52 



THE SKELETON OF THE HORSE 



Variation. — The dorsal border of the orbital wing may come to the surface through a 

 defect in the frontal bone at the place where the horn process is situated in animals which have 

 frontal horns. 



The Ethmoid Bone 

 The ethmoid (Os ethmoidale) is situated in front of the body and orbital wings 

 of the sphenoid. It projects forward between the orbital plates of the frontal 

 bones and enters into the formation of the cranial, nasal, and paranasal cavities.* 

 It consists of four parts — the cribriform plate, two lateral masses, and the per- 

 pendicular plate. 



The cribriform plate (Lamina cribrosa) is a sieve-like partition between the 

 cranial and nasal cavities. Its margin joins the orbital wings of the sphenoid 

 laterally, and the cranial plate of the frontal bones dorsally. Its cranial surface 

 is divided into two parts by a median ridge, the crista 

 i _ galli, which is the intracranial portion of the perpendicu- 



^^^^^■^ lar plate. Each half forms a deep oval cavity, the eth- 



moidal or olfactory fossa, which lodges the olfactory bulb. 

 The plate is perforated by numerous small foramina for 

 the passage of the olfactory nerve filaments, and on 

 either side is the much larger ethmoidal foramen. The 

 nasal surface is convex, and has the lateral masses 

 attached to it. 



The lateral masses or labyrinth project forward from 

 the cril)riform 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 

 ethmo-turbinals 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 ethmo-tur]:)inals are covered with mucous membrane. 



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 perpendicidar 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 

 ectoturbinals, and all are beset with secondary and tertiary coiled lamellae. 



The perpendicular plate or mesethmoid (Lamina perpendicularis) is median, 

 and forms the posterior jjart of tlie septum nasi. Its lateral surfaces are nearly 

 plane, but are marked below l)y 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 ])rojects 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 



' On account of its deep situation, and the fact that it cannot be separated from its surround- 

 ings, the ethmoid must be studied by means of appropriate sagittal and transverse sections of the 

 skull. 



Fig. 29. — Cross-section of 

 Lateral Mass of 

 Ethmoid Bone of 

 Horse. 



