THK XOSE. 383 



IV. THE NOSE. 



The anterior nares or nostrils are two small openings, placed di- 

 rectly in front of the anterior angle of the eye ; the distance between 

 the eye and the corresponding nostril being, in the case of adult 

 animals, about five or six millimeters. The anterior nares are sur- 

 rounded by rims, which are contracted below, and so form very short, 

 tentacular-like prominences. 



The posterior nares (choanae) are about four millimeters from 

 the anterior nares, consequently the long axis of the nose is, approxi- 

 mately, of this length. (For the external muscles of the nose, 

 see p. 59.) 



The boundaries of the nasal cavities are as follows : the roof of 

 each cavity is formed by the dorsal plate of the sphenethmoid, the 

 nasal bone, and the premaxillary bone; the floor is formed by the 

 vomer and the palatine bone, the inner wall by the vertical septum 

 of the sphenethmoid, the outer wall by the premaxillary and max- 

 illary bones, the anterior wall by the premaxillary bone, and the 

 posterior wall by the sphenethmoid. The cartilaginous portion of 

 the nasal skeleton (see also p. 27) projects into the general cavitv 

 and subdivides it into various sinuses, which have been the subject 

 of careful investigation by Born and AY iedersheim. 



a. The nasal cavities are best examined by means of serial trans- 

 verse sections ; in a section through the anterior nares (Figs. 238, 

 239) there will be seen three sinuses on either side : 



(1) The superior sinus (on) is large, rounded, and placed against 

 the cartilaginous septum ; the sinus is lined with olfactory epithe- 

 lium. Posteriorly the sinus extends beyond the posterior nares, and 

 is bounded by the anterior surface of the sphenethmoid. In front it 

 ends in a rounded concavity, likewise lined with olfactory mucous 

 membrane. Opposite the anterior nares the cavity is partially sub- 

 divided, by a longitudinal process on the floor, into two cavities; 

 the anterior nares open into the external chambers, and therefore 

 not directly into the main cavity of the superior sinus. The supe- 

 rior sinus communicates with the inferior sinus by a narrow, almost 

 vertical, slit. 



(2) The inferior sinus (un] is narrow and flattened from above 

 downwards ; it is placed against the maxillary bone, and represents 

 the maxillary sinus of the frog. Externally it possesses a de- 

 scending arm. 



The inferior sinu< is continued backwards and opens on its 



