SENSATIONS OF TASTE AND SMELL. 299 



excite the same taste. Thus, sugar, saccharin, and sugar of lead 

 (lead acetate) all give a sweet taste, while, on the other hand, 

 starch (soluble starch), which stands so close in structure to the 

 sugars, has no effect upon the taste terminals. It is interesting 

 to remember that the taste nerves may be stimulated by sapid sub- 

 stances dissolved in the blood as well as when applied to the ex- 

 terior of the tongue. A sweet taste may be experienced in diabetes 

 from the sugar in the blood, or a bitter taste in jaundice from the 

 bile. 



The Threshold Stimulus. The determination of the threshold 

 stimulus for different sapid substances is made by ascertaining the 

 minimal concentration of the solution which is capable of arousing 

 a taste sensation. The delicacy of the sense of taste is influenced, 

 however, by certain accessory conditions which must be taken into 

 account. Thus, the temperature of the solution is an important 

 condition. Very cold or very hot solutions do not react, that is, 

 the extremes of temperature seem to diminish or destroy the sensi- 

 tiveness of the end-organ. A temperature between 10 and 30 C. 

 gives the optimum reaction. So also the delicacy of the sense of 

 taste is increased by rubbing the sapid solution against the tongue. 

 Doubtless this mechanical action facilitates the penetration of the 

 sapid body into the mucous membrane, but it seems also to in- 

 crease the irritability of the end-organ. It is our habit in tasting 

 bodies with the tongue to rub this organ against the hard palate. 

 With regard to the threshold stimulus such results as the following 

 are reported: 



Salty (sodium chlorid). 0.25 gm. in 100 c.c. H 2 O detectible on tip of 



tongue. 

 Sweet (sugar) 0.50 " " detectible on tip of 



tongue. 

 Acid (HC1) 0.007 " " " detectible on border of 



tongue. 

 Bitter (quinin) 0.00005 " " " " detectible on root of 



tongue. 



The very great sensitiveness of the tongue to bitter substances is 

 evident from this table. 



The Olfactory Organ. The end-organ for the olfactory sense 

 lies in the upper part of the nose, and consists of elongated, epithe- 

 lial-like cells, each of which bears on its free end a tuft of six to 

 eight hair-like processes, while at its basal end it is continued into 

 a nerve fiber that passes through the cribriform plate of the ethmoid 

 bone and ends in the olfactory bulb. These olfactory sense cells 

 lie among supporting epithelial cells of a columnar shape (Fig. 

 122). At the free edge of the cells there is a limiting membrane 

 through which the olfactory hairs project. The olfactory sense 



