MUSCULAR SYSTEM 



^33 



osely the lateral line, being marked by a horizontal partition of 

 connective tissue already mentioned (figs. lo, ;^$). These 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 recog- 

 nizable, but farther forward the hypaxials are greatly reduced, and 

 in the amniotes the reduction is carried so far that the hypaxial 

 muscles, greatly modified, can only be recognized in the cervical and 

 pelvic regions, the 'tenderloin* being hypaxial. 



In the head the developmental conditions are more complicated 

 than in the trunk, our information being 

 more complete with regard to the ichthy- 

 opsida. Here, in the region which develops 

 into the head, ten coelomic pouches are de- 

 veloped (in amniotes the number is ap- 

 parently twelve). These are known by 

 number, except that the most anterior, 

 which was not known when the numbers 

 were applied is called A. These coelomic 

 ' /cavities (also known as 



Fig. 144. — Diagram of 

 head cavities) the eye muscles of the right 



, . eye, seen from the medial 



I differ from the myotomes farther back in side, cr, lateral rectus; t/r, 

 / having no undivided portion of tlie ccelom ^"JC^'rmUSl'rttt; 

 ' below, corresponding to the hypomeral ^^' superior oblique; sr, 



,._ ... , , superior rectus;///, ocu- 



zone, a difference possibly due to the ex- lomotor; iv, trochlearis; 



of visceral clefts in this r^ai^n F/, abducens nerves. 



region 



istence 



(fig- 143)- 



Four of these cavities lie in front of the ear. Of these A disap- 

 pears completely, its cells joining the mesenchyme, while the other 

 three give rise to the 'eye muscles' which move the eye-ball (fig. 

 144). Without going into all of the details, i, which lies in front of 

 the mouth, gives rise to the superior oblique nuscle; ^^ which lies 

 in the region of the jaws, forms fourjQiuscJcs, the inferior oblique 

 and three of the rectus muscles while 3, in the hyoid region, de- 

 velops the lateral (external) rectus (in some forms also a retractor 

 bulbi). This method of origin explains the distribution of the eye- 

 muscle nerves to be described later, each nerve supp ly only t h^ 

 deriva tivesjoLa, singlcjnyoiome (fig. 145). Several of the other head 

 myotomes disappear in development, while the posterior form the 

 so-called hypoglossal musculature (fig. 145). 



In the above account there is given merely the origin of the con- 



