2. WOLFF, GUSTAV. Ueber krankhafte Dissoziation der Vorstellungen. Zeitschrift für Psychologie und Physiologie der Sinnesorgane, Bd. xv., 1897.

3. THORNDIKE, E. L. Animal Intelligence. The Psychological Review, vol. ii., 1898.

4. WHITMAN, C. O. Animal Behaviour. Biological Lectures Delivered at Wood's Holl, 1898. Boston, Ginn & Co.

5. WASMANN, E. Instinct und Intelligenz im Thierreich. Freiburg, 1897.

CHAPTER XIX

ON SOME STARTING-POINTS FOR A

FUTURE

ANALYSIS OF THE MECHANICS OF ASSO

CIATIVE MEMORY

I. The facts have thus far shown that the reflexes are determined chiefly by the structure of the senseorgans, or of the surface of the body, and the arrangement of the muscles. The central nervous system participates in these functions only as a conductor. The true problem with which the physiology of the reflexes is concerned is the mechanics of protoplasmic conductivity. This problem is no longer a biological problem but a problem of physical chemistry.

The only specific function of the brain, or certain parts of it, which we have been able to find is the activity of associative memory. There is at present a tendency to consider the anatomical and histological investigation of the brain as the most promising line for the analysis of these functions. It seems to me that we can no more expect to unravel the mechanism of associative memory by histological or morphological methods than we can expect to unravel the dynamics of electrical phenomena by a microscopic study of cross-sections through a telegraph wire or by

counting and locating the telephone connections in a big city.

If we are anxious to develop a dynamics of the various life-phenomena, we must remember that the colloidal substances are the machines which produce the life-phenomena. But the physics of these substances is still a science of the future. The new methods and conceptions created by physical chemistry give us the hope that a physics of the colloidal substances may be looked for in the near future. At present we can only consider data of secondary importance for the mechanics of associative memory. The first group of these data is furnished by the study of the functions of the sense-organs.

Every

Helmholtz emphasised the fact that our senses only furnish us symbols of the external world. physical process that affects a sense-organ produces changes in the organ. These changes are determined by the peripheral structure or by the specific "energy of the sense-organs, as physiologists since Johannes Müller call it. Whether a blow, an electric current, or ether-vibrations of about 0.0008-0.0004 mm. wavelength stimulate the retina, the sensation is always a specific one, namely, light, while a blow or an electric current produces sensations of sound in the ear. This so-called law of the specific energy of the senseorgans is not peculiar to the sense-organs; it applies, as was emphasised by Sachs, to all living matter; it even holds good for machines. It is in reality only another expression for the fact that the eye, the ear,

and every living organ are able to convert energy in but one definite form- that is, that they are special machines. The determination of the way in which this transformation of energy occurs in the various organs would be the explanation of the specific energy of the various senses.

Physiology gives us no answer to the latter question. The idea of specific energy has always been regarded as the terminus for the investigation of the sense-organs. All the more credit is due Mach and Hering for first having advanced beyond that limit with their chemical theory of colour-sensations. Mach has recently expressed the opinion that chemical conditions lie at the foundation of sensations in general (1).

For the eye we may consider it as probable that light produces chemical effects. Various substances are formed and decomposed in the retina, and the chemical processes of the formation and decomposition of these substances determine the light- and colour-sensations. The ether-vibrations of certain wave-lengths influence these decompositions in a definite manner. The electro-magnetic theory of light will probably in this case lead to further discoveries. Effects similar to those produced by light are also brought about by the electric current. The current itself can pass through the retina only by means of electrolysis, and it may be that the increase in the concentration of ions (wherever their progress is locked) brings about the light- and colour-sensations

caused by the current. It is not impossible that the so-called visual substances-that is, the photo-sensitive substances—are electrolytes. We can thus understand how the electric current produces sensations of light and colour in the eye. But it is more difficult to account for the fact that pressure or a blow on the eyeball produces the sensation of a flash. Carey Lea has found that on photographic plates pressure produces changes of the same character as weak light.

The specific energy of the eye would accordingly amount to nothing more than the fact that an increase in the concentration of ions or certain other chemical substances in the retina causes the sensation of light and colour, no matter whether the changes are caused by vibrations of the ether, by the electric current, or by a blow on the eye. The stimuli which are transmitted to the brain from the eye will hence show exactly the variety and peculiarities which correspond to the variety and peculiarities of the chemical processes in the retina.

The same holds good for the stimuli which are transmitted to the brain from the organs of taste and from the nose. The chemical nature of the causes that produce the sensations of smell and taste is so apparent as to require no proof.

We find greater difficulty in dealing with the senseorgans of the skin. Yet it is conceivable that a chemical basis may also exist for the activity of these senses. This idea finds support in a train of thought, by which I attempted to explain the peculiar influence of

grav