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 the 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 movement. 



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



(Fig- is)- 



The acetabulum (adb), 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 pro- 

 jecting rim round the edge of the cavity. The head of the 

 femur (Jid) is also formed of cartilage, and fits accurately 

 but rather loosely in the acetabulum. 



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

 covered, by a thin skin of connective tissue, the perichon- 

 drium {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 ligament 



