OLFA CTOMETR Y. 1 249 



somewhat similar experiment with the head of a horse, and G. Franke 

 with the human cadaver. 1 They divided the head through the mesial 

 plane, and applied a glass plate, so that it closed one nasal cavity, 

 replacing the septum. On drawing smoke through the nose, they 

 could observe through the glass the direction that it took. 



While these observers are not quite agreed as to the exact height 

 that the main stream of air reaches, they are at one in placing it below 

 that portion of the nose in which von Brunn has localised the position 

 of the olfactory epithelium. 



This olfactory epithelium is, in fact, placed in a narrow chink 

 above the main stream of air, and the question arises, how do the 

 odorous particles reach it ? Diffusion no doubt will account for the 

 entrance of many particles, for this is a rapid process, and will occur 

 during the period of the shortest sniff. The air, however, within 

 the true olfactory region must suffer, at any rate, partial displacement 

 towards the posterior nares, and this air will in its turn be displaced 

 by fresh air entering the nose and laden with odorous particles. The 

 pressure of air during the commencement of inspiration is less at the 

 posterior nares than the pressure of the outside air, and therefore less 

 than that of the air within the whole cavity of the nose. There will 

 consequently be a displacement of the outside air, and of that of the 

 whole cavity of the nose towards the posterior nares. Braune and 

 Clasen 2 have indeed experimentally shown what is a priori a certainty, 

 namely, that the air even within the antrum of Highmore suffers 

 partial displacement during each act of inspiration. We may con- 

 clude, therefore, that although the main stream does not reach the level 

 of the true olfactory mucous membrane, yet over this surface there is 

 nevertheless a definite motion of the air. This motion may be difficult 

 to recognise by the methods of Paulsen and his followers, but it may be 

 quite sufficient to appeal to the exquisitely delicate sense-organs of the 

 olfactory nerve. 



Olfactometry. — It is a matter of great theoretical as well as of 

 practical interest to determine how small a quantity of a substance can 

 excite the sense of smell. It will be remembered that Kirchoff and 

 Bunsen were able to distinguish by means of the spectroscope as little 

 as t.tstt.wo °f a milligramme of sodium. As we shall see, the nose com- 

 pares most favourably with the eye. A grain of musk will scent a 

 room for a very long time, and the actual quantity drawn into the nose 

 during any given inspiration must therefore be inconceivably small. 

 E. Fischer and Penzoldt 3 placed a weighed quantity of an odorous 

 substance, such as mercaptan, in a room. The air was mixed with a fan, 

 and the experimenter entered the room from time to time until he was 

 able to smell the substance. In these investigations it was found that 

 mercaptan, with a dilution of 1 to 50,000,000,000, could be detected. 

 By calculation it was found that the actual weight of mercaptan, in 

 the 50 c.c. of air, inspired by the experimenter, was Tw,75W,7JTnr °f a 

 milligramme. 



By methods such as the above, which give the smallest amount of a 

 substance which can produce sensation, we can arrange odorous bodies 

 in the order of their potency. We can also test the sensibility of the 



1 Arch. f. Larynrjol. u. Rhinol., Berlin, 1893, Bd. i. S. 236. 



2 Ztschr. f. Anat. u. Eiitwcklngsgesch., Leipzig, 1876, Bd. ii. S. 1. 



3 Sitzungsb. d. 2)hys.-med. Soc. zu, Erlangen, 1885-86, Bd. xviii. S. 7. 

 VOL. II. — 79 



