192 



HUMAN ANATOMY. 



the skull. The greatest breadth of the groove is about five millimetres. It nar- 

 rows in front to a point, and thus allows the lateral masses to touch the mediaii 

 plate. It supports the olfactory lobe of the brain, and is perforated by holes for the 

 passage of the olfactory nerves. These are arranged rather vaguely in three rows. 

 There are many in front and few behind. Many of the larger ones, which are near 

 the septum or at the outer side, are small perforated pits. At the front a longi- 

 tudiyial fissicre, close to the crista galli, transmits the nasal branch of the fifth nerve. 

 The lateral masses ^ are two collections of bony plates imperfectly bounding 

 cavities. They are roughly six-sided, the greatest diameter being antero-posterior. 

 The outer surface presents a vaguely quadrilateral plate, the os plaJium,^ forming 

 a large part of the inner wall of the orbit. In its upper border are two notches, 

 which become the anterior and posterior ethmoidal foramina when the frontal bone 

 is in place. The former transmits the nasal branch of the fifth nerve from the orbit 

 to the cranial cavity. The os planum is bounded 

 behind by the body of the sphenoid ; below by the 

 palate bone and superior maxilla, the former of which 

 usually, and the latter always, complete some eth- 

 moidal cells which appear along the lower border. 

 There is a large mass of open cells in front of the J I I 



OS planum. Those nearest to it are completed by 

 the lachrymal and the more anterior ones by the 



Median or perpendicular plate of ethmoid hone in place. The right lateral mass of the ethmoid has been removed. 



nasal process of the superior maxilla. Posteriorly, the lateral mass rests against the 

 body of the sphenoid, the posterior cells being separated from those of the sphenoid 

 by the cornua sphenoidalia. The open cells on the upper surface of the lateral mass 

 are closed by the imperfect cells on the under side of the horizontal plate of the 

 frontal beside the ethmoidal notch. The few cells that open anteriorly are contin- 

 uous with the lateral ones, and are closed by the nasal process of the upper jaw. 

 The numerous spaces within the ethmoid are, for the most part, completed by the 

 neighboring bones, after which they are named. There are some beneath the os 

 planum, however, entirely within the ethmoid. The ethmoidal cells'^ are divided into 

 anterior and posterior, of which the former open into the nose below the middle 

 turbinate bone and the latter above it. The size and shape of the ethmodial cells 

 are very irregular ; sometimes the middle turbinate is hollowed into one, some- 



' Labyrinthus ethmoidalis. - Lamina papyrucea. '^Cellulae ethmoidales. 



