134 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



tractile tissue of the muscles. To this other parts of connective tissue 

 are added. Mesenchyme cells invade the masses of muscle fibers, 

 forming envelopes (perimysium) which bind the fibres into bundles 

 (fasciculi) which, in turn, are united by other envelopes, the fascia. 

 These connective- tissue envelopes are extended beyond the contractile 

 tissue and form the cords or tendons by which the muscle is attached 

 to other parts. Qne-jQoint of attachment, the origin, is fixed, that 

 t o the part to be moved i s 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, as around the eye and mouth of mammals (fig. 149), 

 the parietal muscles are without attachment, but form rings which 

 are usedjto diminish the size of ari^ 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 

 difliculties 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 drived from 

 any one myotome being innervated by branches of the nerve origi- 

 nally 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 con- 

 dition may take place in various ways. A single muscle may split 

 into layers or it may divide longitudinally into two or more distinct 



