120 ELEMENTARY ANATOMY. [LESS. 



between the frontal and the lachrymal, as is the case in 

 Dolichotis. It may or it may not take a share in the forma- 

 tion of the pterygoid fossa. It appears even to form part of 

 the margin of the optic foramen in the Echidna. 



30. The VOMER again is a bone of remarkable constancy. 

 It is much less concealed by other bones in Vertebrates 

 generally than in man, and may appear not only in the palate 

 (where it is indeed normally visible), but also externally on 

 the surface of the skull. 



In the fact that in the adult condition it is a median, 

 azygos bone, the vomer of man agrees with that of all Mam- 

 mals, where it is mostly large. In Birds it may be large, as 

 in the Ostriches, or nearly absent, as in the Pigeons. It is 

 almost always single in them, but may be double, as in the 

 Woodpeckers. In Reptiles it is generally double, but may 

 be single, as in the Chelonians. In Batrachians it is in- 

 variably large and double. In Fishes it is large and single in 

 nearly all, but may be double, as in Lepidosteus and Sudis. 



In Mammals the shape of this bone, as might be expected, 

 varies generally with that of the face. Thus it is extremely 

 elongated in the Dolphins. 



In all above Fishes it contributes to form the partition 

 between the nasal passages, and (except where the facial 

 bones develop palatal plates, as in Mammals and Croco- 

 diles) borders internally their posterior openings. 



31. The inferior maxillary bone, or MANDIBLE, of man is a 

 very characteristic bone, though substantially agreeing with/ 

 that of other Mammals. 



It bears, however, a double relation to parts we find in- 

 lower animals. For, while inasmuch as it forms the whole 

 lower jaw it of course agrees with, and answers to, the whole 

 lower jaw of each bird, reptile, and fish ; yet in development 

 and essential nature, it corresponds with a certain portion 

 only of the lower jaw of each of these animals. 



Thus it is possible for the part answering to the whole 

 lower jaw of man (i.e. the bone called "dentary") not to 

 articulate directly with the cranium, but to be connected with 

 it by a whole series of intermediate parts, as is the case in 

 all Vertebrates below Mammals, and especially in osseous 

 Fishes. 



We may find, as in the Sauropsida, an actual lower jaw 

 consisting of several distinct bones (dentary, angular, sur- 

 angular, coronoid, splenial, and articular) suspended from the 

 skull by a single bone the os quadatrum ; or we may find, 



