igS 



VERTEBRATE SKELETON 



maxillo-turbinal unites with the maxilla. There is always an infra- 

 orbital foramen on the facial part, transmitting a nerve and blood 

 vessel, but in some rodents this is greatly enlarged, allowing passage 

 to a part of the masseter muscle. The bone always forms part of 

 the floor of the orbit. 



The vomer arises from single or paired centres in membrane on 

 the lower side of the nasal septum, to either side of which it sends a 

 plate, while ventrally a vertical plate reaches the hard palate. Its 

 size varies greatly, being very large in Cetacea, sometimes entering 

 the cranial wall between sphenoid and ethmoid. Usually it is hidden 

 from view from below by the hard palate, though visible through the 

 choanae, and in whales between the palatines. Its possible homology 

 with the parasphenoid of Ichthyopsida (for which embryological 

 support is lacking) has been mentioned (p. 77). 



IMonotremes have a membrane bone, variously called prevomer, dximb-bell or 

 paradoxical bone (fig. 206), the homologies of which are uncertain. It lies in the 

 floor of the nasal cavities, medial to the incisive foramina and ventral to the 



paraseptal cartilage, and appears in the osseous roof 

 of the mouth of Ornithorhynchus. In shape it is 

 somewhat like the body of a violin and has a median 

 longitudinal crest extending to the nasal septum. It 

 arises from paired centres. It has been suggested 

 that it is the homologue of the reptilian vomer, and 

 also that it may be a septomaxillary, a bone found 

 elsewhere in mammals only in Tatusia. 



The palatines, in the floor and sides of the 

 naso-pharyngeal ducts, have horizontal (pala- 

 tine) and vertical parts, the former, some 

 whales excepted, forming the hinder part of 

 the hard palate, the vertical parts forming 

 the side walls of the ducts and usually entering 

 the orbits. In rodents the whole hard palate 

 is so short that the choanae are on a level with 

 the last premolar. 



The pterygoid is closely associated with the alisphenoid in the 

 adult. The true pterygoid is a membrane bone developing on the 

 hinder lateral wall of the nasal capsule behind the maxilla. With this 

 is more or less closely associated a pterygoid cartilage, sometimes the 

 median lamella of the ahsphenoid, sometimes a separate cartilage. 

 The bone bounds the postero-lateral part of the naso-pharyngeal 



Fig. 206. — Tip of skull of 

 Echidna (van Bemmeln, 

 '01). <i, dumb-bell bone; ;«.v, 

 maxilla; pm. premaxilla. 



