NASAL CAVITY OF VERTEBEATA. 647 



Nasal Cavity. 

 § 411. 



While tlie nasal cavities are increased in lengthy owing to their 

 being shut off from the buccal cavity by the palate, the increase in 

 the size of the facial portion of the head also affects them ; they 

 increase both in length and height, and thus become large spaces. 

 The olfactory nerve ends in their superior and posterior portion 

 only (i-egio olfactoria), while the infei-ior and anterior portion 

 principally serves as an " air-passage," and consequently comes into 

 relation with the respiratory organs (regio respiratoria). The whole 

 differentiation therefore of the nasal cavity is seen to be connected 

 with the development of the lungs, and their increased physiological 

 importance. The increase in the extent of the internal cavity is 

 effected in various ways. The lateral wall of the nasal cavity, 

 which is developed from the primordial cranium, always takes part 

 in this process; the turbinate bones are lamellar, folded, and 

 coiled processes of this wall. 



In the Reptilia there is only one turbinate bone; this extends 

 backwards from a cavity, which commences at the external nasal 

 orifice, and is generally horizontal in position ; it is feebly developed 

 in the Chelonii, and best developed in the Crocodilini. It is very 

 varied in character in Birds. Sometimes it is simple (Columb^), 

 sometimes complicated by coils (Raptores), or it may be cleft into 

 several lamellae (Struthio). A turbinated structure is connected 

 with the internasal septum in front of, and below this bone, and is 

 by this connection distinguished from the turbinate bones, which 

 are always lateral in position. This pseudo-concha separates the 

 vestibule of the nose from the internal nasal cavity. 



Above the turbinate bone, and, as a rule, at the upper blind end 

 of the nasal cavity, there is another process which corresponds to a 

 depression formed in the wall of the nasal cavity by an air sinus. 

 Part of the olfactory nerve ends on this process, which is not found 

 in the Columbid^. In the Mammalia three turbinate bones may be 

 distinguished. The lower one corresponds to the single bone in the 

 Reptilia and Aves ; it varies very greatly owing to the way in which 

 its lamellas are ramified and variously coiled, e.g. in the Carnivora (it 

 is most complicated in Lutra and Phoca). These bones are least 

 developed in various Marsupials (Macropus, Phascolomys), in the 

 Apes (they are simplest in the Platyrrhini), and in Man. In the 

 Cetacea the cavity has undergone degeneration in consequence of 

 the loss of its olfactory function. The orifice on the upper surface 

 of the skull leads into a vertical canal, which is divided by the inter- 

 nasal septum, and which can be shut off from the pharyngeal cavity by 

 an occlusor muscle; there are no signs of any turbinate bones 

 in it. 



2 N 2 



