216 HISTORY OF THE HUMAN BODY 



a secondary condition induced by an extension of the trans- 

 verse process of the atlas. 



To complete the discussion of the axial muscles there still 

 remains a group to be considered, and that is, the muscles of 

 the eyeball, for the proper estimation of which one must look 

 to embryology, especially that of selachians and amphibians. 

 During development certain pairs of mesodermic somites ap- 

 pear in the head as well as throughout the trunk and tail, and 

 in embryos of such forms as those named, in which the repe- 

 tition of the early stages is best given, these epimeres appear 

 for a time as hollow cavities, the so-called " head cavities." 

 With several of these head cavities dorsal (motor) elements 

 from the cranial nerves become associated, and although cer- 

 tain of them become abortive, after continuing up to the stage 

 of possessing a nerve supply, three pairs remain, those asso- 

 ciated with the third, fourth and sixth pairs of cranial nerves 

 respectively, all motor nerves. From these develop the muscles 

 of the eyeball, in accordance with their innervation; that is, 

 from the walls of the head cavity associated with the third 

 nerve (motor oculi) develop rectus superior, rectus internus 

 (medialis), rectus inferior and obliquus inferior; from that as- 

 sociated with the fourth nerve (trochlearis) there develops 

 the obliquus superior alone ; and from that supplied by the sixth 

 nerve (abducens) arises the rectus externus (lateralis). The 

 retractor bulbi, a muscle which appears first in the amphibians, 

 develops from rectus externus and is consequently innerved by 

 the sixth nerve ; it lies enclosed by the recti and often develops 

 into a hollow cone or a system of slips which act almost as 

 independent muscles. It is well developed in most mammals, 

 but is rudimentary or lacking in the primates. 



Aside from the head cavities from which the muscles of the 

 eyeball are derived, a matter which is in many points still 

 controversial, there are several other pairs of head cavities 

 which come to nothing, and although there is at present no 

 generally accepted theory concerning the primary number of 

 cephalic myotomes, or the exact relationship of those that 

 produce the eyeball muscles, it is evident that primarily the 



