66 THE FROG 



dilute acetic acid (5 per cent.) under the cover glass; this 

 is best done by placing a drop of the acetic acid on the slide 

 in contact with one edge of the cover glass, and drawing out 

 the liquid from the opposite edge by means of a piece of 

 absorbent paper. Observe that there are many nuclei, and 

 that they are elongated and highly refractive. Each nucleus 

 lies in an undifferentiated portion of the protoplasm. . Draw 

 a portion of a fiber, showing details of structure, represent- 

 ing the fiber about twenty-five millimeters in diameter. 



B. General Anatomy of the Skeletal Muscles. A skel- 

 etal muscle consists of a bundle of striated muscle fibers 

 bound together by connective tissue and fastened at each end 

 to different bones, by means of strong cords called tendons. 

 Of the two attachments of a muscle, one is usually to a more 

 fixed and central part, the other to a more movable and 

 peripheral part; the former attachment is called the origin 

 of the muscle, the latter its insertion. 



Since muscles can exert force only while shortening and 

 not while elongating (in other words, since they can pull 

 but cannot push), many of ihe muscles or sets of muscles 

 are arranged in pairs, the members of a pair working in ap- 

 position to each other. 



The many actions of which the frog is capable require a 

 large number of separate muscles, only a few of which can 

 be considered here. To illustrate the mode of attachment, 

 functions and mutual relationships of the skeletal muscles 

 a study will be made of the principal muscles of the hind 

 limbs. 



The motions of which a limb is capable are conditioned, 

 not only by the number and arrangement of its muscles, but 

 by the form of its joints, some of which allow only certain 



