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BACK AND NECK MUSCLES


     Humans possess a number of muscles which allow them to move their backs when they dance, play sports, and get out of bed in the morning.  Humans possess a number of muscles which allow them to move their neck whether they are turning to look at a friend, bowing their heads in prayer, or looking up at the stars.  The muscles which allow humans to perform such movements are not unique to humans, they are typical of vertebrates.

     The ancestors of vertebrates had axial musculature which is divided into segments (myomeres) as in the early Cambrian chordate and jawless fish depicted below

haikouichtys

The axial musculature of Amphioxus and the lampreys retain this organization of myomeres, as is evident in the following image

somites

 

LAMPREY

LAMPREY

HAGFISH
In cartilaginous fish, the myotomes are separated into dorsal epaxial and ventral hypaxial divisions. (Weichert, 1970).  These layers are illustrated in the shark below.
SHARK SHARK
The epaxial muscles act to extend and laterally flex the back.  In fish and salamanders, this epaxial musculature is important for the lateral flexing of the back they depend on for locomotion.  The development of limbs decreased the importance of the epaxial muscles.
DRAWING In amphibians, the epaxial musculature is represented by a single muscle, the dorsalis trunci. In amniotes, this sheet of muscle is called the erector spinae and is divided into medial, intermediate, and lateral groups.  This dorsalis trunci also contains components of logissimus dorsi, intertransversarial, and interneural muscles found in amniotes. (Weichert, 1970, p. 511).  Amphibians and amniotes also possess interspinalis muscles derived from epaxial musculature (Webster, 1974).

FROG

FROG

SALAMANDER

 

SALAMANDER

     In reptiles, the erector spinae is composed of iliocostalis (lateral), longissimus (middle) and transversospinalis (medial) groups.  In the neck region, the longissimus forms the cervicus capitus muscle.  Reptiles also possess transversospinalis muscles from which occipital muscles are derived.  (Webster, 1974).

ALLIGATOR

MUSCLE GROUPS

.   In mammals, the lateral undulation of the vertebral column is greatly reduced in locomotion and the epaxial muscles of the erector spinae become even less important.  Mammalian muscles are similar to those found in reptiles but the anterior muscles are split into a greater number of subdivisions. (Webster, 1974).  The multifidus is a mammalian remnant of transversospinalis muscles in reptiles (Weichert, 1970, p. 513). 

OPOSSUM

ERECTOR SPINAE

 

CAT

ERECTOR SPINAE

ERECTOR SPINAE

GOAT

ERECTOR SPINAE

ERECTOR SPINAE

SHEEP

ERECTOR SPINAE

COW

ERECTOR SPINAE

RHESUS MONKEY

ERECTOR SPINAE