142 ELEMENTARY ANATOMY. [LESS. 



Vertebrates, they may be largely developed, as in many- 

 Birds, e.g. the Ostrich ; or absent, as in Serpents, e.g. the 

 Python. 



The maxillary sinus in man is also in a medium state of 

 development. In very many forms the maxillary bone is 

 solid ; though, in the Elephant, not only the maxilla has air- 

 cells, but the pre-maxilla also. In the New World Rodent, 

 the Paca, the maxillary bone contains a large cavity which 

 communicates with that of the mouth even in the living 

 animal. In the Hare the external surface of the bone is but 

 imperfectly ossified. 



As has been said, other bones may become inflated with 

 air which are not so in man. This, as we have seen, is the 

 case with the median plate of the ethmoid, with the nasals, 

 and with the pre-maxilla in the Elephant, and with the parie- 

 tals and supra-occipitals in the Hog. The pterygoids may 

 become adapted to form air-cavities, as in the Dolphins, the 

 Mole, and some Sloths, and in old Gavials. The greater 

 wings of the sphenoid may be similarly dilated, as in some 

 Insectivora (e.g. Centetes] and Marsupials (e.g. Hypsiprymmis). 



The mastoidal cells of man are very commonly replaced 

 by large inflations of the tympanic or inferior petrous portions 

 of the temporal bone. The postero-external portion of the 

 skull may be very largely inflated in this way, as in Macro- 

 scelides, forming a great mastoidal bulla ; and even the angular 

 part of the lower jaw may be inflated with air, its cells being 

 placed in communication with those of the tympanic region 

 by a special tube, as in the Crocodile. 



The lachrymal bone may also be inflated and form an 

 orbital bulla, as in almost all Ruminants, especially the Giraffe. 



The upper jawbones may be extraordinarily expanded and 

 filled with delicate osseous air-holding cells, as in the beak of 

 the Toucan. 



The process of DEVELOPMENT of the human skull, which has 

 been before noticed, is of very high zootomical interest, as 

 its transitory stages present very interesting resemblances to 

 the permanent cranial structures of very different animals. 



Inasmuch as we find in the first indication of the embryo 

 an indication of the spinal system which is to be, but none of 

 the skull, we are reminded of the headless condition of that 

 lowest of Vertebrates, the Amphioxus or Lancelet. 



In that early cartilaginous condition of the cranium in 

 which we have a cartilaginous mass enclosing the anterior 

 termination of the notochord in the middle, and an auditory 



