MUSCULAR SYSTEM. . 1 29 



other parts. One point of attachment, the origin, is fixed, that 

 to the part to be moved is called the insertion. Tendons may be long and 

 slender, allowing the muscle to lie in or near the trunk, while the part 

 to be moved is in the appendage. Again they may form broad flat 

 sheets (aponeuroses), and these may occur not only at the ends but in 

 the middle of a muscle. Not infrequently parts of tendons may ossify, 

 as in the patella or in the ' drum-stick ' of the turkey. Small rounded 

 ossifications of this kind are called sesamoid bones. In a few cases 

 the parietal muscles are without attachment, but form rings which are 

 used to diminish the size of an opening (sphincter muscles). 



Muscles vary greatly in shape. They are usually short and flat in the trunk, 

 prismatic or spindle-shaped in the appendages. They may be simple or they 

 may have several 'heads' or points of origin (biceps, triceps, etc.), or several 

 points of insertion as in pinnate or serrate muscles. Again, there may be two 

 or more contractile portions (bellies) in a muscle, separated by a tendon or 

 aponeurosis. 



Usually muscles are arranged in antagonistic groups, the action of one being 

 the opposite of its antagonist. Thus there are flexors to bend a limb, extensors to 

 straighten it; elevators to close the jaw, depressors to open it; sphincters working 

 against dilators, etc. 



Only a few points in the progressive modifications of the primitive 

 musculature described above can be mentioned here, partly from lack of 

 space, partly from deficient knowledge. There are great difficulties in 

 tracing exact homologies through the different groups of vertebrates, 

 on account of their very different functions in the separate classes and 

 their great variability, even in the same family. The best test of 

 homology is nerve supply, every muscle derived from any one myotome 

 being innervated by branches of the nerve originally connected with 

 the segment, as is beautifully illustrated in the case of the eye muscles 

 as mentioned above. Next in importance are origin and insertion of 

 the muscles, while the work done by the muscles is of little value. 

 Differentiations from the primitive condition may take place in various 

 ways. A single muscle may split into layers or it may divide longi- 

 tudinally into two or more distinct muscles, or transversely into two 

 successive portions. On the other hand, two muscles, different in 

 origin, may fuse, while with loss of function of a part, its muscles may 

 degenerate or entirely disappear. Muscles may wander far from their 

 point of ontogenetic origin and become connected with parts widely 

 remote, a condition strikingly illustrated in the facial muscles of the 



