HISTORY OF THE EYE MUSCLES 



441 



the well-known text-book formula for the ontogenesis of the 

 eye-muscles as follows : From the first or pre-mandibular head- 

 cavity arise the muscles innervated by the oculomotor nerve, 

 namely the recti superior, anterior and inferior, and the inferior 

 obhque; from the second or mandibular myotome are differen- 

 tiated the superior oblique muscle and the lateral portion of 

 the external rectus; from the third head cavity develops the 

 median portion of the external rectus muscle. A comparison 

 of the old and new formula may be made in tabular form: 



The old formula for the ontogenesis of the eye-muscles 



Myotome 1. 



My tome 2.. 

 Myotome 3 . 



MUSCLES DERIVED 



Rectus superior 

 Rectus internus 

 Rectus inferior 

 Obliquus inferior 

 Obliquus superior 

 Rectus externus 



Oculomotor 

 Oculomotor 

 Oculomotor 

 Oculomotor 

 Trochlearis 

 Abducens 



III 

 III 

 III 

 III 

 IV 

 VI 



Revised formula for the ontogenesis of the eye-muscles 



Myotome Id. 



Myotome Iv. 



Myotome 2d. 

 Myotome 2v. 

 Myotome 3v. 



Rectus superior 

 Rectus internus 

 Rectus inferior 

 Obliquus inferior 

 Obliquus superior 

 Rectus externus 

 Rectus externus 



Oculomotor 

 Oculomotor 

 Oculomotor 

 Oculomotor 

 Trochlearis 

 Abducens 



III 

 III 

 III 

 III 

 IV 

 VI 



But, however interesting and however important from the 

 embryological point of view such an account of the ontogenesis 

 of the eye-muscles in Squalus may be, it tells us little that is 

 morphologically significant. The questions still remain un- 

 answered — What is the morphology of the 'head cavities?' 

 What has been their past history! To the answer to these im- 

 portant questions we may now turn our attention.' 



The true morphology of the head cavities and the first reli- 

 able clue to the phylogenesis of the eye-muscles was revealed 

 by VanWihje ('82) who showed that the head cavities are mem- 



