when cold than when warm. Other facts, including the curious phenomena of coloured hearing, viz., the seeing particular colours on the production of particular sounds, point to a more general tendency in the stimuli of one sense-organ to interfere with or modify the ordinary results of the effect of a stimulation of another organ. Such modifications illustrate the effect of nervous irradiation, that is, the transmission of the state of excitation from particular nervous elements to other and connected ones.1

THE SERIES OF SENSES.

Coming now to the senses in detail we see that they do not exhibit the same degree of definiteness or the same number of distinct presentative aspects or characters. We usually speak of taste and smell as the coarse or unrefined senses, because we cannot sharply discriminate their sensations, whereas hearing and sight are called highly-refined senses for an opposite reason. By attending merely to the number and fineness of the presentative differences we may arrange the senses in the following ascending order: taste, smell, touch, hearing, sight.

A detailed account of the senses, including, as it must do, a description of the peculiar physiological structures involved, would be impossible here. For this the reader can be referred to one of the easily-accessible text-books in Physiology or Physiological Psychology. Here we must content ourselves with a brief résumé of the psychical elements.

§ 13. Sense of Taste. The sense of taste has its own specialised nerve (gustatory nerve) and end-organ, which last has its special seat on a particular posterior area of the tongue and the soft palate. The proper stimulus to the organ of taste (sapid substance) is in every case one of the chemical substances known as crystalloids, which are either liquid or soluble in the mouth. This fact suggests that the immediate excitant of the gustatory end-organ is a chemical process. Hence taste is commonly spoken of as a chemical sense.2

The sensations of taste must be carefully distinguished from other sensations which are wont to accompany them.

1

In the first place then, true sensations of taste are commonly

1 Cf. above, p. 44 f. On this whole subject, see James, op. cit., ii. p. 28 ff. 2 The proper sensation of taste may be excited not only by a sapid substance, but also by electrical stimulation of the peripheral organ. It is doubtful whether mechanical agencies, as by pressing and rubbing the tongue, are capable of exciting the sensation. See Ladd, Elements of Physiol. Psychology, pt. ii, ch. iii. § 12.

accompanied by and confused with organic sensations resulting from the stimulation of the nerve-fibres ending in the alimentary canal or œsophagus. Thus the sensations of relish and disrelish are not pure sensations of taste, but partly organic.

In the second place, sensations of taste must be distinguished from those of touch. The tongue is supplied by nervefibres and end-organs of touch proper, and the tip of the tongue is indeed finely discriminative of tactile stimuli. When we take food, whether solid or liquid, into the mouth we obtain along with sensations of taste proper tactual sensations (including thermal), by which we know the size, shape, softness, grittiness and temperature of the substance. Some of the sensations commonly included among gustatory probably involve a tactual element, as the pungent effect of pepper and other condiments.

Lastly, sensations of taste mingle with, and are not easily distinguished from, those of smell. This is due to the proximity of the organs, and to the fact that many sapid substances give off odorous particles. The impairment of the sense of smell by a cold brings home to us how much the supposed sensations of taste owe to the sister sense.1

The common classification of sensations of taste proper is into four varieties, viz., sweet, bitter, salt and sour. This classification is not, however, universally accepted, some (as Wundt) adding alkaline and metallic, while others (as Valentin) would reduce the number to two, sweet and bitter. These, though undoubtedly characteristic sensations of taste, are probably not the only ones. At the same time it is possible that further analysis may reduce the number of simple gustatory

sensations.

Salt and sour in strong solutions involve the nerves of touch and common sensation, but whether they do so in weak solutions is disputed. Different regions of the tongue appear to be specially connected with different sensations. Thus sweet and sour are said to be tasted chiefly with the tip of the tongue, bitter and alkaline chiefly with the root.2

1 Another element which occasionally combines with gustatory sensation is muscular sensation, or the sensation that accompanies the contraction of the muscles. Any strong stimulation of the nerves of the tongue excites reflex action of the muscles, and so occasions muscular sensation.

2 On the physiological conditions of sensations of taste, see Ladd, op. cit., pt. i ch. v. §§ 6, 7, and pt. ii. ch. iii. §§ 12-15.

The recent researches of Ehrwall appear to show that certain papillæ responded to sour and sweet but not bitter, others sour and bitter but not sweet, others to sweet but not sour or bitter, and so on. This goes to support the hypothesis that there are distinct nervous structures corresponding to difference of quality in our sensations of taste."

It is to be noted that there is no passing from one sensation to another by gradations in the case of sensations of taste. Sweet and bitter are commonly said to be opposed to one another, but this opposition turns largely on the contrast of the agreeable and disagreeable involved. We cannot pass from sweet to bitter by a graduated series of intermediate sensations, as we can from one colour or one musical tone to another. could only produce an artificial transition from the one to the other by mixing the two in such a way that the one taste grows gradually stronger, the other fainter. Still less would it be possible to arrange tastes collectively in a series, as we can arrange colours and tones.

We

This short account of the sense may suffice to show that it has a very limited value as a knowledge-giving sense. The position of the organ at the entrance of the alimentary canal, and the fact that only a certain number of substances, and these only under definite conditions, are sapid, suggest that the original and main function of the sense is to act as a kind of sentinel, testing beforehand the suitability of substances to be taken into the system as nutriment. By our artificial habits of life the range of sensation has been materially extended, but this has been done mainly in the interest not of knowledge but of enjoyment. It is only in restricted lines of observation, as chemical investigation, that this sense becomes an important aid in the discrimination and recognition of objects.

§ 14. Sense of Smell. The sensations of smell, though apt, as we have seen, to be confused with those of taste, are in general sufficiently marked off from other sensations. This differentiation is connected with the peculiarity of the organ involved. The end-organ in which the olfactory nerve terminates and which is situated in a certain region of the nasal passage (regio olfactoria) consists of certain fine appendages

1 Skandinav. Archiv für Physiol. ii. s. 1-69.

"On the whole subject of the sensations of taste, see Ladd, op. cit., pt. i. ch. v. 36, 7, and pt. ii. ch. iii. ; cf. Ziehen, Leitfaden der physiol. Psychologie, pp. 39, 40.

that are acted upon in a way not yet fully understood by the odorous particles or effluvia borne thither by the current of air in the act of inspiration. Only such substances are odorous as exist in a gaseous form or are vaporisable under given conditions of temperature. The process of stimulation being connected with the entering of the current of air is intensified by a voluntary augmentation of the inspiration, as in sniffing.

As in the case of sensations of taste we have to mark off those of smell from others with which they are apt to be confused. Thus a sensation of smell is distinct from the organic sensation given by fresh or stuffy air, and which involves the nerves terminating in the respiratory cavity. Again, olfactory sensations must be distinguished from these mixed sensations which involve elements of tactual and common sensation, as for example those obtained by sniffing ammonia, snuff, and so forth.1

The qualitative variety of odours secms to be far greater than that of tastes; yet the detection and classification of the elementary sensations is even more difficult here than in the case of the latter. Common language contains no words such as sweet, bitter, sour, which point to certain easily distinguishable simple sensations answering to widely-distributed qualitics in things. Such verbal distinctions as are found, as 'fragrant,' point to the concomitant effect of feeling. For the rest we only name sensations of smell by connecting them with particular objects or substances, as the rose, the lilac, the sca, sulphuretted hydrogen, and so forth. There seems so far no possibility of analysing these effects into a few elementary sensations, still less of connecting the manifold variety of olfactory sensation in some simple graduated arrangement, as we can do in the case of colours and tones.

The organ of smell occupies a position at the entrance of the respiratory cavity analogous to that of the organ of taste at the entrance of the alimentary cavity. And the original function of the sense may well have been that of a judge as to the quality of the air inspired as fitted or unfitted for the respiratory organ. This function has, however, in all the higher

1 It is probable that, as in the case of taste, muscular sensations combine with the intenser and more violent forms of olfactory sensation.

animals become a subordinate one. As we may see in the case of some of the lower animals, notably the dog, a fine olfactory sense may become an important means of discriminating and identifying objects. In the case of man this knowledge-giving use of smell is greatly limited owing to the dulness of the sense; which dulness again is connected with the higher development of other senses, more particularly touch. Hence it is only a comparatively small number of objects and substances that we commonly recognise through the sense of smell. And of these, again, it is more particularly those that produce a sensation of smell with a strongly-marked adjunct of agreeable or disagreeable feeling, as certain flowers, garlic, common gas, etc., which come to be customarily recognised and described by means of their characteristic odour.

SENSE OF TOUCH.

§ 15. General Nature of Tactual Sense. The sense of touch, which has for its main element sensibility to pressure, from its higher degrees to bare contact, is in some respects the least specialised of the special senses. It has no definitely circumscribed area of the peripheral surface for its end-organ. All parts of the skin are sensitive to pressure and give us corresponding sensations. Hence touch has been regarded by some as the fundamental mode of sensibility out of which the more specialised kinds have been differentiated.

This view, however, overlooks that highly-specialised form of human touch which is to be found in certain regions of the skin, and particularly the more mobile organs, as the hand and pre-eminently the finger-tips. This special function of certain regions of the skin as the organ of touch is probably connected with the presence in these parts of certain specialised structures or end-organs which are compressed or made to expand as a body presses on the skin or is drawn over it, or, which amounts to the same thing, as the skin is pressed against or drawn over the body.

The precise nature of this terminal apparatus is not clear. Anatomists distinguish among the sensory nerves running to the skin those with free endings which are probably connected with common sensation, and those which end in special terminal structures. These end-organs again have been divided into a number of