THE SKELETAL SYSTEM 37 



of the bones of the skull are flat. The spaces or cavities between 

 these bones are known as air-sinuses. They add to the volume 

 of the skull without increasing its weight. 



The bones of the cranium are the occipital, sphenoid, ethmoid, 

 interparietal, parietal, frontal, and temporal. The first four 

 are single, the others paired. As compared with the body, the 

 cranium of the horse is remarkable for its small size. 



The bones of the face are important for some of them carry the 

 teeth. They are the maxilla, premaxilla, palatine, pterygoid, 

 nasal, lacrimal, malar, superior and inferior turbinals, vomer, 

 and mandible. The last two are single, the others paired. They 

 form the walls of the oral and nasal cavities. The shape of the 

 nose and the coarseness or fineness of the head are determined 

 by these bones. 



The maxillae are the principal bones of the upper jaw and carry 

 the upper molar teeth. They are situated on the lateral aspects 

 of the face. Each presents two surfaces. The external surface 

 is convex, especially in the young animal. The prominent ridge 

 noted on this surface is the facial crest. A little above and in 

 front of this crest is the infra-orbital foramen. The internal sur- 

 face is concave, smooth, and forms part of the wall of the nasal 

 cavity. A ridge gives attachment to the inferior turbinated bone, 

 above which is the lacrimal canal. Springing medially from the 

 lower part is the palatine process, which forms a large portion of 

 the hard palate and meets its fellow at the median plane. The 

 inferior border has six alveoli for the upper molar teeth. 



The premaxillae are situated in front of the maxillae. They are 

 strongly developed anteriorly. Each has three alveoli for incisor 

 teeth. The palatine processes project backward to form the 

 anterior part of the roof of the mouth. 



The palatine bones are small and may be considered as exten- 

 sions to the hard palate. 



The pterygoid bones are thin, strap-like attachments to the 

 palatine bones. 



The nasal bones form most of the roof of the nasal cavity. The 

 dorsal surface is convex, the ventral concave and provides attach- 

 ment for the superior turbinated bone. The anterior extremity 

 is pointed, thin, and free. 



The lacrimal bones are situated in front of the eyes. 



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