48 PHYSIOLOGY l-'OK BKGINNKRS 



layer of cartilage, and this is covered with a smooth trans- 

 parent membrane, as is also the inside of the bag itself. This 

 membrane is called the synovial membrane ; it is moist 

 with a fluid, the synovial fluid, which facilitates the movement. 



Let us examine a few joints in detail. 



The Shoulder Joint. The upper end of the humerus 

 is rounded into a smooth head, fitting into a shallow cup- 

 like depression in the scapula, forming a ball and socket joint. 

 The capsular ligament forms a loose bag, attached all round 

 the cup of the scapula, and all round the humerus just below 

 the head. Band-like ligaments also connect the humerus with 

 the scapula. The synovial cavity, the cavity enclosed by the 

 capsule, is large and the joint very free, so that the arm can 

 be moved in all directions and rotated. When the arm is 

 raised to the horizontal position, the humerus is brought up to 

 the parts of the scapula projecting over the shoulder joint, and 

 so the upward movement between the humerus and the scapula 

 is limited. When the arm is raised beyond this the scapula 

 is moved with the arm. 



The Hip Joint. The upper end of the femur is 

 round, forming a large ball-like head, connected by a neck 

 to the shaft ; this fits into a deep cup-like socket in the hip 

 bone. The capsular ligament is attached round the edge of 

 the cup and round the femur just beyond the head. It is much 

 thicker in front than behind, the front forming, in fact, a strong 

 band-like ligament across the front of the joint. There is also 

 a round cord-like ligament, the round ligament as it is called, 

 passing from the head to the bottom of the cup, inside the 

 joint. The round ligament is sufficiently long to allow the 

 femur a good deal of play. The hip joint, like other ball and 

 socket joints, allows movement in all directions, but the range 

 is not so extensive as at the shoulder joint, because the socket 

 is deeper and the capsule forms a closer bag. Moreover, the 

 hip bone cannot move in the same way as the scapula can. 



The Knee Joint. The knee joint is a hinge joint between 

 the femur and the tibia ; the fibula has no share in it. The 

 upper end of the tibia is broad, and has on its upper surface 

 two shallow depressions side by side, the articular surfaces. 

 These are deepened by a band of fibro-cartilage, semilunar in 

 shape, running round the outer edge of each. The lower end 



