148. Erection of the nerve-points is a movement, or change, which, when strong enough, may become an external impression (31), and on arriving at the brain, may excite a conception similar to external sensations; but having no true material impression as its basis, has consequently no real external circumstance. In this way, remarkably powerful impressions in the brain (from very vigorous conceptions) may produce an external impression on the nerves to which they have reference, as if it came from a real thing without; this reaching the brain will excite the analogue to a real external sensation, and sometimes contains so much of the real, that the mind can only distinguish, by great attention, this class of sensations and their concomitant conceptions, from those which have a true external origin. To distinguish these sensations from the latter class, we shall term them imperfect external sensations. Hitherto they have been confounded with very vivid imaginations; but their extraordinary distinctness shows that they differ from these, inasmuch as often they are in no respect dissimilar from true external sensations. They are necessarily, however, the results only of very powerful impressions in the brain of other conceptions, as shown above; consequently, they can be the actions of a too-active imaginative faculty, and may, therefore, accompany insanity, drunkenness, impressive dreams, strong forebodings, and very strong and violent passions. These imperfect external sensations are known as appearances, apparitions, delusions, spectres, ghosts, &c., and it is of not less importance to pathology and practical medicine than to the present subject, that these be traced to their origin, and distinguished from the feeble images of imaginations and foreseeings. If a vigorous effort be made to see with the eyelids closed, a red colour is seen: if the fear of a sudden fall from a height excites the nerves of vision to give a false impression of surrounding objects, they seem to move, or vertigo occurs a loud stunning noise is heard long after it has ceased, but with decreasing indistinctness: if food be desired with great longing, its taste is perceived beforehand: if a delightful sensation be anticipated earnestly, the nerves become so sensitive, that every slight impression causes it, although previously they would have had no effect. All these are instances of imperfect sensations of this kind, which happen to the most

healthy persons. Consequently, they are not peculiar to an unnatural condition of animal bodies, but occur in the natural.

149. It will be worth the while to consider more closely the origin of imperfect external sensations. A strongly sensational conception makes a forcible impression in the brain, at the point of origin of a certain nerve (144, 26). This is propagated along the nerve to its terminating fibrils, which are thereby stimulated and erected (147). This action imparts an external impression to the fibrils, just as if they had been acted upon from without, which is returned to the brain, and becomes an imperfect external sensation (148). But inasmuch as the real external circumstance is wanting, the question arises, what is really felt imperfectly? The answer is as follows: The sensational conception from which the whole delusion arises, possesses the sub-impressions (merkmale) of a past or future external sensation (66), and their material ideas also partly excite, at the origin of the nerve which they impress, the corresponding material external sensation. This, which is a movement at the cerebral origin of the nerve (43), and is now, in fact, its impression (121), is transmitted downwards to the terminal fibrils, and there makes the baseless external impression, which is returned to the brain, to the same point of the nerve whence it came. The imperfect material external sensation thus excited is a movement at the origin of the nerve, which is partly the same as would have originated if the actual external thing, of which the sensational conception contains sub-impressions, had made an external impression on the terminal fibrils, and which is only defective in that which the external impression alone can supply, to render it a true external sensation. Now, since a true external impression takes place, although without the impression of a real external object, the imperfect external sensation is thereby rendered more similar and equal to a true external sensation, than it previously was by the sensational conception alone. Further, the apparent object of the imperfectexternal sensation is always the object of that external past or future sensation, which was already the basis of the conception that constitutes the first element of the delusion. Consequently, the maniac, the drunkard, the dreamer, the soothsayer, the enamoured, the scorner, &c., each according to their delusion, feel what their imaginations, foreseeings,

expectations, forebodings, instincts, and emotions bring into the mind.

150. When the nerves which feel these imperfect sensations are distributed to mechanical machines, they excite the same movements, as if the sensation had been real. But these imperfect sensations must not be confounded with others arising from circumstances external to the nerves, although not external to the organs of the senses; as for example, when sparks are seen in inflammation of the eye, or singing and humming in inflammation or diseases of the cavity of the ear. These are true external sensations, connected with an erroneous judgment. The nerves of the eye and ear are really impressed by something external to them, but it is within the organ of hearing or of vision itself, and the mind judges falsely, that the sensations arise from objects which have customarily excited them; as, for example, that sparks of fire are seen, that bells are heard ringing, water rushing, &c. (Vide § 378.)

151. Since cerebral impressions excite movements at the terminations of the nerves, where their course is obstructed or reflected (147), it is highly probable, that very vivid sensational impressions are also deflected in their course from the brain downwards to the periphery, either at the bifurcations of the nerves, or in the ganglia, especially where nerves are given off to other parts, or to surround the arteries, and excite such gentle movements, as may have an influence on the contiguous mechanical machines.

CHAPTER III.

ON THE INFLUENCE OF THE ANIMAL-SENTIENT FORCES IN THE MECHANISM OF ANIMAL ORGANISMS.

152. In the foregoing chapter, we have treated of the animalsentient forces abstractedly, without reference to their influence in the mechanical machines of the body. We must now bring this influence under notice (8), as being the second class of actions, which the material ideas excite in the animal economy (16, 113). According to § 117, we have to inquire into:

i. The direct actions of the material ideas, which they alone perform in the mechanical machines of the brain, without the aid of the nerves (153-159).

ii. The indirect actions of the material ideas through the nerves in the mechanical machines generally (311), and specially the material ideas of all the various kinds of conceptions.

Actions of Material Ideas in the Mechanical Machines
of the Brain.

153. The mechanical machines of animal organisms are capable of many kinds of movement, in virtue of their structure; but they are put in motion much more by animal than physical and mechanical forces. Since the animal forces are not regulated in their operation according to physical or mechanical laws (7), we cannot bring them into the same category; and as their laws are unknown, we can only infer their actions in the animal machines from observation.

154. The influence of the soul on the body is exercised by means of the material ideas, or the impressions of their conceptions in the brain (25, 121), and all the movements of mechanical machines which are actually effected through these, are sentient actions (97). All sentient actions in the mechanical machines are excited either directly, by the material ideas developed in the brain itself, or indirectly, by means of

the nerves along which conceptional or internal impressions are transmitted from the brain.

155. To understand this matter rightly, the meaning of the term mechanical machines used in this whole treatise, must be properly comprehended; it must be understood to include all the organs of the body, except the brain and nerves, which are proper animal machines. Consequently muscles, tendons, membranes, vessels, glands, bones, cartilages, viscera, &c., are all mechanical machines. Although many of these are endowed with animal moving forces, they are so endowed, only through, and in virtue of, the nerves with which they are supplied. So also the organs of the external senses, although endowed with sensitive nerves, are only mechanical machines, and even the brain and nerves contain the latter.

156. There are no other mechanical machines in direct connection with the brain, than certain glands, the meninges, and the circulating and lymphatic vessels. Consequently, these are the only machines, in which material ideas can develope sentient actions directly in the brain itself, without the aid of the nerves.

157. The uses of the glands situate on the membranes of the brain, and in close proximity to it, are altogether unknown; but they probably constitute a part of its mechanical structure.

158. The membranes enclosing the brain, and partly the nerves, belong to the same class (the mechanical) as tendons, ligaments, &c. They are not only insensible, but the brain itself seems to have no influence on them, except in communicating the mechanical movement (24) which it possesses itself.

159. The tubes, and especially the vascular system, remain only to be noticed among the mechanical machines which are in connection with the brain, and in these, material ideas can develope true sentient actions. The cortical substance is almost wholly a tissue of tubes, it is not the organ of the conceptive faculty, nor the seat of the animal-sentient forces, but the organ for secreting the vital spirits, which it supplies to the medulla of the brain, and thence to the entire system of animal machines (the nervous system) (11); in this respect it can be considered as a viscus of the head, whose proper function is the secretion of certain fluids from the blood; which fluids constitute, however, an essential part of the animal machines (9); it is, therefore, really a mechanical machine (155), but in virtue