30 OUTLINES OF EQUINE ANATOMY. 



ing from above downwards, continuous anteriorly as 

 septum nasi. On either side it is jagged, and continues 

 as far as the anterior margin of the Avings by the cribri- 

 form pjates, perforated thin layers of bone through 

 which the fibres of the olfactory nerves pass, and which 

 posteriorly form the boundaries of depressions olfactory 

 fossa, in which the olfactory bulbs rest ; anteriorly have 

 attached to them certain thin plates of bone curled upon 

 one another in a complex manner, forming the ethmoid 

 cells, a direct continuation of the ethmoidal sinuses in the 

 body. These are portions of bone which derive their nutri- 

 ment directly from the mucous membrane which covers 

 them, having no special periosteum. In that mucous 

 membrane (and also that of the superior turbinated bone) 

 the fibres of the olfactory nerve are distributed, rendering 

 it the organ of the special sense of smell. 



OS SPHENOIDES forms the major portion of the floor 

 of the cranium, and is situated between the occiput and 

 OS ethmoides. It consists of a body and two wings with two 

 crura or legs. The upper surface of the body is irregularly 

 concave, receiving the name sella turcica. Centrally it 

 supports the pituitary body, and on either side of this is a 

 depression in which rests the cavernous sinus, while extend- 

 ing on to the upper surface of each wing on each side we 

 see a groove for the superior maxillary division of the fifth 

 cranial nerve, running forwards to join the suhsphenoid 

 foramen i7i forming foramen rotundum. This is separated 

 anteriorly by a thin j)late of bone from a smooth groove, 

 which unites with another on os ethmoides to ioYv^ foramen 

 lacerum orhitale. The posterior extremity presents a rough- 

 ened ovoid surface for articulation with the anterior ex- 

 tremity of the basilar process of the occiput. Anteriorly is 

 a similar surface for apposition with the posterior part of 

 the body of the ethmoid, which is separated on each side 

 from foramen rotundum by a thin plate of bone, which 

 presents a small squamous surface for the antero -internal 

 part of the wing of the ethmoid. The under surface of 

 the body is convex, tending to become smooth anteriorly. 

 Posteriorly it is rough for insertion of a rectus capitis 

 anticus major on either side. The wing of os sphenoides 

 superiorly is concave and presents an irregular surface, 

 helping to form the cerebral cavity. Its posterior margin 

 is internally sharp, externally rounded, forming the anterior 



