in.] THE CRANIAL SKELETON. 83 



base of each is perforated by the foramen, which transmits 

 the optic nerve ; behind the foramen projects backwards a 

 small "anterior clinoid process." 



Each orbital wing forms the hindermost and innermost 

 part of the roof of the orbit. 



The greater ala and the orbital wing of each side are 

 separated by a long but narrow space ; termed the sphcnoidal 

 fissure. This transmits the third, fourth, and sixth nerves and 

 the first branch of the fifth nerve. In addition to the junc- 

 tions already noted, each ala unites with the malar bone of 

 its own side, and the body of the sphenoid is in contact with 

 a bone, the vomer, hereafter to be noticed. 



At an early period the sphenoid bone may be said to be 

 made up of ten parts: (i) the bulk of the body; (2) the 

 anterior part of the body, or pre-sphenoidal part ; (3 and 4) 

 a pair of great wings and external pterygoid processes, or 

 two alisphenoidal parts ; (5 and 6) a pair of lesser wings, or 

 orbito-spherioidal parts; (7 and 8) the pair of internal ptery- 

 goid processes, or proper pterygoid bones ; and (9 and 10) 

 the lingulae sphenoidales. 



7. The ETHMOID, 1 or sieve-like bone, is of singular deli- 

 cacy of structure and complexity of 

 shape. It is placed between the skull 

 proper and the face, hanging down 

 between the orbits. It consists of a 

 transverse cribriform plate and of 

 three vertical portions. The cribriform 

 plate is so called from its sievelike 

 condition (being perforated with nu- 

 merous small holes for the nerves of 

 smell); it extends between the orbital FlG . 87 .1 MAN , S ETHMOID 

 plates of the frontal and the three BONE. 



vertical portions. The first of these ^, Crista gain, rising verti- 

 three is median, and its summit, called caliy above the horizontal 

 crista galli, projects a little upwards S~^52T3 

 into the cranial cavity, while the main the ethmoid; me, median 

 part is below the cribriform plate, and ethmoid, 

 forms the upper and front part of the 



partition between the nares, whence it is termed the " median 

 ethmoid" 



The two other parts hang down from the under surface of 

 the cribriform plate, and each is termed a " lateral ethmoid" 

 The outer surface of each of these is smooth, and appears 



1 From r,0/uo, a sieve. 

 G 2 



