FROG : EXTERNAL FEATURES AND BODY- WALL 41 



There is, generally speaking, also a difference in fine 

 structure between voluntary and involuntary muscle, but we 

 shall postpone discussion of this for the present. A muscle 

 has a belly of muscular tissue which is attached by tendons 

 of a peculiar kind of connective tissue. One of the two 

 attachments is called the origin, and this is made to a 

 relatively fixed part ; the other, called the insertion, is made 

 to a more movable part. Parts of the skeleton which are 

 thus movable upon one another must be provided with 

 joints. When the amount of movement which is possible 

 is small, the joint consists of an intervening layer of cartilage 



ar.c- — 



Fig. 20. — A diagram to illustrate the structure of "perfect " joints. 



ar.c, Articular cartilage ; bn., bone ; lig., ligament ; med., medulla or 

 marrow ; syn.c, synovial capsule. 



or ligament, and is said to be imperfect. This kind of joint 

 is found, for instance, between the bones of the frog's 

 shoulder girdle. When free movement is possible there is 

 a perfect joint. Here a convex surface of one structure 

 plays within a concave surface of another, the two 

 surfaces being separated by a fibrous bag, the synovial 

 capsule, which contains a watery fluid, the synovia, and serves 

 as a cushion. Outside the joint, ligaments hold the mov- 

 able pieces together. The muscular system of the frog 

 is complicated, and we shall therefore only give an 



