56 THE FROG CHAP. 



(cps. lg\ forming a short, fibrous tube. The other end of 

 this tube is fixed to the femur, just below the head, the 

 ligament being continuous, in each case, with the perichon- 

 drium. There is thus a space between the head of the 

 thigh bone and the acetabulum, closed all round by the 

 capsular ligament. This space is filled with a delicate, fibrous, 

 closed bag, the synovial capsule (sy. cps}, one side of which 



med 



chct actb 



FIG. 15. Horizontal section of the Frog's hip-joint. 



cictb. acetabulum ; chd. perichondrium ; cps. Ig. capsular ligament ; lid. head of 

 femur ; il. ilium ; med. marrow ; p. ost. periosteum ; pu. pubis ; sh. shaft of 

 femur ; sy. cps. synovial capsule. 



fits closely into the acetabulum, while the other as closely 

 invests the head of the femur. The capsule is filled with a 

 watery fluid, the synovia^ and thus forms a buffer or water- 

 cushion between the adjacent bones. Thus the synovial 

 capsule keeps the two bones slightly apart and prevents 

 friction, while the capsular ligament keeps them together 

 and prevents dislocation. 



It is obvious that, in such a joint as this, movement is 

 possible in all directions. The femur can be inclined either 

 upwards, downwards, or sideways, and is capable of a certain 

 amount of rotation. The joint is, in fact, a cup-and-ball 



