52 ON EMBRYOLOGY AS AN AID TO ANATOMY 



round it so as to invest it like a cup. From 

 its walls are developed the rectus superior, rectus 

 inferior, rectus internus and obliquus inferior 

 muscles i.e. } all the muscles supplied by the third 

 nerve. At last we see why the third nerve supplies 

 these and no other muscles ; it does so because it 

 is the nerve belonging to the segment in which the 

 first head cavity lies; and therefore supplies the 

 muscles that are formed out of the walls of that 

 cavity. This reason is a complete and sufficient one. 



Concerning the obliquus superior we are still in 

 the dark ; it does not appear to have any connection 

 with the first head cavity, and this is sufficient 

 reason why it should not be supplied by the third 

 nerve. The development of the nerve that does 

 supply it the fourth is at present absolutely 

 unknown. Concerning the rectus externus muscle 

 we are in a better position. The sixth nerve which 

 supplies it bears the same relation to the seventh 

 nerve that the anterior root of a spinal nerve does to 

 the posterior root. The rectus externus has no 

 connection with the first head cavity, but lies 

 altogether superficially to it, and for some time 

 behind it ; it appears to be developed partly from 

 the third head cavity and possibly in part from the 

 second. We see now clearly why it is not supplied 

 by the third nerve, the reason being that it does 

 not belong to the segment which is supplied by that 

 nerve, but to one further back. 



Thus, we have not quite a complete, but quite a 

 novel and, I think, an intelligible clue to the solution 

 of our problem. 



