MUSCULAR SYSTEM II

I.   MUSCLE HOMOLOGIES

     A. Some muscles may have diverse functions; may respond simultaneously to several 
        different and opposing selective pressures.

     B. Skeletal and muscular systems = single coadapted mechanism that evolves as a unit.

     C. Phylogenetic instability:
        1.  Muscles not as stable (phylogenetically) as skeletal system.
        2.  Extensively modified in each class of vertebrates --> difficult to determine 
            homologies.
        3.  Problems:
            a.  Fusion or splitting of muscles.
            b.  Changes in points of attachment (which also changes function).
            c.  Loss of some muscles.
            d.  Muscles leave no direct fossil record; positions must be inferred from 
                attachment scars on bones.
        4.  Criteria for determining homology:
            a.  Embryonic anlagen; however, embryonic differences may make difficult.
            b.  Innervation - seems to exhibit some phylogenetic permanence.
            c.  Vascularization (not as good).
        5.  It is more reasonable to examine modifications of muscle groups rather than 
            changes in individual muscles.  These groups are established during embryonic 
            development.

II.  MUSCLE GROUPINGS

     A. Postcranial Musculature:
        1.  Appendicular.
        2.  Axial (divided by horizontal septum):
            a.  Epaxial.
            b.  Hypaxial.

     B. Cranial Musculature:
        1.  Jaw muscles:
            a.  Hypobranchial - from trunk somites, tips grow forward into throat; runs 
                between ventral gill elements.
            b.  Branchiomeric - from head somitomeres, innervated by cranial nerves; not 
                visceral muscle.
        2.  Extrinsic eye muscles - move or rotate eye ball; derived from somitomeres.

III. COMPARATIVE ANATOMY OF MUSCLES

     A. Postcranial Musculature:
        1.  Axial:
            a.  Fishes:
                (1)  Myomeres retain segmentation; separated by myosepta.
                (2)  Muscle fibers are short, but zig-zag folding of myomeres gives them 
                     control over extended lengths of body.
                (3)  Well-suited for undulatory movements.
            b.  Tetrapods:
                (1)  Axial musculature reduced (appendicular muscles increase in bulk to 
                     take on role of locomotion).
                (2)  Remaining axial muscles differentiate into specialized muscles for more 
                     complicated control over flexion of vertebral column.
                (3)  Hypaxial musculature involved in breathing & moving trunk; abdominal 
                     muscle sheets arranged into antagonistic layers (eg. obliques).
        2.  Appendicular:
            a.  Fishes: simple - fin elevators and depressors.
            b.  Tetrapods:
                (1)  Tendency of dorsal & ventral muscles to split & divide forming more 
                     distinct muscles for more complex control over limbs.
                (2)  Axial and branchiomeric musculature also contribute to muscles of forelimb, 
                     especially shoulder.

     B. Cranial Musculature:
        1.  Branchiomeric:
            a.  Fishes: associated with control of gill arches and jaws.
            b.  Tetrapods: jaw muscles & facial muscles; cucullaris gives rise to trapezius 
                and mastoid muscles (move scapula).
        2.  Hypobranchial:
            a.  Fishes - coracoarcuals (open jaws, expand buccal cavity).
            b.  Tetrapods - associated w/ branchial arch derivatives such as throat, larynx, 
                tongue.

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