CHAPTER IV 



THE FROG (continued) : THE JOINTS AND MUSCLES 



IN the previous chapter the bones more than 150 in 

 number which together constitute the greater part of 

 the skeleton of a frog have been considered as so many 

 separate parts, fitting into or against one another in 

 certain ways. We must now see how they are joined 

 together in the entire animal so as to afford the requisite 

 support, and, at the same time, to allow of free move- 

 ment. 



The Hip-joint. Let us begin by a study of the hip- 

 joint (Fig. 15). 



The acetabulum (actb), as you have already seen (p. 50), 

 is a hemispherical depression on the outer surface of 

 the hip-girdle. It is formed of cartilage, continued 

 into a projecting rim round the edge of the cavity. The 

 head of the femur (hd) is also formed of cartilage, and 

 fits accurately but rather loosely in the acetabulum. 



The acetabulum is lined, and the head of the femur is 

 covered, by a thin layer of connective-tissue, the 

 perichondrium (p. chd), which, in both cases, is continued 

 on to the adjacent bone, where it receives the name of 

 periosteum (p. ost). 



Attached all round the rim of the acetabulum is a 

 strong sheet of connective-tissue called the capsular liga- 



