THE SPECIAL SENSES 2f>9 



froin the skull, but rests upon a thin bone which separates 

 it from the cavity of the nose ; and the branches which proceed 

 from it pass through this bone by means of numerous small 

 openings. The engraving represents the outer surface of the 

 right nasal cavity ; the three wave-like inequalities, upon which 

 the nervous network is spread out, are due to the spongy bones. 

 The left cavity is supplied in the same manner. 



41. The nerves which ramify over the lower part of the 

 membrane, and which endow it with sensibility to touch and 

 pain, are branches of the 



"fifth pair" of nerves. 

 An irritation applied to 

 the parts where this nerve 

 is distributed occasions 

 sneezing that is, a spas- 

 modic contraction of the 

 diaphragm, the object of 

 which is the expulsion of 

 the irritating cause. The 

 manner in which the ol- 

 factory nerve-fibres termi- Fm ' 62 ' ~ SECTION OP RlOHT NA8AL AV1TT 

 nate is peculiar. Unlike the extremities of other nerves, which 

 are enclosed by a greater or less thickness of tissue, these come 

 directly to the surface of the mucous membrane, and thus are 

 in very close contact with the odorous particles that are carried 

 along by the respired air. The surface is at all times kept in 

 a moist condition by an abundant flow of nasal mucus; other- 

 wise it would become dry, hard, and insensitive from the con- 

 tinual passage of air to and fro in breathing. Birds, which 

 respire more actively than men, have a special gland for secret- 

 ing a lubricating fluid, located in the air-passages of the head. 



42. The Uses of the Sense of Smell. Smell is the special 

 sense which enables us to appreciate odors. Touch, as we have 

 seen, is largely concerned with solid bodies, and taste with 



41. Branches of the " fifth pair " of nerves ? Nasal mucus ? Birds ? 



42. Smell ? Touch ? Taste ? Design of smell ? Invisible and gaseous particles ? The 

 extreme fineness of the particles f Musk ? In other cases ? 



