62 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 68. — The frontal bone seen from below (f). 



Fig. 69. — The frontal bone, the greater part of the ethmoid, and the nasal bones in place, seen from 



below (f). The frontal bone is white, the ethmoid yellow, and the nasal pink. 

 Fig. 70. — The ethmoid bone seen from above ( T ). 

 Fig. 71. — The ethmoid bone seen from the side ( T ). 



Fig. 72. — The ethmoid bone together with the eonchae sphenoidales, which are united with it, seen 

 from above and partly from behind ( T ). 



situated between the frontal spine and the ethmoidal notch. The development of the frontal 

 sinuses is subject to great individual variation; they are generally larger in advanced life than 

 during youth, and they are always situated at the base of the frontal portion, especially behind 

 the superciliary ridges, the prominence of which is actually dependent upon the size of the sinuses. 

 They frequently also extend into the orbital plates for a varying distance, and are sometimes 

 enormously developed and markedly distend the frontal bone in the region of the superciliary 

 ridges. 



The frontal bone is developed entirely in membrane from two completely separated portions, which grow from 

 two centers of ossification in the frontal eminences; these centers, like those of the parietal bone, appear toward the end 

 of the second month. Even in the new-born the two halves of the frontal bone are completely separated by the frontal 

 suture (Fig. 105), which does not disappear until the second year of life and may sometimes be present in the adult, and 

 is then also termed the metopic suture. At about the time it disappears the frontal sinuses begin to develop and they 

 enlarge quite gradually until the time of puberty, when they increase more rapidly. 



THE ETHMOID BONE. 



The ethmoid bone (Figs. 70 to 73) forms the median portion of the nasal skeleton; and 

 its cribriform plate aids in the formation of the floor of the anterior cerebral fossa. In the 

 articulated skull the largest portion of the ethmoid is concealed by other bones; it is quite 

 centrally placed and articulates with several of the cranial bones and with the majority of the 

 facial bones. 



It has, as a whole, an irregularly cubical form, and presents a median and two lateral por- 

 tions. The former consists of a small horizontal plate, the cribriform plate, and of a larger 

 vertical plate, which consists of a small thickened portion situated above the cribriform plate, 

 the crista galli (Fig. 71), and of a larger portion, the perpendicular plate, situated below the 

 cribriform plate, which aids in the formation of the bony nasal septum (Fig. 73). 



If the median portion of the ethmoid bone be observed from in front or from behind, or, still better, in cross- sec- 

 tion (Fig. 98), it will be seen to be shaped like a dagger, the handle of which is formed by the crista galli, the guard by 

 the cribriform plate, and the blade by the perpendicular plate. 



Attached to the lateral margins of the lamina cribrosa are the two lateral masses, also termed 

 the ethmoidal labyrinths, which are air-containing structures with thin bony walls, and form 

 a part of the outer walls of the nasal fossse and a part of the inner wall of the orbit. 



The crista galli (Figs. 70, 71, and 73) is a pointed bony ridge situated in the sagittal plane; 

 it is high in front and low behind, and gives attachment to the falx cerebri. The cribriform 

 plate (Figs. 69, 70, and 72) is an approximately rectangular plate situated between the cranial 



