I 4 o ELEMENTARY ANATOMY. [LESS. 



The floor of these fossae may, instead of forming with the 

 basi-cranial axis an angle opening downwards (as in man, 

 the Hare, and Sheep), be parallel with it (as in the Dog), or 

 form an obtuse angle opening upwards, as in the Elephant, 

 or even a slightly acute angle, as in the Dolphins. 



The nares are exceptionally high in proportion to their 

 length in man. Their length may be enormous, as in the 

 Great Ant-eater and Crocodile, where pterygoid plates follow 

 behind the palatine bones and so prolong the bony palate. 



The nares, on the contrary, are in most Vertebrates 

 much shorter than in Mammals ; for there are no palatal 

 plates to prolong the fossae backwards, and their posterior 

 border is formed by the palate bones. Thus the posterior 

 nares in such animals (e.g. Birds, Lizards, Frogs) answer 

 rather to the middle portion of the human nasal fossae. 



The bones which form the anterior and posterior boun- 

 daries of the nasal passage have been already described, as 

 also the asymmetrical form of the anterior nares in the 

 Cetacea. 



The median division of the fossae, or septum narium, need 

 not be partly osseous and partly cartilaginous, as it is in man. 

 It may be quite unossified, as in Chelonians and Serpents, or 

 it may be ossified to the ends of the nasals, as in certain 

 extinct species of Rhinoceros and in some Dolphins and 

 Seals (e.g. the species Leonina), and in one species of Tapir, 

 where ossification advances even in front of the nasals. The 

 septum may be ossified continuously with the lesser wings of 

 the sphenoid, as in the Frog, where it forms the middle part 

 of the os en ceinture. 



As to \hefrontal, sphenoidal, and maxillary sinuses, it is a 

 general character of air-breathing Vertebrates to have some 

 or other part of the cranial bones furnished with cavities 

 containing air. In this respect, therefore, man is no excep- 

 tion to the rule, and indeed he occupies an intermediate 

 position, as cranial air-cells may be more restricted or much 

 more developed than they are in him, and this not only as 

 regards the relative size of the air-cavities, but also as regards 

 the number of cranial bones so inflated. 



The frontal sinuses (which are not constant in man, being 

 absent at least in some Australian skulls) may be much less 

 or more developed than in him in members of his own order, 

 e.g. in different Apes. In hollow-horned Ruminants they may 

 extend into the substance of the horns, or backwards into the 

 parietals and supra-occipital. The latter condition exists in 



