184 



Discussion of the Asymmetry. 



We have now ascertained the facts which justify a 

 fuller discussion of the causes and course of the asymmetry 

 of the head. To satisfactorily understand the considera- 

 tions which we put forward, the reader will find it essen- 

 tial to follow our argument with dried crania both of a 

 Plaice and Cod before him. We have already described 

 the cranium in detail, but it will be useful to summarize 

 those features relevant to the present discussion. 



Note then that (a), the bony interorbital septum is 

 formed by the right frontal alone, that it is a thin flattened 

 bone lying in the morphological horizontal plane of the 

 head, and that in front it is in contact with the ethmoid 

 cartilage which is perforated by a wide fenestra ; (b), that 

 the left prefrontal has lost its primitive connection with 

 the left frontal* but is still attached to the parasphenoid 

 bar, while the right prefrontal, though still in contact 

 with the right frontal has lost its connection with the 

 parasphenoid bar, and is further distinctly anterior to the 

 left bone ; (c) that all the oblique muscles take origin from 

 the left prefrontal. 



Note now the differences between these relationships 

 and those of the corresponding structures in the Cod's 

 skull. In the latter (a), the fused frontals form a broad 

 roof to the cranium, and internally and below two bony 

 ridges form the bony portion of the interorbital septum, 

 whilst the ethmoid cartilage is not perforated ; (b), the 

 right and left prefrontals are in contact with the frontals 

 of their own side and with the parasphenoid bar ; (c), the 

 oblique muscles take origin from the upper portion of a 

 strong membranous partition joining the frontal ridges 



* The present connection between the left frontal and left prefrontal is 

 purely secondary, as mentioned elsewhere. 



