435. If we would comprehend more distinctly how the nerve-actions of an external impression can resemble all the sentient actions, whether direct or indirect, of its external sensation, we must take into consideration that the direct nerveactions in the part irritated result immediately from the application of the external impression, and before it can be felt; to this class the internal impression is not required. With regard to the indirect nerve-actions of external impressions, the matter is as follows:

436. When an external sensation, either from sympathy or natural deflexions, develops direct sentient actions, or excites incidental sentient actions by means of subordinate conceptions, imaginations, foreseeings, desires, &c., through other branches of the nerve which has received the external impression, or by means of nerves quite different (131), it is requisite to these series of phenomena, that the external impression be changed into a conceptional internal impression before it can suitably excite the cerebral origin of the nerve, or be reflected in its course downwards on the other branches or nerves, as an internal impression (123, 124). If the same nerves be suitably excited in the same manner, by a reflexion or turning back of the external impression, exactly the same phenomena are excited, as if they had resulted from secondary sensational conceptions, foreseeings, desires, &c.

437. It is thus we comprehend, how it is possible for headless animals to exhibit on a stimulus being applied (as proved by experiment), the same adapted movements as are produced by sensation, and by the ideas, foreseeings, desires, &c., resulting therefrom; as when a fly deprived of its head, flies away if irritated, or as when a headless snake quickly withdraws its body from whatever comes in contact with it, or a headless fish strikes the boiling water it is put into with its tail, &c.

438. It appears really wonderful that a blind external impression is so reflected on other nerve-fibrils in its way to the brain, as to produce those movements which the mind produces in virtue of its sensational volitions. The wonder arises, however, from our ignorance of the laws of animal nature, and from our prejudice in concluding that all which results from sensation, can result in no other way. The nerve-actions produced by external impressions, are referred by the mind to

their external sensations, and to the pleasure or pain they excite, or to the secondary conceptions they produce. There must be then a sub-element [Merkmal] for the mind in the external sensation of an external impression, so that it feels co-ordinately the reflexion of the latter, and also the force required for the resulting indirect nerve-action; and it is thereby led by the sensational secondary desires to effect the movements volitionally, which then take place the more readily, because the mind can of itself satisfy the desires for volitional movements (283). In this way, the indirect nerve-action becomes at the same time the incidental sentient action of the external sensation excited by the same external impression (97, 221). But having observed, that adapted motions excited after decapitation were always previously volitional, we are led to presuppose that they must be volitional in their nature, and, therefore, always volitional; and are astonished when we find that they can take place independently of the will. It is so in every instance; but it is those motions only that excite astonishment, which from their nature we have always been accustomed to consider as wholly dependent on the will. We are little surprised at seeing a muscle in a decapitated animal contract when irritated, because we often feel the irritation without at the same time feeling or observing the contraction; but when we see the animal rise up and leap away when strongly irritated, we are surprised, because a similar sensation was always previously connected with the determination to escape, made by a sensational act of will. If the movement to escape were not always connected with the painful sensation in the uninjured animal, we should see it produced in the decapitated animal without thinking it resembled a volitional movement, and without being surprised.

439. It is clear from the preceding statements (366, and 398-401), that brainless animals, although without sensation, because not endowed with mind, nevertheless by means of external impressions which operate incessantly in them, perform all the acts and manifest all the activity of the sentient animal; everything, in short, that is effected sensationally and volitionally they effect by means of the organic forces of the impressions; and since they can act as orderly, judiciously, and rationally as it were, as if they thought, it has been inferred

erroneously, that the apparent voluntariness of these acts depended on sensational conceptions, even although they might be only external sensations. That which is termed voluntary motion, we term so only in ourselves; the voluntariness is not in the movements-which remain the same whether sensational conceptions produce them or not-but simply in this, that we produce them by spontaneous sensational conceptions. But who has proved that animals thus produce their movements? or who can demonstrate, in the face of ocular proof to the contrary, that these movements can be effected by no other animal force than conceptions (400)? That these animals act in this way according to the preordained objects of nature, especially in the instincts, is undeniable (262); but of these objects even the greater number of thinking animals themselves know nothing (265). They are the objects of nature, not theirs (266); and nature has so provided when their adapted acts should take place, or their instincts ought to be in operation, that certain external impressions are imparted in a naturally necessary manner, which pass along their nerves, and are so reflected and changed into internal impressions (399), that the animal must perform those apparently adapted and volitional movements; and which are intended also for the gratification of an instinct, if it exist, but which are nevertheless just as fully effected without it (269). Thus it is from erroneous views that our astonishment arises, inasmuch as we think that these acts cannot be developed by any other animal force than the conceptive force of the mind. For the same reason we erroneously infer, that because the acts of bees, ants, flies, polypes, and other insects and worms, are regulated to ends and in agreement with preordination of nature, they are dependent upon the conceptive force (266). It is quite possible, however, that the external impressions manifestly provided by nature for the instincts in a preordained manner, and which excite the organs according to a pre-established order of sequence, cause in them all those wonderful and apparently voluntary acts, without a conception being at all necessary thereto (286, 292, 293).

440. We know as little how and wherein external impressions on the nerves differ from each other as we know with regard to the various external sensations which they excite (413). An external impression produces as nerve-actions the same movements

which it would excite if it were felt and caused a painful or pleasing sensation; but we know no more in what it differs from an impression which produces the contrary effect, than in what sensations themselves differ (190). Nevertheless, it is very probable, that the external impression which would have excited pleasing sensations, operates upon the nerves connaturally or in a way that is in accordance with their normal and appointed functions; while on the contrary, those which would have caused a painful sensation, act upon the nerves contra-naturally [widernatürlich]; consequently, the resulting nerve-actions themselves are either connatural or contra-natural (191, 195). Experiment supports this view. If a decapitated animal be irritated, so that in its ordinary state the irritation would have caused pain, it fights with its natural weapons, as if the pain were really felt; a headless wasp stings, a headless earwig attacks with its nippers, &c. All these movements are violent, convulsive, and contra-natural, just as they are in the ordinary state of the animal. An acrid irritant causes a convulsive contraction in the excised intestine of an animal, just as usually occurs in the painful colic excited by ths same acrid poison. Gentle stimuli, on the contrary, excite in headless animals gentle movements only; when a decapitated cricket receives the external impressions which ordinarily excite the insect to the act of sexual congress, a disordered and half-convulsive manifestation of the sexual instinct is excited, which borders closely on a contra-natural state, because its sensational stimulus is a titillation of the sexual organs (274). It chirps incessantly with its wings, and allures to sexual congress with unusual energy and activity.

441. Consequently, just as external impressions follow upon each other, so also the same movements result as nerve-actions, which take place as sentient actions, when the external impression excites pleasing or unpleasing external sensations, pain or tickling; and these nerve-actions are in like manner, either in accordance with the natural destination of the mechanical machines, or opposed thereto.

442. It is not necessary that an external impression shall always develop indirect nerve-actions in the mechanical machines supplied with fibrils from the nerve which received the impression, or from others in connection with the latter, for

the same reasons as laid down in § 201 with reference to external sensation.

443. The nerve-actions of an unfelt external impression may be felt, and induce external sensations (184). This constitutes in sensational animals, a new link between the nerve-actions of external impressions and the sentient actions of external sensations. A loaded state of the stomach, worms, or poisons cause therein external impressions, which usually are not felt. These have their direct nerve-actions in the stomach, producing in it a contraction and contra-natural movement, and this nerve-action it is which we feel, when we say that we are ill. Vomiting follows upon this external sensation, as its sentient action, and as the result of a nerve-action of an unfelt impression. So the headaches accompanying disorder of the stomach, are felt nerve-actions from unfelt external impressions (419). Examples of this kind are of daily occurrence.

In specially considering the nature of the nerve-actions in the different mechanical machines, it must not be forgotten that the latter are by nature peculiarly adapted to certain movements, and to none other (193).

SECTION II.—THE VIS NERVOSA OF AN EXTERNAL IMPRESSION IN SPECIAL RELATION TO DIRECT NERVE-ACTIONS.

444. The nerve-actions, produced indirectly by an external impression, are really nerve-actions of a non-conceptional internal impression (422). As these will be considered in the next Chapter, our inquiry as to the vis nervosa of an external impression need not extend beyond the direct actions it produces in the mechanical machines (418).

445. Muscular fibre of all the tissues is most eminently that in which an external impression excites direct nerve-actions. The structure is peculiarly adapted to the latter, since the fibrilli are easily excited to movement at any point of their length, when the two ends either approach or separate from each other (161). An external irritation which duly excites the nervemedulla distributed through the muscle can therefore very readily impart an obvious movement at the irritated point, and produce a direct nerve-action (418). An external impression cannot so easily excite motion in other mechanical machines,