No. i.] MORPHOLOGY OF THE PETROSAL BONE. \\ 



brates as well, is naturally subject to much variation in form 

 and size, according to the extent and manner in which its 

 membranous walls adhere to each other and are chondrofied or 

 ossified. 



Further important evidence in support of these several con- 

 clusions is found in Jacoby's statement (No. 16, p. Si) that a 

 certain dorsal process of the cartilaginous orbitosphenoid of 

 human embryos indicates an earlier cartilaginous connection of 

 the orbitosphenoids with the Parictalplattcn ; cartilage thus 

 taking the place, in the early ancestors of man, of the thick, 

 so-called dural membrane of Amia. In those early ancestors 

 the alisphenoids must necessarily have been excluded from the 

 bounding side walls of the cranial cavity proper, as they are in 

 Amia. 



The orbital and interorbital openings of the skull of fishes 

 must now be considered. 



The optic fenestra of Sagemehl's descriptions is considered 

 by him as an enlargement of the foramen by which the optic 

 nerve, on each side of the head, pierces the side wall of the 

 skull (No. 25, p. 202). In the fresh skull this opening, in 

 Amia, consists of two parts, one of which is closed by the 

 anterior portion of the tough, fibrous dural membrane, which 

 here forms a part of the side wall of the skull, while the other 

 part of the opening is apparently open to the orbit and is the 

 tall orbital opening of the eye-muscle canal of my descriptions. 

 The posterior portion of the tough fibrous membrane lies inside 

 the skull, internal to the alisphenoid, and is not exposed to the 

 outer surface. 



The optic nerve and the arteria ophthalmica pierce the 

 anterior, exposed portion of the tough fibrous membrane, and 

 enter the orbit at once. The other nerves that pierce the 

 membrane pierce it in its posterior portion, or in the limiting 

 region between its two portions, and enter the eye-muscle 

 canal, from which they issue, by the orbital opening of the 

 canal, into the orbit. The vena ophthalmica and the external 

 rectus muscle, which enter the eye-muscle canal from the orbit 

 by the orbital opening of the canal, do not pierce the tough, 

 fibrous membrane at all. 



