ON EMBRYOLOGY AS AN AID TO ANATOMY 45 



vertebrate having well-developed eyes, there we find 

 this same arrangement of muscles and nerves. 

 The exceptions to this rule that have been quoted 

 are probably only apparent. The few exceptions 

 that are quoted are worth noticing. In Amphioxus 

 the eye is a mere spot of pigment and has no 

 muscles at all; in Myxinoids the eyes are small 

 and the eye-muscles imperfect, but all three nerves, 

 though very small, are found to be present and with 

 their usual distribution ; in Lepidosiren there are 

 no oblique muscles, and the third, fourth, and sixth 

 nerves are said to be absent ; the observations on 

 this point are, however, imperfect. In some of the 

 lower Amphibia, the newt for instance, all three 

 nerves are present, at any rate proximally, although 

 distally they may become closely bound up with 

 branches of the fifth nerve. With these exceptions, 

 in all fish, amphibia, reptiles, birds, and mammals 

 these six muscles are all present and all have their 

 typical nerve supply. Let me again remind you 

 that these nerves supply nothing else ; the third 

 nerve supplies no other parts than those I have 

 mentioned ; the fourth nerve never supplies any- 

 thing besides the superior oblique muscle ; the 

 sixth nerve invariably supplies the external rectus, 

 but may also supply two other muscles when these 

 are present viz., the retractor muscle of the eye, 

 which is found in some newts, many reptiles and 

 most mammals, except the monkeys and ourselves, 

 and the special muscle of the nictitating membrane, 

 which is met with in birds and reptiles. 



Such a state of things so widely spread must 



