128 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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 hypaxial muscles, greatly modified, 

 can only be recognized in the cervical and pelvic regions. 



In the head the developmental conditions are more complicated 

 than in the trunk, our information being most complete with regard to 

 the ichthyopsida. Here, in the region which develops into the head, 

 ten ccelomic pouches are developed (in amniotes the number is appar- 

 ently twelve) . These are known by number, except that the most anter- 

 ior, which was not known when the numbers were applied is called A. 

 These ccelomic cavities (also known as head cavities) differ from the 



myotomes farther back in having no undivi- 

 ded portion of the ccelom below, correspond- 

 ing to the hypomeral zone, a difference possi- 

 bly due to the existence of visceral clefts in 

 this region (fig. 136). 



Four of these cavities lie in front of the ear. 

 Of these A disappears completely, its cells 

 joining the mesenchyme, while the other three 

 FIG. i 37 . Diagram of give rise to the 'eye muscles' which move 

 the eye muscles of the right th e ye-balL Without going into all of the 



eye, seen from the medial t. 



side, er, external rectus; ifr, details, i, which lies in front of the mouth, 



inferior rectus; io, inferior 



oblique; itr, internal rectus; 



gives rise to the superior oblique muscle ; 2, 



so, superior oblique ;sr, S upe- wm 'ch lies in the region of the jaws, forms four 



rior rectus; III, coulo mo tor; , ,,. , , c 



/F,trochiearis;F/,abducens muscles, the inferior oblique and three oi 



nerves. 



the rectus muscles (in some forms also a 

 retractor bulbi), while 2, in the hyoid region, develops the external 

 rectus. This method of origin explains the distribution of the eye- 

 muscle nerves to be described later, each nerve supplying only the 

 derivatives of a single myotome. Several of the other head myo- 

 tomes disappear in development, while the posterior form the so-called 

 hypoglossal musculature (fig. 138). t 



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

 tractile tissue of the muscles. To this other parts of connective tissue 

 are added. Mesenchyme cells invade the masses of muscle fibres, 

 forming envelopes (perimysiurn) which bind the fibres in to bundles 

 (fasciculi) which, in turn, are united by other envelopes, the fascia. 

 These connective-tissue envelopes are extended beyond the contractile 

 tissue and form the cords or tendons by which the muscle is attached to 



