ii i. J THE CRANIAL SKELETON. 139 



as Birds and many Reptiles, they are only divided from each 

 other by a thin interorbital septum, made up of bone or carti- 

 lage or membrane. They may, on the contrary, be so widely 

 separated (as in the Hammer-headed Shark) that the cranium 

 is much broader between them than it is anywhere else. 



Although it is peculiar to man's order to have the orbit 

 enclosed as in him, yet there are other creatures which have 

 the orbit protected by a bony rim, as is the case in Ruminants 

 (e.g. the Sheep), in the Crocodiles and Turtles, some Lizards, 

 and some Frogs. This enclosure is effected by the junction 

 of the malar with the true frontal bone (in Mammals, except 

 in the Horse, where the zygomatic process of the tem- 

 poral bone intervenes) or with a post-frontal bone. In many 

 fishes, however, the orbit is bounded inferiorly by a chain of 

 skinbones (dermal ossicles), the suborbital bones, which seem 

 to be serial repetitions of the lachrymal ; and in some 

 Birds (e.g. the Woodcock and the Macaw, Calyptorhyncus] 

 there is an analogous formation, and the orbits are completely 

 encircled by bone. 



In the immense majority of man's class, however, the orbit 

 is not even encircled by bone, and its separation from the 

 temporal fossa is not in many even marked by a post-orbital 

 process of the frontal. The lachrymal foramen (which exists 

 in most terrestrial forms) need not open within the orbit, but 

 may be, as in Lemurs, upon the cheek. 



The orbits may be continued backwards (as in many osseous 

 Fishes, e.g. the Pike) into a prolonged conical canal situate 

 beneath the cerebral cavity, and protected inferiorly by the 

 para-sphenoid. 



In the possession of nasal fosscz limited and defined by 

 osseous structures man agrees with the whole of the Vertebrate, 

 above Fishes, except the very lowest of the Batrachians. 



In the possession of two ruch fossae separated by a wide 

 or narrow septum, and separated from the mouth by an 

 osseous plate, so that they open posteriorly only into the 

 pharynx, 1 man agrees with all the members of his class, and 

 with 'the Crocodiles also. 



The sheltering of spongy bones (or turbinals) is a character 

 which the nasal fossae possess in almost all Mammals, though 

 such parts may be entirely wanting, as in the Dolphins. 

 These spongy bones may be represented only by cartilagi- 

 nous structures, as we find in the classes Reptilia and 

 Batrachia, 



1 For the pharynx see Lesson XI. 



