Chap, ix.} FORAMINA OF SKULL. 333 



more important to the brain, which it contains and 

 protects ; of these the most important is its relation to 

 the cerebral nerves and outgrowths that pass out 

 from it. The distribution and arrangement of these 

 nerves form, in disputed cases, one of the best criteria 

 of the homologies of the different parts of the cranium. 

 Seen in its most elaborated condition, as it is found in 

 Mammals, the cranial bones and cerebral nerves have 

 the following relations. The so-called olfactory nerve 

 (Fig. 140 ; 1) perforates the cribriform plate of the 

 ethmoid ; the optic nerve passes through the optic 

 foramen in the orbitosphenoid bone (os) ; the third, 

 fourth, and sixth nerves, that go to the muscles of the 

 eye,- pass with the most superior division of the fifth 

 through the irregular or jagged space (/la) that lies 

 between the orbitosphenoid and the alisphenoid. The 

 two other branches of the fifth (or trigeminal) pass 

 through the round (fr) and oval (fo) foramina in 

 the alisphenoid, while the seventh has a passage at the 

 outer side of the periotic (PER), while between the 

 periotic, basioccipital, and exoccipital there is a pos- 

 terior foramen lacerum (flp) for the glossopharyngeal, 

 vagus, and hypoglossal (9, 10, 11) nerves; lastly, the 

 twelfth nerve passes through the conclylar foramen (cf), 

 while, as we have already learnt, there is a great 

 foramen at the hinder end of the cranium which 

 serves as the means by which the medulla oblongata 

 is allowed to be continuous with the spinal cord. 



An examination of the interior of the cranium 

 similarly reveals the close connection that obtains be- 

 tween the containing case and the contained brain ; 

 and, indeed, our knowledge of the characters of the 

 brains of extinct forms is absolutely dependent on casts 

 of the internal configuration of such skulls as have 

 been preserved to us in the form of fossils. 



Where, as in the lower Mammals, the cerebral 

 hemispheres are of no great size, and do not overlap 



