CHAPTEE V. 

 TASTE AND SMELL. 



SEC. 1. THE STRUCTURE OF THE OLFACTORY 

 MUCOUS MEMBRANE. 



857. The organ of smell resembles the organs of sight and 

 hearing in that it consists of a special nerve, ending in a specialized 

 epithelium. The nerve is the olfactory nerve ( 674) and the epi- 

 thelium forms part of the mucous membrane which lines the upper 

 region, comprising the upper and middle turbinal bones and the 

 upper third of the septum, of each nasal chamber. This region of 

 the nasal mucous membrane is called the olfactory mucous membrane, 

 the remaining lower region being called the respiratory mucous 

 membrane, or sometimes the Schneiderian membrane. In ordinary 

 breathing the currents of inspired and expired air are mainly 

 limited to the lower respiratory region, the upper region being 

 reached by means of diffusion or of eddies. 



The respiratory nasal mucous membrane closely resembles that 

 of the trachea, and consists of a ciliated epithelium, of several 

 layers, resting on a vascular dermis fairly rich in lymphatic 

 elements. Goblet cells are abundant in the epithelium, and 

 numerous small branched mucous or albuminous glands are found 

 in the deeper layers of the dermis. Filaments from the fifth nerve 

 are distributed to this region, but none from the olfactory nerve. 



The olfactory mucous membrane is thicker than the respiratory 

 portion, and has to the naked eye a yellowish colour. The thick- 

 ness is chiefly due to the development in the epithelium of a nuclear 

 layer similar in general appearance to that of a crista or macula 

 acustica ; indeed the resemblance between the olfactory epithelium 

 and the auditory epithelium is in many ways striking. A vertical 

 section discloses, very much as in a macula acustica, a layer of 

 cylindrical or columnar cells, the cylinder cells, resting on a thick 



