STRUCTURE OF THE VERTEBRATES 



181 



Diagram III will show how muscle levers may be changed 

 by shifting the function and therefore the fulcrum. If the heel 

 is raised and the floor is tapped with the toe, the calf muscles 

 function as in the first lever, W:F:P. In this case the lever has 

 remained the same, while the fulcrum has been shifted. Other 

 cases involve a shift in the lever itself. Thus, a monkey may 

 stand on the ground and pull the limb of a tree downward; or, 

 he may hold a limb and pull his body upward. Essentially the 

 same muscles are brought into play although the weights and 

 levers have been changed. 



]\Iuscle groups are named for their more prominent function, 

 although at times their action may be different. For example, 

 the muscles on the palmar (ventral) side of the forearm are 

 called flexors. Those on the back (dorsal) side are the exterisors 

 because their major function is to straighten an angle caused by 

 the contraction of the flexor group. But when the hand is flexed 

 at the wrist, the extensors may draw it into the position of a 

 straight angle and then continue until the hand is in a position 

 of dorsal flexion. 



C. Axial IMuscles 



Amphioxus has the simplest possible arrangement of the axial 

 muscles. Each segment is approximately equal to every other, 

 and is a V-shaped band extending from the dorsal to the ventral 



Myotomes^ 



—Myoseptum 



A. Amphioxus 



B. Cyclostx)me 



C. Dogfish 



Fig. 102. Axial Muscles of Primitive Chordates (Diagrammatic). Note 

 the bending and elongation of the myotomes in the Cyclostome and 



Dogfish. 



