CHAP. IX.] NEBFOUS SYSTEM AND OEGJNS OF SENSE. 315 



retained in the sensitive memoiy, cohering and being retained the 

 more firmly, the more frequently they are repeated. 



The sense of taste can only bo exercised upon matters in a state of 

 solution. It is closely connected with the sense of smell, insomuch 

 that various flavours, which are commonly supposed to be perceived 

 through taste, become imperceptible if no use be made of the sense 

 of smell. Sight even has its effect upon taste, as the rehsh for various 

 pleasant tasting substances may be diminished by darkness. This sense 

 is very capable of education, and pleasurable feelings can be induced 

 by perseverance with respect to substances which are at first distaste- 

 ful. One kind of taste will for a time paralyze the power of 

 appreciation of other flavours, while, j:)<?r contra, certain tastes (e.g., 

 wme and cheese) reciprocally stimulate the intensity of gustatory 

 sensations they respectively give rise to. After-tastes are sometimes 

 observed, such as a taste of sweetness following the intensely bitter 

 taste of tannin. 



The se)ise of smell needs for its exercise the presence of some 

 influence in the air drawn in through the nostrils, the Schneiderian 

 membrane being moist and the olfactory nerves in a normal condi- 

 tion. Olfactory impressions do not persist as long as do those of 

 taste, and of course much less than those of touch, but they are very 

 delicate, a tt.o-uV.-o-u-o" P^-i'^ of musk being perceptible when mixed with 

 common air. The sense of smell can be strengthened by use, and it 

 tends to act both as a deterrent and also as a guide to what is 

 attractive to the organism. Odours become easily associated in 

 memory with sensations of other kinds, so that a recurrence of a 

 smell Tvall often recall the latter. Smell may be considered as guard- 

 ing respiration, as so many noxious atmospheric conditions are 

 attended with disagreeable odours. 



The sense of hearing gives information as to the intensity, quality, 

 and direction of sounds. It has been a widely received doctrine 

 that the nerve-branches supplied to the semi-circular canals enable 

 us to appreciate the direction of the sounds heard, while the cochlear 

 filaments serve to make known the qualities of sounds ; these are 

 purely speculative opinions, and their truth cannot be relied on. 



The sense of sight, as regards the mechanism of its organ, has 

 been already explained. It remains but to speak of the curious fact 

 that the co-existence of inverted images on the back of two eyes 

 produces but a single perception of an external world which is not 

 inverted. This phenomenon has been explained by attributing the 

 exercise of sight not to the eyes themselves, but to certain of the 

 grey masses vdthin the cerebrum, or to the corpora quadrigemina. 

 It is, however, to say the least, no less inexplicable that such interior 

 grey masses should be the seat of sight, than that the retina itself 

 should be that seat. It is objected that irritation of the cut optic 

 nerve produces a sense of light, and it is thence argued that percep- 

 tion of light must be interior, as also that in persons who have lost 

 their eyes, diseased conditions may produce sj)ectral illusions. But 

 to such objections it may be replied (analogously to the reply before 



