TASTE AND SMELL 597 



epithelial layer with the loose underlying connective tissue there are 

 found supporting " basal cells." These are irregularly cubical, being 

 broadest at the base and tapering towards the surface. 



The Mode of Excitation. Smells are not destroyed by passing 

 the air containing them through a long tube packed with cotton-wool. 

 It is estimated that such a tube removes particles TD ^oo cubic milli- 

 metre in size. It is known that a gramme of musk will give off its 

 odour for years, and not weigh appreciably less at the end of the time ; 

 0-01 milligramme of meicaptan diffused in 230 centimetres of air is 

 perceptible to the sense of smell i.e., 0-00000004 milligramme per 

 litre, or a dilution of 1 in 50,000,000. A hound will follow every 

 zigzag that a fox takes across country. 



It is obvious, therefore, that the mode of excitation is as subtle 

 as that of the retina by light. Substances of low molecular 

 weight either have no odour or tend to irritate the nose rather than 

 evoke the true sense of smell. Increase in molecular weight often 

 increases the property of smell for example, of the paraffins. In 

 the series of alcohols also there is an increase in -the intensity of 

 odour as the molecular weight increases. 



The Investigation of the Sense of Smell. Keenness of smell may be 

 investigated roughly by preparing a series of solutions of camphor 

 from 1 in 1,000 to 1 in 1,000,000, and placing them in flasks holding 

 10 to 15 c.c., and having an opening of 17 millimetres diameter. For 



FIG. 335. THE OLFACTOMETER. 



more accurate work, the instrument known as the olfactometer is used 

 (Fig. 335). This consists of two concentric cylinders, the inner one 

 of which ends in a nose-piece. The outer cylinder (A) is lined with an 

 odorous substance. It will be seen that, when the inner cylinder (B) 

 is pushed in level with the outer one, air inhaled through the instru- 

 ment does not pass over this substance; but the more the cylinder 

 is drawn out, the greater the area of the odorous substance exposed 

 to the indrawn air. The extent to which it is necessary to draw 

 out the inner cylinder to recognize the odour is the measure of the 

 responsiveness of the nose to that particular stimulus. 



A classification of different smells is very difficult, almost impracti- 

 cable. Such classifications have been made, but they cannot be re- 

 garded as satisfactory. There is an antagonism between certain 

 odours e.g., iodoform is masked by balsam of Peru, musk by bitter 

 almonds, ammonia by acetic acid. 



The sense of smell may be fatigued. After smelling tincture of 

 iodine, alcohol and copaiba balsam are odourless. Some people do 



