HEAD CAVITIES AND NERVES OF ELASMOBRANCHS. 97 
the only muscle in this region, it becomes very probable that 
the rectus externus is really developed from the wall of these 
cavities. Whether both cavities take part in it is uncertain, 
but it is evident that, if this be the mode of its development, the 
third cavity must at any rate form part of it, inasmuch as from 
its earliest appearance part of the muscle (figs. 19 and 36) is 
situated behind the fifth nerve, and therefore behind the second 
cavity. In this fact we probably get the explanation of the fact 
that the nerve supply of the rectus externus is by means of the 
anterior root of the seventh nerve. 
The morphology of the eye muscles and their nerves has long 
been recognised as a curious and interesting problem. I 
venture to hope that the account given above of their 
development in a very typical Vertebrate will be accepted as 
giving us at any rate the clue to the solution of the problem 
—a clue I hope to follow up more closely in a further paper. 
VOL, XXI1L—NEW SER. G 
