544 ON THE THEORY OF THE VERTEBRATE SKULL 



spinal cord passes out behind the posterior margin of the basi- 

 occipital. The olfactory nerves leave the skull on each side of the 

 ethmovomerine division of the craniofacial axis. 



The walls of the cranial cavity are formed by a number of bones, 

 which are divisible into two series, a superior and a lateral. Of the 

 latter, four pairs of bones, separated by natural lines of demarcation,, 

 or sutures, are distinguishable, three of which abut directly upon the 

 cranio-facial axis, while the fourth pair are only indirectly connected 

 with it. Behind are the exoccipitals,^ united with the basioccipital, 

 and forming the lateral boundaries of the occipital foramen. In 

 front of these are the petromastoids, complex bones which contain 

 the auditory labyrinth, and are connected with the anterior part of 

 the basioccipital and the posterior and superior part of the basi- 

 sphenoid, only by cartilage. 



Next come the alisphenoids, which are attached to the infero- 

 posterior and the anterior portions of the basisphenoid. And, lastly, 

 the orbitosphenoids articulate with the upper margins of the vertically 

 elongated presphenoid. 



In the superior series only four bones can be counted, of which 

 two are single and two are pairs. The hindermost is the supra- 

 occipital bone. It articulates with both the exoccipitals and the 

 petromastoids. The next, in front, is the parietal, single in the 

 adult sheep, but composed of two symmetrical halves in the lamb. 

 It articulates with the petromastoids and with the alisphenoids. 

 The frontals, or anterior paired bones, lastly, unite with the orbito- 

 sphenoids, and, in front of them, with the ethmoid". 



Most important relations exist between the contents of the 

 cranium and these constituent elements of its walls. The par vagum 

 makes its exit between the exoccipital and the petromastoid ; the 

 portio dura and portio mollis enter the petromastoid ; the third 

 division of the trigeminal passes through the large " foramen ovale," 

 which, in the sheep, has the exceptional peculiarity of being situated 

 nearly in the middle of the alisphenoid ; the optic nerve passes 

 through a foramen included between the orbito- and pre-sphenoids, 

 while, as has been mentioned above, the olfactory nerve passes out 

 beside the ethmoid and in front of the orbitosphenoid. The relation 



' In speaking of these bones I shall avail myself, for the most part, of the useful trans- 

 lation of the Cuvierian nomenclature adopted by Prof. Owen. It is, doubtless, more 

 convenient to say "alisphenoid" than " grande aile" or "aile sphenoidale," and "orbito- 

 sphenoid" instead of "aile orbitaire,'' the slightness of the real change thereby effected 

 being one of its principal recommendations. The adoption of the terms will, of course, not 

 be held to imply any recognition of the justice of the views of either their inventor or their 

 adopter. 



