THE SKULL 



43 



a transverse perforated plate, the cribriform plate (lamina 

 cribrosa), attached to the caudal end of the lamina perpendicu- 

 laris and the labyrinths. 



The lamina perpendicularis (Fig. 43, ;/; Fig. 42, /) 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 perpendic- 

 ularis, 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 



Fig. 31.— Ethmoid and Vomer, Fig. 32.— Ethmoid and Vomer, 



Side View. 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 cribri- 

 form plate, 



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. 



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 



