182 STRUCTURE OF THE VERTEBRATES 



side. The dorsal portion is thick, and thins out ventrally. Sep- 

 arating each myotome from its neighbor is a myoseptum, a band 

 of connective tissue, uniting adjacent muscle bands with each 

 other and with the notochordal sheaths. The muscle fibers ex- 

 tend longitudinally, being attached with connective tissues to 

 the myosepta. Therefore, when one side contracts the animal 

 forms the arc of a circle. As the anterior end of the animal is 

 more rigid than the caudal end, the alternate contraction of the 

 two sides sets up a propeller-like motion and sends the animal 

 forward. 



The cyclostomes have more complex myotomes. The middle 

 portion of the ''V" is retained in place, the apex being at the 

 axial line, but the dorsal and ventral portions are bent forward, 

 giving a sigma shape to each myotome. Contractions thus tend 

 to give a dorso-ventral as well as a lateral motion. 



The myotomes of the dogfish are more complicated by the 

 further bending and elongation of the dorsal part of the epaxial 

 group. Each myotome is attached to several vertebrae, losing in 

 part the simple metameric structure. The contraction of the 

 myotome affects more than one segment of the body, giving 

 greater flexibility to vertebral movements. 



Future evolution, particularly in the land vertebrates, is in 

 correlation with (1) the lack of a supporting water environ- 

 ment which diffuses shocks, and (2) the flexibility required by 

 land life where locomotion is supplied by the legs rather than 

 axial movements of the body. There is a progressive elongation 

 and splitting of the myotomes, attaching them to a greater num- 

 ber of vertebrae, and giving a nicety of individual function im- 

 possible in the simpler forms. 



The embryology of the mammal demonstrates many of these 

 changes. The first myotomes are simple metameric structures, 

 broken at the axial line. The epaxial group elongate, split, and 

 become largely longitudinal in position. A great number of 

 muscles result, some remaining as short muscles of small diam- 

 eter, others fusing to form long muscles extending more than 

 half the length of the vertebral column. These back muscles are 

 classified into several distinct groups. In the anterior region are 

 short occipital muscles which are attached to the head and 

 cervical vertebrae. These are structurally continuous with the 



