MUSCULAR SYSTEM 



82? 



muscle so that there are as many primitive muscles as there were 

 myotomes. 



The somatic wall of the myotomes does not become muscle but 

 changes into mesenchyme from which the corium of the skin develops. 

 Some of the mesenchyme protrudes between the various myotomes and 

 there forms fibrous connective tissues that later become the ligaments 

 which connect the various muscles of a side. 



This primitive muscle segmentation can still be seen in the inter- 

 costal and rectus abdominis muscles. 



The myotomes lie close to the level of the notochord and spinal cord r 

 but they grow both dorsally and ventrally, working their way between 

 the skin and the walls of the coelom to become an actual part of the 

 somatopleure. 



Ventrally the muscles from both sides grow toward each other and 

 practically meet at the mid-ventral line, the direct mid-ventral line which 

 is filled with connective tissues being known as the linea alba. 



In the fishes the trunk and tail muscles are arranged in myomeres 

 which take a zig-zag course. (Fig 401). The muscles are divided hori- 

 zontally into dorsal and ventral portions (Fig. 423), the epaxial and hy- 

 paxial muscles, a line of division which follows more or less closely the 

 lateral -line. The plates of muscle do not retain their flat ends in the 

 adult, but one end becomes conical and fits into a corresponding hollow 

 in the next plate. In the tail of the amphibia epaxial and hypaxial 

 muscles are clearly recognizable, but farther forward the hypaxials are 

 greatly reduced, and in the amniotes the reduction is carried so far that 

 the epaxial muscles, greatly modified, can only be recognized in the cer- 

 vical and pelvic regions, the "tender-loin" being epaxial. 



The developmental conditions are more complicated in the head than 

 in the trunk. In the head region of fish and birds ten coelomic pouches 



develop while in amniotes the 

 number is apparently twelve. 

 These are known by number, with 

 the exception of the most anterior, 

 which was not known when the 

 numbers were applied and is 

 called A. These coelomic or head 

 cavities differ from the myotomes 

 farther back by having no un- 

 divided portion of the coelom be- 

 low, corresponding to the hypo- 

 meral zone. This difference is 

 possibly due to the existence of 

 visceral clefts in this region. 



Four of these cavities lie in 

 front of the ear of which A dis- 

 appears completely, its cells join- 



rig. 466. 



The head of a dogfish, seen from above with 

 the right orbit opened, e., eyeball ; o.f., o.s., in- 

 ferior and superior oblique muscles ; r.e., r.int., 

 r.s., external, inferior, internal, and superior recti 

 muscles; e.p., spiracle; //., optic nerve; IV., 

 fourth nerve. (From Borradaile). 



