56 THE SKELETON OF THE HORSE 
perpendicular plate and are termed endoturbinates. These diminish in size from above down- 
ward; the largest is attached to the nasal bone, and is hence usually called the dorsal or nasal 
turbinate; the second is much smaller, and is very commonly termed the great ethmoid cell. 
The cavity enclosed by this communicates laterally with the maxillary sinus, but not directly 
with the nasal cavity. Between the endoturbinates are twenty-one small ectoturbinates, and 
all are beset with secondary and tertiary coiled lamelle. 
The perpendicular plate (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 
Frontal Posterior end Supraorbital 
suture of frontal sinus process 
(sawn off) 
Frontal bone (temporal part) 
Orbital wing of sphenoid 
Foramen orbitale 
Foramen rotundum 
— Alar canal 
Body of sphenoid Pterygoid process 
Fic 33.—CRross-sEcTION OF CRANIUM OF Horse. Tue Secrion ts Cur Jusr 1n I’Ronv oF THE TEMPORAL CONDYLE 
AND Is VIEWED FROM BEHIND. 
1, Internal plate of frontal bone; 2, crista galli; 3, cribriform plate; 4, ethmoidal foramen; 5, ethmoidal spine of 
sphenoid bone; 6, optic foramina. 
crista galli. The dorsal border joins the frontal bones at their line of junction. 
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 ecribriform plates 
are 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. 
THE INTERPARIETAL BONE 
This bone (Os interparietale) is centrally placed between the squamous part 
of the occipital and the parietal bones. It is usually described as a single bone, 
although it ossifies from two chief lateral centers, and is often distinctly paired in 
skulls of young foals. 
