44 ON EMBRYOLOGY AS AN AID TO ANATOMY 



The movements of the eyeball are effected by 

 six muscles four recti and two obliqui. Of these, 

 speaking roughly, the superior rectus turns the eye 

 upwards ; the inferior rectus turns it downwards ; 

 the internal and external turn it inwards and 

 outwards respectively ; the superior oblique rotates 

 the eye outwards and downwards, the inferior 

 oblique outwards and upwards. Of these six 

 muscles four are supplied by the third cranial nerve, 

 while each of the remaining two has a separate 

 and distinct cranial nerve to itself, the fourth 

 cranial nerve supplying the superior oblique muscle, 

 and the sixth cranial nerve the external rectus. 



Now it is a very remarkable fact that this 

 small group of muscles should have three cranial 

 nerves to supply them, and still more remarkable 

 that these nerves should do little or nothing else. 

 The third nerve, besides supplying the muscles 

 mentioned, also supplies the elevator muscle of the 

 upper eyelid and sends branches which penetrate 

 the eye and supply the ciliary muscle and iris. 

 The fourth and sixth nerves, however, do nothing 

 else, and form unique instances of nerves with 

 separate origins from nerve centres distributed ex- 

 clusively to single muscles, and it is very curious 

 that these two muscles should both be connected 

 with the eye. In order to complete the conditions 

 of the problem it is only necessary to add that the 

 distribution of the nerves and muscles I have 

 just noticed is not peculiar to man but occurs 

 throughout the whole vertebrate series almost 

 without exception. Wherever we meet with a 



