MUSCULAR SYSTEM. 



the ventral half of the body, thus giving rise to the musculature 

 of this region, while dorsally their extension is less marked 

 (Fig. 1 1 3). In this process the myocommata also participate, so 



that the whole body is enveloped on 

 either side by a series of muscle-plates, 

 the fibres of which have a generally 

 longitudinal direction, and are inter- 

 rupted at regular intervals by the in- 

 termuscular ligaments, the derivatives 

 of the earlier myocommata. This 

 primitive condition can readily be rec- 

 ognized in the trunk region of a fish, 

 but it becomes greatly modified in the 



FIG. 117. Illustrating the 

 downward growth of the 

 myotomes. <?, epimere (myo- 

 tome) ; ///, mesomere. 



birds and mammals ; yet 

 even here traces of the primi- 

 tively segmented condition 

 can be made out in the 

 ventral abdominal region and 

 in the intercostal muscles. 



In the fishes the result- 

 ing muscles of the trunk and 

 tail become subsequently di- 

 vided into dorsal and ventral 

 or epiaxial and hypaxial sys- 

 tems, the line of division 

 between the two following 

 more or less closely the lat- 

 eral line, and being marked 

 by a partition of connective 

 tissue. In the amphibia 

 these epi- and hypaxial por- 

 tions are clearly visible in the tail, bt^t farther forward the 

 hypaxial system is reduced. This reduction is carried to a 

 greater extent in the aminotes, where almost the sole traces 



FIG. 118. Transverse section through 

 young Amblystoma^ showing the final re- 

 sult of the downward growth of the myo- 

 tomes (deeply shaded) ; a, aorta ; c, car- 

 dinal vein ; /, liver ; o, oesophagus ; />, 

 peritoneum ; pg, pectoral girdle ; r, rib ; 

 s, subvertebral vein. 



