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HEAD MUSCLES 1

MUSCLES OF THE HEAD

       Muscles of the head allow humans move their eyes, chew their food, express emotion, and speak.  The muscles which allow humans to perform such movements are not unique to humans.

     During early development, a series blocks of embryonic tissue known as somites form bands of muscle along the body axis known as myotomes.  The somites of a developing chick are depicted below. A series of myomeres separated by bands of connective tissue develop from these somites and are the major axial muscles of lancelets and fish.

SOMITESlancelet

lanceletlamprey

hagfishlamprey

  Myotomes from head region of jawless and cartilaginous fish embryos suggest that the ancestral vertebrate condition included somites in the head region (pro-otic somites) which produced the muscles which move the eye.  The first pro-otic somite forms the superior rectus, inferior rectus, internal rectus, and the inferior oblique which are innervated by cranial nerve III.  The second pro-otic somite forms the superior oblique and is innervated by cranial nerve IV.  The third becomes the external rectus and is innervated by cranial nerve VI (Weichert, 1970).  These pro-otic myotomes may also give rise to levator palpebrae superioris and retractor bulbi of frogs. (Weichert, 1970).   The six extrinsic eye muscles occur in all gnathostomes and are the most highly conserved muscles of the vertebrate body  (Webster, 1974, p.126). 

EYE

EYE

HUMAN

LAMPREY

GILLS

SHARK

GILLS

   

    Branchial muscles develop from the pharyngeal pouches and move the gills in fish.  Branchial muscles form from the splanchnic mesoderm of hypomeres of embryos rather than from myotomes as do most muscles.  As a result, branchial muscles are innervated by visceral motor neurons rather than somatic motor neurons (Weichert, 1970).

      With the loss of gills in tetrapods, these branchial muscles were modified to perform new functions.  In amphibians, modified branchial muscles form the levator mandibuli, interhyoideus, larynx muscles, and the trapezius.  (Webster, 1974).  In amniotes, all the muscles corresponding to dorsal gill muscles and gill levators are involved with chewing such as the masseter, temporalis, and pterygoids.  (There is only one exception to this: the tensor tympani is located in the mammalian middle ear where it functions in hearing.  Two bones of the mammalian middle ear were derived from ancestral bones of the jaw joint.)  The tetrapod depressor mandibulae is retained only as the stapedius in mammals (Kardong, p. 393).

      The ventral branchial musculature forms the intermandibularis in amniotes (which forms intermandibularis and mylohyoid in mammals) and the digastricus.  In mammals, the ancestral digastricus forms part of the digastric, the platysma, and many muscles of facial expression. (Webster, 1974).  In humans, monkeys, and prosimians, the digastric may not pass through the stylohyoideus (Hartman, 1933). In tetrapods, the ancestral ventral gill constrictor forms the interhyoideus and sphincter coli (Webster, 1974). 

 

     Fish, including jawless fish, possess axial muscles which run underneath the gills forming the hypobranchial musculature.     Hypobranchial muscles are absent in lancelets (Weichert, 1970).  In tetrapods, these muscles are modified to form throat muscles.  In mammals, remnants of the cranial portions of this ancestral muscle include the sternohyoid, sternothyroid, geniohyoid, omohyoid, and thyrohyoid. (Weichert, 1970).   The ancestral condition of the rectus abdominis was a long muscle which stretched the entire length of the body.   The rectus cervicis is a continuation of rectus abdominis.  (Weichert, 1970, p. 514).In vertebrates, the MYH16 is expressed only in chewing muscles and other muscles derived from the first pharyngeal arch. In addition to humans, a number of mammals have lost the expression of this heavy chain myosin (such as artiodactyls, rodents, and kangaroos) (McCollum, 2006).

DRAWING
      Branchial muscles make up all other muscles of face.  Turtles and birds possess a sheet of muscles known as the sphincter colli deep to the integument of the head.  In mammals this sheet of muscles differentiates into facial muscles although they are poorly developed in most mammals.  The outer layer of the sphincter colli (the platysma layer) forms the auricularis frontalis, orbicularis oculi, mentalis, quadratus labii inferioris, and the zygomaticus.  The deep layer of the sphincter colli gives rise to muscles of the nose, the orbicularis oris, and buccinator muscles. (Weichert, 1970, 529).  Throat muscles attaching to the hyoid bone have roles in swallowing and/or speech and all the hyoid muscles used in human speech are present in monkeys.   A comparative look at he mylohyoid (red) and geniohyoid (orange, deep to the mylohyoid) are pictured in the adjacent drawing.

TONGUE MUSCLES

SHARK

SHARK

FROG

FROG

FROG

TURTLE

JAW MUSLCES

SALAMANDER

ALLIGATOR

JAW MUSLCES

OPOSSUM

STERNOTHYROID

JAW MUSLCES
JAW MUSCLES

CAT

JAWMUSCLES

JAWMUSCLES
JAWMUSCLES

 

JAW MUSCLES

     The masseter and temporalis muscles developed from the adductor musculature in gnathostome fish (Kardong, p. 393; Romer, p. 312).  The pterygoid muscles were probably derived from the masseter muscle (Hartman, 1933, p.96)

 

SHARK AND SALAMANDER

sharksalamander

FROG

FROG

OPPOSSUM

JAW MUSCLES

JAW MUSCLES

CAT

CAT

GOAT

JAW MUSCLES

JAW MUSCLES
JAW MUSCLES

MONKEY

JAW MUSCLES

HUMAN MODEL

JAW MUSCLES