64 THE FROG CHAP. 



Thus all the complex and accurately adjusted move- 

 ments of the frog are performed by the contraction of 

 its numerous muscles, acting either singly or in concert. 

 The contractions of these muscles are brought about by 

 nervous impulses sent from the brain or spinal cord 

 along nerves which branch out and are distributed to 

 the muscles, thus bringing the whole of the complex 

 machinery which affects the movements of the animal 

 under the direct control of its will. 



PRACTICAL DIRECTIONS 



The Joints and Ligaments. 



1. The hip-joint. Strip off most of the muscles from the 

 thigh and the adjacent parts of the pelvis ; decalcify in weak 

 acid and then wash thoroughly. Cut the femur through 

 lengthwise and continue the section through the pelvic 

 girdle (Fig. 15). The details are more easily made out if the 

 femur of a larger animal (e.g., rabbit) be used. 



Note : a. The cartilage of the acetabulum and head of the 

 femur ; the perichondrium and periosteum. (b) The capsular 

 ligament and its relations to the synovial capsule. Observe 

 that the hip-joint is a cup-and-ball joint. Sketch. 



In a prepared wet skeleton 



2. Examine and compare a hinge-joint (e.g., elbow or knee). 



3. Examine the cartilaginous union between the bones of 

 the shoulder- or hip-girdle (immovable or imperfect joints). 



4. Examine the joints and ligaments of the vertebral 

 column. 



The Muscles. 



1. Remove the skin from part of the body and legs of a 

 preserved frog (the one you have already dissected will do). 

 Then clear away the fascia here and there and separate some 

 of the muscles by dissecting away the connective-tissue 

 which binds them together. Notice the different forms of 

 the muscles in different parts (Fig. 16 and p. 63). 



2. In the hind-leg carefully dissect away the connective- 

 tissue investing the gastrocnemius muscle (Figs. 16 and 17), 

 and trace it upwards towards the thigh and downwards 

 towards the foot, cutting away any of the other muscles 



