It consists of a median vertical portion, the =lamina perpendicularis= (Fig. 43, _n_; Fig. 42, _p_), forming a part of the nasal septum, of two lateral portions made of thin sheets of bone variously folded and united--the =labyrinths= (or ethmoturbinals), which fill the greater part of the nasal cavity; and of a transverse perforated plate, the =cribriform plate= (lamina cribrosa), attached to the caudal end of the lamina perpendicularis and the labyrinths.
The lamina perpendicularis (Fig. 43, _n_; Fig. 42, _p_) is a flat four-sided bone. By its caudal margin it is continuous with the cribriform plate; by its ventral margin it is enclosed by the halves of the vomer; by its dorsal margin it unites with the crest formed by the vertical portion of the nasal bone craniad and with the vertical lamina of the medial margin of the frontal caudad, while its cranial margin is continued into the septal cartilage of the nose. Its lateral faces are smooth and free.
The lamina cribrosa or cribriform plate (Fig. 42, _o_) is elongated heart-shaped, with the apex of the heart ventrad. Its caudal face is concave and looks into the cranial cavity. It presents three irregular longitudinal rows of holes, one median and two lateral, for the passage of the olfactory fibres from the cranial cavity into the nasal cavity. Its cranial face is continuous along the medial line with the lamina perpendicularis, and at the sides with the labyrinths.
The notch in the heart is directed dorsad and receives the vertical lamina of the medial border of the frontal bone. The apex of the heart articulates with the cranial end of the dorsal surface of the presphenoid. Its lateral margins are articulated with the ethmoidal ridges on the medial surface of the frontal bone.
[Illustration: FIG. 31.--ETHMOID AND VOMER, SIDE VIEW.
FIG. 32.--ETHMOID AND VOMER, VENTRAL VIEW.
_a_, vomer; _b_, vertical cells of the labyrinth of the ethmoid; _c_, horizontal cell of the same; _d_, part of the ethmoid that forms the lamina papyracea; _e_, edge of cribriform plate.]
The labyrinths (Figs. 31 and 32) are attached to the cranial face of the lamina cribrosa, one on each side of the lamina perpendicularis. Each is made of thin bony plates irregularly folded so as to enclose spaces, the =ethmoid cells=. In each may be distinguished a cranial portion (_b_), in which the cells are nearly vertical, and a caudal portion (_c_), in which the cells are nearly horizontal.