CRISTA CALLI 



ALAR PROCESS 



48 THE BONES OF THE HEAD. 



It consists of a central vertical plate, and of two lateral masses, united at their 

 superior borders by the horizontal cribriform plate. It articulates with thirteen 

 bones : the frontal, sphenoid and vomer, the nasal, lachrymal, superior maxillary, 

 palate, and inferior turbinate bones. 



The vertical plate lies in the median plane, and forms the upper third of the 

 septum of the nose (fig. 71, p. 69). Its superior border appears in the cranial 

 cavity, above the cribriform plate, in the form of a ridge, rising anteriorly into a 

 thick process, the crista galli, to which the falx cerebri is attached. The posterior 

 margin of the crista galli is thin and smooth. The anterior is in its lower part 

 broadened out, and divided into two alar processes, which project laterally, and are 

 rough in front for articulation with the frontal bone : between them there is usually 

 a median groove completing the foramen csecum. Below the level of the 



cribriform plate, the anterior 

 border of the vertical plate 

 slopes much forwards, and 

 articulates with the nasal spine 

 of the frontal and with the 



,ANT. ETHM. CELLS 11 ml P 



nasal bones. ihe inferior 

 margin articulates in front, and 

 sometimes even in its whole 

 extent, with the septal carti- 

 lage of the nose : in -its pos- 

 terior half, in the adult, it is 

 more or less completely joined 

 by osseous union on one or 

 both sides to the two plates 



of the vomer. The posterior 



Fig. 47. THE ETHMOID BONE, FROM THE RIGHT SIDE. . , -, . 



(Drawn by D. Gunn.) margin is very thin, and is 



united to the crest of the 



sphenoid. This plate presents superiorly a number of grooves and minute canals, 

 leading from the foramina of the cribriform plate, for the transmission of the 

 olfactory nerves. 



Each lateral mass or labyrinth encloses a number of spaces of irregular 

 form, arranged in three sets, the anterior, middle, and posterior ethmoidal cells, which 

 in the recent state are lined with prolongations of the mucous membrane of the 

 nose. On its external aspect is a thin, smooth lamina, of an oblong form, the 

 orUtal plate or os planum, which closes in the middle and posterior ethmoidal cells, 

 and forms a considerable part of the inner wall of the orbit (fig. 69, p. 66). The 

 circumference of the orbital plate articulates in front with the lachrymal, behind with 

 the sphenoid, above with the frontal, and below with the superior maxillary and palate 

 bones, which often complete two or three ethmoidal cells. At the lower part of this 

 aspect is a deep groove, which belongs to the middle meatus of the nose, and is 

 limited below by the rolled margin of the inferior turbinate process. Anteriorly, the 

 groove curves upwards, and is continued into a passage named the infundibulwn, 

 which leads through the fore part of the lateral mass into the frontal sinus. Into 

 the horizontal part of the groove the middle ethmoidal cells open, and into the 

 ascending part the anterior ethmoidal cells. In front of the orbital plate, the 

 lateral mass extends forwards under cover of the lachrymal bone, which closes over 

 the open anterior cells seen in the disarticulated ethmoid bone ; and from this part 

 descends the uncinate process, a long thin lamina, which curves backwards, down- 

 wards and outwards in the groove of the middle meatus. In the complete skull the 

 uncinate process lies across the orifice of the antrum of the superior maxilla, and 

 forms part of the inner wall of that cavity ; at its extremity it articulates, by means 



