746 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



smaller will be the surface capable of sending odoriferous particles 

 into the air inspired through this tube. This method enables the 

 experimentor accurately to grade the stimulating quantity. 



The amount of substance necessary to excite the olfactory mechan- 

 ism, is extremely small. Thus, 0.01 mg. of mercaptan may be per- 

 ceived if diffused through 230 c. cm. of air, so that each liter of the latter 

 contains only 0,000,000,04 mg. of this substance. ^ The threshold value 

 of ether and oil of wintergreen is 0.0005 mg. per liter of air. Cam- 

 phor stimulates in a dilution of 1 part to 400,000, musk in the propor- 

 tion of 1:8,000,000 and vanilla in the proportion of 1 : 10,000,000. 

 The acuity of the sense of smell differs in different persons, and is 

 subject to various exherent factors. It is said that women, and espe- 

 cially children, are more sensitive than men; moreover, it is a matter 



Fig. 371. — Single Olfactometer. (Zwaardemaker.) 



of common experience that this sense is easily fatigued, but if fatigued 

 so as to be no longer excited by one kind of odorous substance, it 

 is still in a condition to receive other modalities. Quite similarly, 

 while the persons seated in a poorly ventilated room, are quite unable 

 to perceive the foulness of the air, one who has just entered immedi- 

 ately notices its quahty. Furthermore, some persons are absolutely 

 insensitive to certain odors; at least, they fail completely in recognizing 

 their respective qualities. 



In this connection, attention should again be called to the fact 

 that the sense of smell is absent in some animals and is very unequally 

 developed in others. For this reason, the osmatic? group of animals 

 is commonly divided into a microsmatic and macrosmatic, the latter 

 class including such animals as the dog and rabbit. Clearly, the ability 

 of the dog to follow the trail of his master must depend upon a very 

 acute recognition of individual odors, the stimulating quantity of 



^ Fischer and Penzoldt, Liebig's Annalen, 1887, 131. 



