370 Prof. W. Thomson on Steenstrup's views 



But the right eye is in a closed orbit whose right border con- 

 sists of the right prefrontal ; and we must account for the posi- 

 tion of the eye within this bone, if possible, without resorting to 

 the extreme view that it passed through beneath it. It must 

 always be remembered that the prefrontal bone has no definite 

 relation in position to the eye, whose capsule is essentially con- 

 nected with the bones of the frontal arch of the skull only. 

 The prefrontal is an extremely variable bone of the face, in rela- 

 tion with the olfactory sense-capsule, if with any. Prof. Steen- 

 strup's diagrams of the path of the eye from the blind to the 

 eye side beneath the cranial bones are all taken from mature 

 distorted skulls ; but at the time of the transit of the eye the 

 fish was symmetrical, or nearly so. The eyes were nearly sym- 

 metrical ; and it would be only natural to conclude that the 

 bones of the head (or their potential positions) were nearly 

 symmetrical likewise. The left border of the skull remains 

 normal throughout, the parts occupying nearly the same relative 

 positions which they do in the Cod ; i. e., the left eye is opposite 

 the lower edge of its own frontal bone, the comparatively small 

 prefrontal merely finishing the anterior edge of its orbit. It is 

 clear that the left eye of the Turbot or either of the eyes of the 

 Cod might migrate across the head under the skin, merely ab- 

 sorbing or pressing before it the frontal bone, without coming 

 in contact with the prefrontal at all. In the Turbot the left 

 prefrontal is nearly normal in size, and not more than half the 

 length of that of the right side; and I think we may conclude 

 that in the symmetrical young both bones were normal and 

 alike, and that the right eye was placed opposite the edge of the 

 frontal bone, which at that time formed a portion of the right 

 edge of the skull. When the change in the position of the eye 

 occurred, this exposed portion of the right frontal, whether po- 

 tential or actually developed, was pushed or absorbed before the 

 migrating eye and its nervous and muscular connexions, and 

 reduced to the crescentic plate which, in the mature head, lines 

 the left wall of the orbit, still retaining its original position with 

 reference to the eye. At a subsequent stage in the development 

 of the oblique head, the right prefrontal shot out a process 

 backwards across the gap through which the nerves and muscles 

 of the eye had passed, and became articulated to the frontal 

 bone, forming the beam whose immediate relation to the new 

 condition of equilibrium — that is to say, to the obliquity of the 

 fish — is so accurately pointed out by Prof. Steenstrup. 



From these considerations we are forced to conclude that the 

 eye of the blind side passed to the eye side, not through the 

 vault of the head, but under its integument, displacing in its 

 progress the frontal bone of its own side — the space through 



