424 THE OLFACTOKY ORGAN. 



this faculty for the pursuit of their prey. But even in the herbivora it 

 is well marked, and furnishes them, though less exactly, similar indica- 

 tions. In man, though this sense is less acutely developed, it applies 

 itself to a greater variety of objects, and doubtless enables him to appre- 

 ciate differences among odors in a more correct manner than in the case 

 of the lower animals. 



The general principle involved in the construction of the organ of smell 

 Mechanism of * s to ex P ose an extensive and constantly moistened surface 

 the olfactory to the air brought in by the respiratory current. Of course, 

 other things being equal, the larger the surface, the more per- 

 fect the sense. The object of gaining a great extent of superficial ex- 

 posure under a relatively small volume is accomplished by spreading 

 the sensitive mucous membrane on projections or shelves, which also 

 serve the purpose of intercepting the incoming current of air. It is in 

 reptiles and birds that turbinated processes first make their appearance. 

 In air-breathing animals, the organ of smell is essentially an appendix 

 to the respiratory mechanism, its action depending entirely upon the 

 play thereof. But, though the material submitted to the olfactory mem- 

 brane in this manner is presented in the vaporous or gaseous state, it is 

 intermediately dissolved, as has been stated, in the liquid mucus which 

 covers that membrane, before it can affect the ramifications of the olfac- 

 tory nerve. 



The nose, thus constituting the commencement of the respiratory 

 tract, forms a characteristic feature of the countenance. It is composed 

 in part of bones and in part of cartilages, covered over with muscles and 

 integument. Its five cartilages give to it shape in its inferior portion, 

 and, by their elasticity, enable it to resist external injury. The whole 

 surface of the nasal cavities is covered over with mucous membrane, to 

 which the names of pituitary or Schneiderian membrane have been given. 

 This mucous membrane likewise extends into the maxillary antrum, 

 ethmoid, and sphenoid cells, or sinuses which are adjacent, and open into 

 the same nasal cavity. The Schneiderian membrane is highly vascular, 

 and receives its nervous supply from the nasal branches of the fifth 

 pair, which give it common sensibility, but its olfactory function de- 

 205. Fig. 206. pends on the distribution which a 



certain portion of it receives from 

 the first, or olfactory nerve. 



Fig. 205 illustrates the distribu- 

 tion of the olfactory nerve on the 

 septum of the nose. Fig. 206 is 

 its distribution on the outer wall of 

 the nasal fossa. 



That the function of the first pair of nerves is olfactory is proved by 



