The =lateral masses= or =labyrinth= project forward from the cribriform plate into the posterior part of the nasal cavity, which they nearly fill. Each mass is somewhat conical in shape, with the base attached to half of the cribriform plate. The =inner surface= is separated by a narrow space from the perpendicular plate. The =outer surface= is convex and faces chiefly into the frontal and maxillary sinuses, but is attached behind to the inner wall of the orbital cavity; it is covered by a very thin layer of bone, the =lamina papyracea.= The mass consists of a large number of delicate, scroll-like plates of bone, termed =ethmoturbinals= or =ethmoidal cells.= These are attached to the lamina papyracea, and are separated by narrow intervals termed =ethmoidal meatuses=, which communicate with the nasal cavity. In the living animal the ethmoturbinals are covered with mucous membrane. [Illustration: FIG. 29.—CROSS-SECTION OF LATERAL MASS OF ETHMOID BONE OF HORSE. ] The lateral mass is a very complex structure, the arrangement of which may be studied on cross-sections of decalcified specimens with the mucous membrane retained. Each mass consists of six turbinals which extend almost to the perpendicular plate and are termed =endoturbinals=. These diminish in size from above downward; the largest is attached to the nasal bone, and is hence usually called the naso-turbinal or superior turbinal; the second is much smaller, and is very commonly termed the great ethmoid cell. Between the endoturbinals are twenty-one small =ecto=turbinals, and all are beset with secondary and tertiary coiled lamellæ. The =perpendicular plate= or =mesethmoid= (Lamina perpendicularis) is median, and forms the posterior part of the septum nasi. Its =lateral surfaces= are nearly plane, but are marked below by some grooves and ridges; they are covered by the nasal mucous membrane. The =anterior border= is irregular and is continuous with the septal cartilage. The =posterior border= projects into the cranial cavity as a ridge, the =crista galli.= The =dorsal border= joins the frontal bones at their line of junction—the frontal suture. The =ventral border= is received into the groove of the vomer. =Development.=—The ethmoid develops in cartilage from five centers, two for each lateral mass, and one for the perpendicular plate; from the latter ossification extends into the cribriform plate. At birth the perpendicular and cribriform plates are entirely cartilaginous. By the time ossification is complete the ethmoid has united with surrounding bones to such an extent that it cannot be separated intact for study. [Illustration: FIG. 30.—SKULL OF HORSE, DORSAL VIEW. 12, Supraorbital process; 14, parietal bone; 14′, external sagittal crest; 15, frontal bone; 15′, frontal crest; 21, lacrimal bone; 22, nasal bone; 26, maxilla; 24, incisor teeth. (After Ellenberger-Baum, Anat. für Künstler.) ]