46 ON EMBRYOLOGY AS AN AID TO ANATOMY 



have a reason for its existence, but hitherto no 

 satisfactory or even intelligible answer has been 

 given to the problem, and anatomy still leaves us 

 in total ignorance as to why the external rectus 

 muscle has a special nerve all to itself. Explan- 

 ations have of course been attempted, for the 

 problem cannot fail to strike any one who is 

 acquainted with the anatomy of the parts in 

 question. A common attempt is to say that the 

 movements of the eye are so complex and have 

 to be so extremely nicely adjusted that a complex 

 nerve supply is necessary. This, I would submit, 

 is no explanation at all ; it does not tell us in the 

 least why a small group of six muscles should 

 have three cranial nerves to supply them. For we 

 find the same nerve supply in the lowest vertebrates, 

 in which we cannot suppose these movements to be 

 very complex. Nor does it suffice to say that the 

 nerve supply is due to the two eyes having to work 

 together, and the internal rectus of one eye having 

 to work habitually with the external rectus of the 

 other, for in these lowest vertebrates the eyes are 

 situated at the sides of the head, so that they have 

 totally distinct fields of vision and hence do not 

 work together at all. Sir Charles Bell, the illus- 

 trious anatomist, who was keenly alive to the 

 interest of the problem before us, attempted an 

 explanation as follows : Observing that during sleep 

 and also during certain involuntary acts, such as 

 sneezing, the eyeball is turned upwards beneath 

 the upper eyelid, and finding by experiment that 

 the recti muscles are voluntary, he was led to 



