STRUCTURE OF THE VERTEBRATES 177 



of evolution a single muscle may divide to form several; and 

 conversely, several separate muscles may fuse to form a single 

 one. The three bases of homology are (1) comparative anatomy, 

 (2) embryology, and (3) innervation. The last is, perhaps, the 

 soundest basis of judgment. 



The names of muscles are also confusing to students. The 

 earliest study of anatomy was human, and the muscles were 

 named accorchng to their shape, position or function in man. 

 These names rarely apply to the muscles of other animals. The 

 more recent method of naming muscles by their points of attach- 

 ment is an advance in the direction of simplicity. However, as 

 the skeletal structure of the vertebrates has changed, the muscu- 

 lar attachments have changed with them; and a muscle which 

 can be accurately homologized will have different names in the 

 various classes of vertebrates. The result is that scientific accu- 

 racy is incompatible with simplicity of nomenclature, and the 

 subject of homologies has been left largely in the hands of spe- 

 cialists in the field. 



Structurally a muscle consists of muscle fibers bound together 

 and attached at either end with connective tissues. Each fiber 

 is surrounded with a microscopically thin sheet of tissue, and 

 groups of fibers are bound into bundles. Several of these bundles 

 make a muscle, the entire structure being surrounded by a strong 

 covering of connective tissues. These fibers completely interlace, 

 and in this way bind the muscle into a strong functional unit. 



A muscle is usually attached by a broad base known as the 

 origin. It then widens to form, the belly of the muscle, and from 

 this point narrows almost to a point at its insertion on a bone. 

 The insertion is defined as the point of attachment on the part 

 which is to be moved. Frequently the function of a muscle 

 changes with the position of the animal, because of the change 

 of leverage. The rectus abdominis, which extends from the pubic 

 symphysis to the lower border of the ribs, illustrates the point. 

 In man the contraction of this muscle tends to pull the ribs, 

 and therefore the upper part of the body, downward; but in a 

 rabbit, or a man when he is suspended by his arms, the muscle 

 pulls the pelvis upward. Either end might be called the inser- 

 tion. Structurally, however, the muscle arises on the pelvis as a 

 thick, rounded mass, and spreads out on the ribs as a thin 



