THE MOTIONS OF ANIMALS 75 



body curve to the same side and thus gives a stroke of 

 the tail fin. A similar muscle on the opposite side pro- 

 duces a stroke in the opposite direction. These alternating 

 strokes are the propelling power which forces the fish 

 through the water. At the base of each of the other 

 fins are found a few small strips of muscle. These give 

 their varied and more gentle movements which keep the 

 body in any particular position and aid in directing its 



Fig. 44. — Side view of a dogfish (shark) with a strip of skin removed to 

 show muscles. (After Parker.) 



locomotion. About the head are other muscles which 

 move the jaws, eyes, and gill-covers. 



If we take the skin from the side of a body of a dog 

 and dissect out all the muscles we find a large number of 

 well-developed ones. The great muscular mass on the side 

 of the fish's body used for one or a few motions is replaced 

 in the dog by a great number of muscles used to produce 

 a number of various movements. The few comparatively 

 weak muscles of a ventral fin are replaced by the many 

 large, strong, and definite ones of the hind limb of the 

 dog, while the small group working the pectoral fin finds 

 its representative in the large and varied group moving 

 the fore leg. 



Further differences are seen in comparing the head of 

 the fish with that of the dog, as well as in every other 

 part of the two bodies. These differences all show that 

 in the dog there is a separation of the muscular system 

 into more numerous and more definite forms of muscles, 

 with the possibility of more numerous and more accurate 

 motions than in the fish. If we compare the muscles 



