366 COMPARATIVE ZOOLOGY 



straight parallel fibers, not in bundles, but distinct, and 

 usually flat, thin, and soft. 



The great majority of the muscles of vertebrates are 

 attached to the bones, and such are voluntary. The 

 fibers, which are coarsest in fishes (most of all in the 

 rays), and finest in birds, are bound into bundles by 

 connective tissue ; and the muscles thus made up are 

 arranged in layers around the skeleton. Sometimes 

 their extremities are attached to the bones (or rather to 

 the periosteum) directly ; but generally by means of 

 white inelastic cords, called tendons. In fishes, the 

 chief masses of muscle are disposed along the sides 

 of the body, apparently in longitudinal bands, reaching 

 from head to tail, but really in a series of vertical flakes, 

 one for each vertebra. In proportion as limbs are de- 

 veloped, we find the muscles concentrated about the 

 shoulders and hips, as in quadrupeds. The bones of 

 the limbs are used as levers in locomotion, the fulcrum 

 being the end of a bone with which the moving one is 

 articulated. Thus, in raising the arm, the humerus is a 

 lever working upon the scapula as a fulcrum. The 

 most important muscles are called extensors and flexors. 

 The latter are such as bring a bone into an angle with 

 its fulcrum as in bending the arm while the former 

 straighten the limb. Abductors draw a limb away from 

 the middle line of the body, or a finger or toe away 

 from the axis of the limb, while adductors bring them 

 back. 



2. Locomotion. All animals have the power of vol- 

 untary motion, and all, at one time or another, have the 

 means of moving themselves from place to place. Some 

 are free in the embryo life, and fixed when adult, as 

 the sponge, coral, crinoid, and oyster. There may be 

 no regular, well-defined means of progression, as in the 

 amoeba, which extemporizes arms to creep over the sur- 



