III.] 



THE CRANIAL SKELETON. 



141 



the Hog, but attains its maximum of development in the 

 Elephant, where air-cavities extend even into the nasal bones ; 

 and the vertical section of the area of the cranial sinuses 

 exceeds, in the adult, that of the cranial cavity itself. 



BE- 



FIG 120. A section of the Cranium of a full-grown African Elephant, taken to 

 the left of a middle line, and including the vomer (Vo), and the mesethmoid 

 (ME) : an, anterior, and/, posterior narial aperture, f z . 



This section shows the enormous thickness of the skull wall, and the prodigious 

 development of air-cells in the frontal above and in front of the cerebral cavity, 

 and above and behind the anterior nares, an. 



(From Flower's " Osteology.") 



In Birds also they may be well developed, while, on the 

 contrary, an extraordinarily dense projection of bone may 

 take place from the frontals, as in the Cassowary. Frontal 

 sinuses may be entirely wanting, as in many Mammals and 

 Reptiles, and as in the Frog, and this though other cranial 

 elements have large air-cavities, as in the Crocodile. 



The sphenoidal sinuses may be much less developed than 

 in man, as is the case in common Monkeys and many Mam- 

 mals, e.g. the Sheep and Manatee. They may, however, be 

 very greatly developed and extend into the median plate of 

 the ethmoid, as in the Elephant. Similarly, in oviparous 



