64 THE FROG 



CHAP. 



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 hgament 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 which obscure the view. Notice the 

 belly i and the tendons of origin and insertion (p. 60). Sketch. 



Bend the foot upon the shank as in the ordinary sitting position of the 

 frog. Hold the thigh firmly with one hand, and with the other take hold 



