966 A MANUAL OF PHYSIOLOGY 



of onion, garlic, asafoetida ; (6) empyreumatic, or burning odours, as 

 of burnt coffee or tobacco smoke ; (7) caprylic or goat odours, as 

 of sweat ; (8) repulsive odours, as the odour of the disease ozaena ; 

 (9) nauseating odours, as of faeces or putrefying material. _ f 



The most interesting form of inadequate stimulation is electrical 

 excitation of the olfactory mucous membrane, which causes a 

 sensation like the smell of phosphorus. The sensation is experienced 

 at the kathode on closure and the anode on opening. As to the 

 manner in which the multitudinous adequate stimuli excite the 

 olfactory nerves, we can only suppose that they act as chemical 

 stimuli. Smell and taste are pre-eminently the ' chemical ' senses, 

 as sight and hearing are pre-eminently ' physical ' senses. But little 

 is known of the relation between the chemical constitution or physical 

 properties of substances and the quality of the odoriferous sensation 

 which they excite, although Hay craft has pointed out some inter- 

 esting relations between the atomic weights of certain elements and 

 their power of exciting odours. The number of distinct odours 

 which can be perceived is so great that it is scarcely conceivable that 

 each is subserved by special - olfactory fibres. Marked changes 

 occur in disease, and all odours need not be affected to the same 

 extent. Some may be almost normally perceived, while relative or 

 complete loss of smell exists as regards others. These and other facts 

 have given rise to the idea that there are several groups of olfactory 

 fibres, each concerned in the appreciation of a particular odour or 

 group of odours. Yet it has not proved possible to reduce them to 

 a limited number of fundamental odours and their combinations. 



Acuteness of smell may be measured by arrangements called 

 olfactometers. Zwaardemaker's olfactometer consists of a piece of 

 indiarubber tubing fitted inside a glass tube, through which air 

 is drawn into the nostrils. Another glass tube just fitting the 

 rubber tube is pushed inside it, so as to cover a portion of it. The 

 minimum amount of surface of the indiarubber tube which must be 

 left exposed so that the smell of the rubber may be perceived is a 

 measure of the acuteness of smell. To investigate other odours 

 tubes of the corresponding odorous substances can be constructed. 



Taste. The sense of taste is not so strictly localized as the 

 sense of smell. The tip and sides of the tongue, its root, the 

 neighbouring portions of the soft palate, and a strip in the centre 

 of the dorsum, are certainly endowed with the sense of taste ; 

 but the exact limits of the sensitive areas have not been defined, 

 and, indeed, vary in different individuals. 



The nerves of taste are the glosso-pharyngeal, which innervates 

 the posterior part of the tongue, and the lingual, which supplies 

 its tip (see p. 821). The end-organs of the gustatory nerves are 

 the taste-buds or taste-bulbs, which stud the fungiform and cir- 

 cumvallate papillae, and are most characteristically seen in the 

 moats surrounding the latter. They are barrel-like bodies, the 

 staves of the barrel being represented by supporting cells ; each 

 bud encloses a number of gustatory cells with fine processes at their 

 free ends projecting through the superficial end of the barrel. They 

 are surrounded by the end arborizations of the fibres of the gustatory 

 nerves. Taste-buds are also found on the posterior surface of the 

 epiglottis and in the larynx. It has been suggested that these form 

 the afferent end-organs of a reflex apparatus which guards the glottis 



