HIP-JOINT, KNEE-JOINT. 5! 



form by their union part of an arch upon which rests the 

 upper portion of the cotyloid cavity, and thus divides the 

 force acting upon the lower limbs. 



The head of the femur represents nearly two-thirds of a 

 sphere. It exactly fills the cotyloid cavity, but is not itself 

 all inclosed in it, as the depth of the cavity does not exceed 

 half the diameter of the sphere to which it belongs. A very 

 elastic, circular, fibro-cartilaginous ring surrounds the edge 

 of the cotyloid cavity, increasing its size, embracing the head 

 of the femur; and acts as a valve and hermetically closes the 

 articular cavity in which the head of the femur is retained by 

 atmospheric pressure alone. In fact, if we place this articu- 

 lation, properly prepared, under the receiver of an air-pump, 

 we shall see, as a vacuum is produced, the head of the 

 femur gradually slip down and leave the cotyloid cavity as 

 far as the ligaments will permit, and when air is again ad- 

 mitted into the receiver, it resumes its place in the cavity. 

 This beautiful experiment of E. Weber shows in a striking 

 manner the direct and constant influence of external agents 

 on the functions of the organism. 



The inclosing of the head of the femur in the cotyloid 

 cavity gives the articulation of the hip great solidity, which 

 is augmented by the muscles and ligaments which hold the 

 parts together as well as give them motion, so that it is only 

 by the greatest violence that the head of the femur can be 

 forced out of this cavity. This articulation, which is of the 

 same nature as that of the shoulder, permits the movement 

 of the lower limb in every direction, though to a less ex- 

 tent than that of the arm. We shall again have occasion to 

 discuss this movement. 



The lower extremity of the femur ends in two oblong, 

 rounded masses, which are called the condyles of the femur, 

 which rest in two cavities in the superior portion of the princi- 

 pal bone of the leg, or tibia, and form with them the articula- 

 tion of the knee. The semi-lunar cartilages, in terposed between 

 the two bones, diminish the pressure of the femur on the 

 tibia, and prevent the displacement of the former by increas- 

 ing the surface and depth of the articular cavity. In front 

 of the knee-joint is placed fas patella or knee-pan, the largest 



