THE ETHMOID BONE. 35 



which, in the disarticulated bone, appear as cavities on 

 the surface, but with similar depressions on the adjoining 

 bone all these broken cells are completed. The lateral 

 mass presents for examination an external and an in- 

 ternal surface, and an anterior inferior and posterior 

 border. The external surface presents a number of 

 broken cells, and a smooth, rectangular plate of bone, 

 the os planum, which forms part of the inner wall of 

 the orbit. This plate, at its upper border, presents two 

 notches, which, with similar notches on the frontal bone, 

 form the anterior and posterior ethmoidal foramina. The 

 os planum articulates along its superior border with the 

 frontal; anteriorly, with the lachrymal; the inferior 

 border, with the superior maxillary ; and the posterior, 

 with the sphenoid and the palate. Below the os planum 

 is an unciform or hook-like process of bone, which passes 

 downward and articulates with the superior maxillary, 

 and helps to close the antrum of Highmore. The inner 

 surface of the lateral mass is very irregular. It is rough, 

 and grooved for the passage of branches of the olfactory 

 bulbs. Running obliquely downward and backward from 

 the upper posterior portion is the superior turbinated 

 process (rudimentary in man), while below, at its inferior 

 border, is the middle turbinated process, a scroll-like, 

 elongated plate of bone, with its convexity presenting 

 upward and inward. The cells of the ethmoid are 

 divided into anterior and posterior, separated by a more 

 or less complete bony partition ; they communicate with 

 the frontal and sphenoidal sinuses, and with the nasal 

 fossa. 



The ethmoid develops from three centres; one for 

 the vertical plate and one for each of the lateral masses 

 and its attached horizontal portion. The centres first 

 appear about the fifth month. 



