THE SPECIAL SENSES. 



graving 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 distrib- 

 uted occasions sneezing, that is, a spasmodic contraction 

 of the diaphragm; the object of which is the expulsion of 

 the irritating cause. The manner in which the olfactory 

 nerve-fibres terminate is peculiar. Unlike the extremities 

 of other nerves, which are covered in by a greater or less 

 thickness of tissue, these come directly to the surface of 

 the mucous membrane, and thus come into very close con- 

 tact 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 ; otherwise 

 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 secreting 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 fluids, or with solids in solution. 

 Smell, on the other hand, is designed to afford us informa- 

 tion in reference to substances in a volatile or gaseous 

 form. Invisible and subtile particles emanate from odor- 

 ous bodies, and are brought by the respired air in contact 

 with the terminal filaments of the olfactory nerve, upon 

 which an agreeable or disagreeable impression is produced. 



4 1 . Branches of the " fifth pair" of nerves ? Nasal mucus ? Birds ? 

 42. Smell? Touch? Taste? Design of smell ? Invisible and gaseous part* 

 cles ? The extreme fineness of the particles ? Musk ? In other cases ? 



