OCCIPITAL BONE. 45 



jutting into it. The anterior chamber is subdivided into two 

 jDarts, the internal of which is prolonged into the inferior turbinal 

 bone, the external, which is the smallest, having an elevation on 

 its floor caused by the root of the fourth molar tooth. 



The Sphenoidal sinus is a small irregular cavity formed in the 

 sphenoid, ethmoid, and palatine bones. It is subdivided by 

 incomplete partitions into several compartments, especially into 

 an anterior included between the plates of the palatine, and a 

 well-marked posterior one, excavated in the pre-sphenoid bone ; 

 the latter is separated from its fellow on the opposite side by a 

 perforated lamella. 



The Ethmoidal sinus, the smallest of these cavities, is a space 

 included in the volute of the ethmoid bone. It communicates by 

 a small opening with the maxillary sinus. 



The sinuses, which contain air, and are larger in the adult than 

 in the young animal, are partially divided by imperfect septa, 

 which run across their interior ; in the fresh state they are lined 

 with mucous membrane. 



Separate Bones of the Skull. 



CRANIAL bones. 



The bones of the Cranium which we now proceed to describe 

 separately, are the occipital, sphenoid, ethmoid, and Wormian 

 bones, which are single, and the parietal, frontal, and temporal 

 bones, arranged in pairs ; the temporal bone is, in the horse, 

 divided into two parts, the squamosal and petrosal bones. 



OCCIPITAL BONE. 



(Fig. 10. L ; 12. 1.) 



This bone occupies the posterior part of the skull, and is in 

 the young animal divisible at first into four pieces, and after- 

 wards into two, which finally unite to form one bone ; it is very 

 irregular, and presents an external and an internal surface and 

 borders. 



The external surface may be thus described. The upper or 

 supraoccipital portion being very much exposed, is thick and strong ; 

 it terminates superiorly in a prominent crest, the crest of the 

 occiput, which is convex anteriorly, and gives attachment to the 



