822 STRUCTURE OF THE OLFACTORY ORGAN. 



end-organs of the olfactory nerve. The long narrow olfactory cells (fig. 603, N) are distri- 

 buted between the ordinary cylindrical epithelium (E) covering the regio oliactona. The 

 body of the cell is spindle-shaped, with a large nucleus containing nucleoli, and it sends 

 I upwards between the cylindrical cells a narrow (0*9 to 1'8/x) smooth 



rod, quite up to the free surface of the mucous membrane. In the frog 

 (n) the free end carries delicate projecting hairs or bristles. In the 

 deeper part of the mucous membrane, the olfactory cells pass into, and 

 become continuous with, varicose fine nerve-fibres, which pass into the 

 olfactory nerve ( 321, I., 1). According to C. K. Hoffmann and Exner, 

 after section of the olfactory nerve, the specific olfactory end-organs 

 become changed into cylindrical epithelium (frog), and in warm-blooded 

 animals they undergo fatty degeneration, even on the 15th day. V. 

 Brunn found a homogeneous limiting membrane, which had holes in it 

 for transmitting the processes of the olfactory cells only. 



[The respiratory part of the nasal mucous membrane is lined by cili- 

 ated epithelium stratified like that in the trachea and resting on a base- 

 ment membrane. Below this there are many lymph-corpuscles arid 

 aggregations of adenoid tissue.] 



[The organ of Jacobson is present in all mammals, and consists of two 

 narrow tubes protected by cartilage, and placed in the lower and anterior 

 part of the nasal septum. Each tube terminates blindly behind, but 

 anteriorly it opens into the nasal furrow or into the naso-palatine canal 

 (dog). The wall next the middle line is covered by olfactory epithelium, 

 and receives olfactory nerves (rabbit, guinea-pig), and it contains glands 

 similar to those of the olfactory region ; the outer wall is covered by 

 Fig. 603. columnar epithelium ciliated in some animals (Klein).] 



N min a ) C -? C from ( the 421. OLFACTORY SENSATIONS. Olfactory sensations 



frog ; E, 'epithelium are produced by the action of gaseous, odorous substances, being 



of the regio olfac- brought into direct contact with the olfactory cells, during 



t0Yia - the act of breathing. The current of air is divided by the 



anterior projection of the lowest turbinated bone, so that a part above the latter 



is conducted to the regio olfactoria. Odorous bodies taken into the mouth and then 



expired through the posterior nares are said not to be smelt {Bidder). [This is 



certainly not true, as has been proved by Aronsohn.] 



[It is usuallv stated that only odorous particles suspended in air excite the sensation of smell. 

 This is certainly not the whole truth otherwise, how do aquatic animals, like fish, smell ? More- 

 over the mucous membrane is always moist, and in some cases where there is a profuse secretion 

 from the olfactory mucous membrane, there is no impairment of the sense of smell.] 



During inspiration, the air streams along close to the septum, while little of it passes through 

 the nasal passages, especially the superior (Paulsen and Exner). [The expired air takes almost 

 the same course as the inspired air.] 



The./zrtf moment of contact between the odorous body and the olfactory mucous 

 membrane appears to be the time when the sensation takes place, as, when we wish 

 to obtain a more exact perception, we sniff several times, i.e., a series of rapid in- 

 spirations are taken, the mouth being kept closed. During sniffing, the air within 

 the nasal cavities is rarefied, and as air rushes in to equilibrate the pressure, the 

 air, laden with odorous particles, streams over the olfactory region. Odorous 

 fluids are said not to give rise to the sensation of smell when they are brought into 

 direct contact with the olfactory mucous membrane, as by pouring eau de Cologne 

 into the nostrils (Tourtual, 1827; E. H. Weber, 1847). [Aronsohn has, however, 

 shown that these experiments are not accurate, for one can smell eau de Cologne, 

 clove oil, (fcc, when a mixture of these bodies with 73 per cent. NaCl is applied to 

 the olfactory mucous membrane ; the most suitable medium is -73 per cent. NaCl 

 and its temperature 40-43 C] Even water alone temporarily affects the cells. 

 We know practically nothing about the nature of the action of odorous bodies, 

 but many odorous vapours have a considerable power of absorbing heat (Tyndall). 

 [Odorous bodies diminish the number of respirations (Gourewitsch).] 



The intensity of the sensation depends on 1. The size of the olfactory surface, 

 as animals with a very keen sense of smell are found to have complex turbinated 



