THE SKELETON OF TIIK VERTEHUATE HEAD. 100 



plane of its u[)per wall. This direction of the posterior nares 

 is due to the following circumstances : — 1. That the iuter- 

 niaxillaries, although completing their arch below, are 

 principally developed upwards and backwards ; 2. That the 

 maxillaries, even when they meet partially across the middle 

 line, have the space which they enclose occupied by the 

 neurapophyses, centrum, and sense-capsules of their own 

 sclerotome — in other words, they are in contact with the 

 central and neura]»oi)hyseal aspect of their own sclerotome ; 

 3. That the palatines do not form an arch at all, but lie in 

 the horizontal plane of the under surfaces of the centi-ums of 

 the cephalic sclerotomes behind them. 



The bird, in fact, does not possess nasal fosste in the same 

 sense as the mammal — that is, it does not present nasal 

 chambers, formed by the completed haemal arches of a certain 

 number of sclerotomes. Its nasal fossa? consist only of the 

 catacentric-ha^mal or nenro-hremal spaces of the vomerine 

 sclerotome, and of the combined neural and " sense-capsule " 

 spaces of the ethmoidal sclerotome, which occupy the space 

 enclosed by its haemal arch. They differ therefore from the 

 mammalian nasal fossae not only in wanting rhinal compart- 

 ments, but also in the deficiency of ethmoidal and prc- 

 sphenoidal ha'mal spaces. The palate of the bird, instead of 

 being, like that of the mammal, situated in a plane inferior 

 and parallel to that in which the vei-tebral column lies (?), is in 

 the plane of the latter, like that of the fish. The palate of the 

 fish is in the horizontal plane of the vertebral column, because 

 its nasal fossa? are absent, the constituent ha-mal arches being 

 all incomplete ; and because the cavities of its olfactory 

 capsules open externally. The palate; of the bird is in the 

 horizontal plane of the vertebral coluuni for reasons already 

 stated, and also because the olfactory capsules, instead of 

 being situated external to the cavities of their sclerotome, as 

 ill the fish, or in its ha-mal cavity, as in the mannnal, huM- 



