266 ESSENTIALS OF ZOOLOGY 



extremity, to the palatine processes of the premaxillae. It 

 expands posteromedianly into a thin plate, and throughout 

 its length it provides a groove on the upper surface for the 

 reception of the septum nasi. 



The nose is bounded by the nasals, maxillaries, and the 

 palatines, and is divided into two chambers by the septum 

 nasi and the vomer. Its outer and posterior walls are occu- 

 pied by spongy bones developed in cartilage and called turbinals. 

 Posteriorly and fused with the cribriform plate are the lateral 

 masses, the ethmo turbinals. The nasals attach the long 

 nasoturbinals, and the maxillae the large maxilloturbinals. 

 The lachrymal duct opens below the last. The nose is expanded 

 into sinuses which invade the neighbouring bones. The frontal 

 sinuses have been observed. The sphenoid bone is sometimes 

 similarly hollowed, and a large cavity, the maxillary sinus or 

 antrum of Highmore, occurs in the maxilla on each side. 



With the exception of the ethmoid and the turbinals all 

 the above bones entering into the formation of the face are 

 membrane bones. 



The mandibles are articulated to the squamosal, and both 

 rami are usually separate in front. Each presents above the 

 large coronoid process and posteriorly the articular process 

 and the posterior projection or angle. They bear all the 

 teeth of the lower jaw. 



The hyoid consists of a transverse rod, the basihyal, which 

 gives attachment to anterior and posterior cornua. The 

 anterior cornu consists of three bony elements, which from 

 below upwards are the hypo-, cerato-, and stylo-hyal, and after 

 encircling the pharynx with its neighbour it is articulated to the 

 periotic just in front of the styloid foramen. It represents 

 with the stapes the hyoid arch. The posterior cornua, or 

 thyrohyals, are connected with the thyroid cartilage of the 

 larynx and represent the remains of the first branchial arch. 

 They are cartilages and cartilage bones and are seldom 

 preserved in the dried skull. 



If other skulls, as that of the horse, are used, differences will 

 be seen, e.g. the presence of a foramen in the frontal for the 

 ophthalmic artery and nerve, the uniting of the supraorbital 

 process of the frontal with the zygomatic arch, the foramen 



