130 ON THE VERTEBRATE SKULL. 



cribriform plates, leaving the ethmoid proper, or lamina perpen- 

 dicularis, upon their inner side, and the lateral masses of the eth- 

 moid, or superior and middle spongy bones, upon their outer sides. 



The optic nerves pass out through the optic foramina, situated 

 between the roots of the orbito-sphenoids, from the chiasma, 

 which rests, as has just been stated, upon the posterior and upper 

 part of the presphenoid. Hence it follows, that the presphenoid 

 lies in front of, and between, the optic nerves, which embrace 

 it, as in a fork, from behind. 



The third and fourth pairs are not of so much morphological 

 importance that I need dwell upon them, but the trigeminal 

 affords first-rate cranial landmarks bv its nasal branch and its 



ml 



whole third division. The nasal nerve enters the orbit by the 

 foramen Jacerum anterius, passes to the inner side of the eye, 

 and then, traversing the anterior of the two ethmoidal foramina, 

 perforates the " lateral mass of the ethmoid," and entering the 

 cavity of the bony cranium, though it always lies beneath the 

 dura mater, skirts the olfactory aperture, and passes out into 

 the nasal cavity, by an aperture in the front part of the cribri- 

 form plate. We shall find this irregular perforation of the 

 " lateral mass of the ethmoid," bv the nasal division of the fifth 

 nerve, to be an excellent guide to the determination of the 

 homologue of the bone in the lower Vertebrata. 



The third division of the trigeminal traverses the foramen 

 ovale in the posterior part of the alisphenoid, so that it makes 

 its exit behind the greater part of that bone, and altogether in 

 front of the periotic bone. 



The portio dura enters the internal auditory foramen in the 

 periotic mass, runs along its canal, situated above the fenestra 

 ovalis, and eventually passes out by the stylo-mastoid foramen. 

 It therefore perforates the fore part of the periotic, passino- in 

 front of the membranous labyrinth. The portio mollis also enters 

 the periotic bone by the internal auditory foramen, and it termi- 

 nates in the membranous labyrinth. 



The eighth pair passes out through the foramen lacerum 

 posterius completely behind the periotic (which thus lies between 

 the exits of the fifth and of the eighth pairs), and in front of 

 the ex-occipitals. 



