apart. The separation 

 produces a characteristic sound, and the articular capsule and 

 surrounding tissue are intraflected to fill the space left by the 

 displacement of the surfaces of contact. 



Besides the atmospheric pressure, the contact of the articular 

 surfaces is aided by the ligaments which are attached chiefly to 

 the capsule. This is apparent in the amphiarthrosis of the carpus 

 and tarsus, which, owing to the shortness, strength, and tension 

 of the accessory ligaments which strengthen the capsules as well 

 as to the complex and irregular form of the articular surfaces, are 

 movable only to a very limited extent. In' the arthroses and 

 diarthroses, on the contrary, which are more freely movable, the 

 capsules and ligaments serve, not to keep the articular heads in 

 contact, but rather to limit the movements. In fact they are not 

 tense when the muscles are at rest, but are thrown into tension 

 when the moving limb reaches a certain extreme position. In 

 order to understand the mechanism of the articulations in general, 

 it is also necessary to take into consideration the tone and state 

 of contraction of the muscles which surround the joints. Even 

 in the resting state the muscles are never so relaxed in the normal 

 individual, as not to contribute to the support of the joints. The 

 articular contact is opposed by the weight of the limb, as well as 

 by the pressure at which the synovial juice is secreted, which 

 cannot be less than that at which the blood circulates in the 

 capillaries of the synovial tissue. And there must always be 

 equilibrium between these antagonistic forces. It is not possible 

 to calculate exactly to what extent atmospheric pressure helps to 



