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 lamellae. 



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 bj^ 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 



Posterior end Supraorbital 

 of frontal sinus process 



(sawn- off) 



Frontal bone (temporal part) 

 Orbital wing of sphenoid 



Foramen, orbitale 

 Fornmni rotundum 

 Alar canal 



Fig 33.- 



Body of sphenoid Pterygoid process 



-Ckoss-section of Cranium of Horse. The Section is Cut Just in Front of the Temporal Condyle 

 AND Is Viewed prom Behind. 



1, Internal plate of frontal bone; 2, crista galli; 3, cribriforin plate; 4, ethmoidal foramen; 5, ethmoidal spine of 



phenoid 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 cribriform 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. 



