THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



271 



Although the transformation of head segments into muscles has not been 

 followed in detail in mammalian embryos, it may be inferred from the study of 

 lower forms that three segments are involved in the formation of the eye muscles. 

 The most cephalic (anterior) segment gives rise to the recti superior, inferior 

 and medialis (internus) and to the obliquus inferior, all of which are innervated 

 by the occulomotor (III) nerve. The next segment gives rise to the obliquus 

 superior which is innervated by the pathetic (IV) nerve. The most caudal 

 segment gives rise to the rectus lateralis (externus) which is innervated by the 

 abducens (VI) nerve. 



The development and innervation of the other muscles of the head and" of 

 the hyoid musculature present certain peculiarities which have caused these 

 muscles to be considered as more closely related to the visceral musculature 

 than to the myotomic musculature. In the first place they are derived from. 



Eighth cervical 

 myotome 



Upper limb 

 bud 



Somatopleure 



Mesonephric 

 duct 





FIG. 231. Transverse section through the eighth cervical segment of a human 

 embryo of 2.1 mm. Lewis* 



the branchial arches (hence are often called branchiomeric muscles), and not 

 directly from the myotomes of the neck region. This places them in closer 

 relation to the visceral muscles, although they are structurally and functionally 

 different from the latter. In the second place the nerves which supply them 

 are fundamentally different from those which supply the myotomic muscles 

 (Chap. XVII). 



The first branchial arch on each side gives rise to the temporalis, masseter 

 and pterygoidei, to the mylohyoideus and digastricus (venter anterior) and to the 

 tensor tympani and tensor veli palatini. All these muscles are innervated by the 

 trigeminal (V) nerve. 



The second arch, which is often called the hyoid arch, gives rise to a large 

 sheet of myogenic tissue which produces many of the facial muscles, such as the 



