one kind of sensation without the other kinds being thereby abolished. The only thing which the facts attest is, that the three kinds of sensations have special conditions, and that these conditions may be singly destroyed.-What are these conditions? We may conceive many kinds.-They may be anatomical: this is the answer of the physiologists, of Landry, Brown-Séquard, and Lhuys. In fact, it is sufficient for the explanation of these isolated abolitions that there be three kinds of nerves; this solution is a manifest one, we are tempted to adopt it. But there are others, for the presence of a special nerve does not necessarily follow from the fact of a special condition.-There are two other possible explanations. First, the condition may be a special state of the same nerve, which would appear to be the case from the experiment in which the frozen knee becomes bloodless. Secondly, the condition may be a special state of the parts surrounding the nerve, and through which the external stimulus acts on the nerve; in this case, the same nerve, under the influence of the same external stimulus, would transmit different sensations, according as the parts between it and the stimulus were in different states. These last solutions are more abstract, but they agree better with the facts.

Weber's experiments appear to me conclusive as to this.*-If we dip into cold water a large nervous trunk, for instance, the cubital nerve, where it springs from between the two bones at the elbow, we find, in accordance with a wellknown law, a sensation in the arm and two last fingers of the hand, occasioned by the nervous action going on at the elbow; now this sensation is not of cold, but simply of pain. Consequently, when we have a sensation of cold, it is not

* Article Tastsinn, 498, in "Handbuch der Physiologie," by Rudolf Wagner. Cf. Fick, "Anatomie und Physiologie der Sinnes Organe," 28, 30, 42, 43. From the anatomical structure of the organs of touch, he shows, by approximation and hypothesis, the different types of action which excite in the same nerve different sensations, that of heat or cold, that of pressure or contact. "It is probable that the stimulation of the nerves, in the sensation of heat or cold at the sensible periphery of the skin, is not immediately developed by a change of temperature of the nervous substance itself, but by the simultaneous changes supervening in the mechanical relations of the terminal corpuscles."

owing to the immediate action of cold on the nerve ; for it was not felt just now, when the cold was acting immediately on the cubital nerve. In order to feel it, the cold must act indirectly; that is to say, through certain parts adjacent to the nerve, certain organs disposed for this purpose; these act directly on the nerve; the cold modifies them, and their modification impresses on the nerve a special type of action, which excites in us the special sensation of cold.-If, on the contrary, without paralyzing the nerve, we simply destroy in these adjacent parts the property they have of impressing this rhythm of action on the nerve, we shall cease to have the special sensation of cold; when the cold begins to act on the nerve, it will no longer excite the special sensation of cold, and will only excite, as we found just now in the case of the cubital nerve, the sensation of pain. This is what happens in certain illnesses. M. Axenfeld writes as to this "With ataxic persons, whose cases are among those in which want of sensibility is most complete, I have often observed that cold was disagreeable without its being recognised as cold. When we question them as to the nature of their perception, all we can get from them is, 'It hurts me.'"-We are led to the same conclusion by studying the sensations of persons whose bodies present large cicatrices, consequent on amputations or other wounds. "The parts of the skin," says Weber," in which the tactile organs have been destroyed, and are not completely reproduced, cannot distinguish heat and cold."-Similar experiments point out the presence of similar media in the case of the sensation of pressure. If the cubital nerve between the two bones of the elbow be pressed with the finger, the sensation in the fingers and forearm is not of pressure, but solely of dull pain. "Therefore," says Weber, "the sensation of pressure, and the power of distinguishing its numerous and different degrees, are only possible when the pressure acts on the organs of touch, and, through them, on the extremities of the tactile nerves; this sensation does not arise when the tactile nerves are directly compressed."-Consequently, the

sensation of pressure has for its special condition, not the pressure of the nerve, but a certain modification of certain organs or parts surrounding the nerve. If these organs be alone destroyed, or their capacity for undergoing this modification be alone suppressed, the sensation of pressure will alone be abolished.

Thus, in all cases, what is excited in us is a special type of action in the nerve, and what excites this special type of action in the nerve is a special modification of its appendages and dependent parts.-Consequently, to explain the three kinds of tactile sensations, and to comprehend how they may be singly abolished, there is no necessity of supposing them excited in us by distinct nerves of three different kinds this is a gratuitous hypothesis which no vivisection or microscopic observation comes in to confirm. It is enough to admit that the same nerve or group of nerves is capable of many different types or rhythms of action, and that each of these rhythms is directly excited by the special modification which the external agents impress on the parts surrounding the nerve, whether on the tubes containing it, or on the blood washing it, or on some other of its internal accompaniments.

It is not impossible to form a notion of the differences of these rhythms. "Each tactile nervous fibre," says Fick, can only transmit one and the same sensation which is capable only of degrees. . . . . But ordinary external stimuli do not arrive at these isolated elementary fibres; they come in contact with a group of fibres taken together. We may suppose that heat reaches these fibres in a different order from pressure."—" In fact, the nearer we draw to the true elementary sensation, the more does the difference between the sensation of heat and that of a mechanical stimulus seem to vanish. For instance, we can hardly distinguish a prick with a very fine needle from the touch of a spark of fire."-There is a further analogy; we know that the sensations of heat and cold, like those of pressure, become, when carried beyond a certain point, pure pain.— Lastly, place on the skin some imperfectly conducting

body; for instance, a piece of paper, pierced with a hole of from 2 to 5 millimetres in diameter; through this hole apply to the skin, first, a mechanical stimulus, a pointed piece of wood, a pencil or a flock of wool; then, a heated stimulus, such as the radiation from a piece of hot metal." The two sensations, when thus limited to a minimum of nervous elements, are so similar that the subject of the experiment frequently mistakes the sensation of heat for one of contact, and that of contact for one of heat.-On the contrary, when the nervous elements are numerous, as when a large surface of the skin is subjected to the same experiments, there is not this confusion.-Plainly, then, here as before, the ordinary sensation is a whole; and, here as before, two whole sensations may be apparently irreducible to one another, though their elements may be the same; for this it is enough if the little composing sensations differ in number, magnitude, order and duration; their wholes form masses indivisible by consciousness, and seem simple facts, differing in essence and opposed in quality.

It is very probable that the sensation of pain is no more than a maximum; for all the others, pressure, tickling, cold, heat, change into pain when carried beyond a certain limit. -It is very probable that the sensation of pressure only differs from contact because in pressure "the terminal corpuscles of the deep-seated system are also engaged, while in contact they are not so."*--The sensation of tickling is most probably nothing more than a high degree of the sensation of touch; for, writes M. Axenfeld, "I have always found it disappear with the sensation of touch.” And, in fact, the contact producing it, though apparently feeble, is actually excessive; the feather of the quill, or the piece of string which, when drawn slowly along the cheek or across the nose, grazes imperceptibly the extremity of a nervous papilla, evidently excites considerable activity in the terminal molecule of the papilla, for the sensation is a most

*See Fick and Gratiolet at the places mentioned. Cicatrices have no sensation of heat, and only a dull sensation of pain, but they retain the sensation of presThis arises from the terminal epithelial corpuscles being lost, while the deep-seated corpuscles of Pacini are still there.

sure.

vivid one and lasts for some seconds after the touch.

The

alteration of equilibrium in the nerve indicated by it is greater and slower in disappearing than when a pressure drives back uniformly a whole group of papillæ; if the whole displacement of flesh is then much greater, the relative displacement of the nervous molecules is much less. This is why the final sensation, if of less extent, is far more vivid.

To sum up; all that observation shows us in the nerves of touch, are different systems of transmissible molecular displacements. Composed of similar elements, they constitute dissimilar types or rhythms; undefinable by us in the present state of science, they are, like all displacements, definable in themselves, by the speed, magnitude, and order of their elements; and we may admit that, according to the order of their elements, they arouse in us sensations, sometimes of temperature, sometimes of contact or pressure; that at their minimum of speed and magnitude, they arouse feeble sensations of pressure, contact, and temperature; at their maximum of speed and magnitude, they excite in us the sensation of pain.

V. Let us attempt to cast a general glance over all these facts. A sensation of which we are conscious is a compound of more simple sensations, which are themselves composed of others still simpler, and so on. Thus the sensation produced by a harmony of thirds, ut mi, is made up of two simultaneous sensations of sound, ut and mi. Again, the sensation of ut, like that of mi, is made up of a comparatively strong sensation, that of ut, with the addition of other comparatively feeble simultaneous sensations, those of the superior harmonics. As to this comparatively strong sensation and these comparatively weak ones, each ist made up of shorter successive sensations, which, when isolated, are still perceptible by consciousness, and whose number is equal to that of the vibrations of the air divided by two. Each of these little sensations is, in its turn, composed of two successive elementary sensations, which, taken singly, are not perceptible to consciousness. Finally, each of these elementary sensations is itself an infinite series of successive

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