434 H. V. NEAL 



mammals a retractor bulbi (oculi) makes its appearance as a 

 derivative of the external rectus muscle (Johnson, '13; Fraser, 

 '15), but in the great majority of vertebrates the number and 

 the nerve relations of the eye-muscles remain identical and un- 

 changed. Nature seems to have pursued with regard to them 

 the pohcy of 'letting well-enough alone.' Their 'evolutionary 

 potential' appears to be approximately zero. 



Were we therefore dependent upon comparative anatomical 

 evidence alone for our conclusions concerning the history of 

 the eye-muscles, we should be obliged to consider them as an 

 isolated and peculiar group, the pre-craniote history of which 

 is unknown. While we should not feel forced to assert that 

 they arose, Minerva-like, full-formed, nevertheless it, would 

 remain a matter of uncertainty or of speculation whether they 

 were exotic or endogenous, whether visceral or somatic, in their 

 origin. Comparative embryology, however, appears to justify 

 the assertion that the eye-muscles are a remnant of the lateral 

 trunk muscles which, in the ancestors of vertebrates, extended 

 in an unbroken series throughout the entire length of the body. 

 Of the parietal muscles anterior to the ear they alone have per- 

 sisted, through their functional relation^ to the eye-ball. Their 

 isolation is associated with the growth and enlargement of the 

 otic capsules and of the cranial skeleton. 



The details of this story have been slowly gathered. First, 

 Balfour ('78) discovered the extension of the body cavity into 

 the head region of Elasmobranch embryos, thereby demonstrat- 

 ing the fundamental similarity of head and trunk regions in 

 Vertebrates. He also showed that the mesoderm of the head 

 undergoes a segmentation independent of that of the visceral 

 arches, resulting in the formation of the so-called head-cavities. 

 Later, Marshall ('81) proved that the eye-muscles arise from the 

 walls of these head-cavities. He asserted that four of the eye- 

 muscles (those innervated by the oculomotor nerve) develop 

 from the first head-cavity, the superior oblique muscle from 

 the second and the external rectus muscle from the third. Sub- 

 sequent investigation has repeatedly confirmed these results 

 for all classes of vertebrates — Cyclostomes (Koltzoff '01), Elas- 



