MUSCULAR SYSTEM. 129 
other parts. One point of attachment, the origin, is fixed, that 
to the part to be moved iscalledtheinsertion. Tendons may belong 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. 5 
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, acondition strikingly illustrated in the facial muscles of the 
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