Muscle

1. Muscle: classification based on histological criteria.Smooth, cardiac, and striated

2. Categories of muscles: based upon their functional role in an organism, muscles may either be visceral or somatic, skeletal or
    nonskeletal, or, voluntary and involuntary. 3. Skeletal Muscles: consist of muscular and tendinous elements. Tendons attach muscle to bone and are extensions of  the
    muscle fascia (epimysium) which wraps individual skeletal muscles. 4. Origins, insertions, and actions: denote the structural and functional relationship of muscles. Origin: site of attachment
     that remains fixed and does not move. 5. Names and homologies of skeletal muscles:


Axial Muscles

1. Skeletal muscles of trunk and tail, hypobranchial, tongue, and  extrinsic eye muscles. Characteristically, these muscles exhibit metamerism.

2. Metamerism is due to origin of these muscles in the segmental  mesodermal somites. Somite mesenchyme moves lateral and then  ventrally to fuse in the midventral line as the linea alba. Myosepta may seperate muscle of one body segment from another.

3. Fishes: axial muscles consist of myomeres seperated by  myosepta. Except in the agnathans, these are divided into dorsal (epaxial) and ventral (hypaxial) masses by the horizontal skeletogenous septum (fiberous connective tissue  sheet between vertebral column and lateral body wall).

 Metamerism of hypaxial muscles is interrupted by pelvic and  pectoral girdles, and by the gills. Dorsal to gills are the  epibranchials which extend to the skull. Beneath gills, the hypobranchials extend to the lower jaw.

4. Tetrapods: except for the urodele amphibians, tetrapods have lost most axial metamerism due to locomotion on land. Amniotes develop epaxial muscles arranged as straplike or pinnate bundles above the transverse processes. Similar modifications below the transverse process allowed for much greater  flexibility of the vertebral column.

 Hypaxial myomeres were also replaced by broad muscular sheets whose fibers ran in different directions (except the  intercostals).

 Epaxials of the trunk: extend from base of skull into tail.  Collectively called the dorsalis trunci in amphibians. In amniotes, they form 4 groups: intervertebrals, longissimus, spinales, and iliocostales.

 Hypaxial muscles of the trunk: divided into 4 groups  (subvertebrals, oblique, transverse, and rectus abdominus). 5. The mammalian diaphram: formed by a contribution of mesenchyme  from several somites which form a dome-shaped muscle  separating the thorax from the abdominal cavity.

6.  Muscles of the tail: continuation of epiaxial and hypaxial  muscles of the trunk which also contribute to sphincter  muscles of cloaca and anus.

7. Hypobranchial and tongue muscles: originate from mesenchyme of  postbranchial somites which moves forward in floor of pharynx  beneath the branchial arches (anterior extensions of hypaxial  muscles).

8. Extrinsic eye muscles: allow voluntary movement of the eye in  all directions. Include superior and inferior rectus, lateral  and medial rectus, and superior and inferior obliques. Muscles of the eyelid also belong to this group.