fications. The peculiar connexion of this system of nerves with the organs of vegetative life, has caused it to receive the designation of the Nervous System of Organic Life; the Cerebro-Spinal system being termed the Nervous System of Animal Life. It is also not unfrequently termed the ganglionic system; on account of the separation of its centres into scattered ganglia, which forms a striking contrast to the concentration that is so evident in the Cerebro-spinal system. But this term is objectionable, as leading to a supposed analogy between this system and the general nervous system of Invertebrata, whose centres are equally scattered; an analogy which is completely erroneous, since as we have seen, this last is chiefly the representative of the Cerebro-Spinal system of Vertebrated animals. The term Sympathetic is perhaps the best; although it must not be supposed that this system of nerves is the instrument of by any means all the sympathies, which manifest themselves between different organs.

927. The sympathetic system contains both classes of nervous fibres; -the ordinary white tubular fibres, all of which are probably derived from the Cerebro-Spinal system; and the gray or gelatinous fibres, part of which seem to belong to itself (§ 375). Thus we may consider each system as intermingling itself with the other;-the Cerebro-Spinal system transmitting some of its fibres, both motor and sensory, into the Sympathetic; whilst the Sympathetic is represented in the CerebroSpinal system, by certain fibres and collections of vesicular matter of its own. The trunks that. proceed from the Semilunar ganglia, are almost entirely composed of gray or organic fibres; whence it is evident that these ganglia are to be regarded as the true centres of the Sympathetic system. On the other hand, the trunks which issue from the chain of spinal ganglia, contain a large admixture of white or tubular fibres.

928. The Sympathetic nerves possess a certain degree of power of exciting Muscular contractions, in the various parts to which they are distributed. Thus by irritating them, immediately after the death of an animal, contractions may be excited in any part of the alimentary canal, from the pharynx to the rectum, according to the trunks which are irritated,-in the heart, after its ordinary movements have ceased, -in the aorta, vena cava, and thoracic duct,-in the ductus choledochus, uterus, fallopian tubes, vas deferens, and vesiculæ seminales. But the very same contractions may be excited, by irritating the roots of those Spinal nerves, from which the Sympathetic trunks receive their white fibres; and there is, consequently, strong reason to believe that the motor power of the latter is entirely dependent upon the Cerebro-spinal system. Whatever sensory endowments the Sympathetic trunks pos sess, are probably to be referred to the same connexion. In the ordinary condition of the body, these are not manifested. The parts exclusively supplied by Sympathetic trunks do not appear to be in the least degree sensible; and no sign of pain is given when the Sympathetic trunks themselves are irritated. But in certain diseased conditions of those organs, violent pains are felt in them; and these pains can only be produced through the medium of fibres communicating with the sensorium through the spinal nerves.

929. It is difficult to speak with any precision, as to the functions of the Sympathetic system. There is much reason to believe, however, that it constitutes the channel through which the passions and emotions of the mind affect the Organic functions; and this especially through its power of regulating the calibre of the arteries. We have examples of the influence of these states upon the Circulation, in the palpitation of the heart which is produced by an agitated state of feeling; in the Syncope, or suspension of the heart's action, which sometimes comes on from a sudden shock; in the acts of blushing and turning pale, which consists in the dilatation or contraction of the small arteries; in the sudden increase of the salivary, lachrymal, and mammary secretions, under the influence of particular states of mind, which increase is probably due to the temporary dilatation of the arteries that supply the glands, as in the act of blushing; and in many other phenomena. It is probable that the Sympathetic system not only thus brings the Organic functions into relation with the Animal, but that it also tends to harmonize the former with each other, so as to bring the various acts of secretion, nutrition, &c., into mutual conformity. For whilst the quantity of a secreted product, or the amount of tissue generated in a part, may be affected by an increase or diminution in the calibre of the vessels supplying it, the quality of the secretion, or the character of the tissue, may be likewise affected (there seems valid reason to believe) by that Nervous force, whose relations to the Physical and Chemical, as well as to all other Vital forces, are so intimate (§ 396).

CHAPTER XIII.

OF SENSATION, GENERAL AND SPECIAL.

1. Of Sensation in general.

930. ALL beings of a truly Animal Nature possess (there is good reason to believe), a consciousness of their own existence, first derived from a feeling of some of the corporeal changes taking place within themselves; and also a greater or less amount of sensibility to the condition of external things. This consciousness of what is taking place within and around the individual, is all derived from impressions made upon its afferent nervous fibres; which, being conveyed by them to the central sensorium, are there felt (§ 390). Of the mode in which the impression, hitherto a change of a physical character, is there made to act upon the mind, we are absolutely ignorant; we only know the fact. Although we commonly refer our various sensations to the parts at which the impressions are made,-as, for instance, when we say that we have a pain in the hand, or an ache in the leg,-we really use incorrect language; for, though we may refer our sensations to the

parts where the impression is first made on the nerves, they are really felt in the brain. This is evident from two facts;-first, that if the nervous communication between the part and the brain be interrupted, no impressions, however violent, can make themselves felt; and second, that if the trunk of the nerve be irritated or pinched, anywhere in its course, the pain which is felt is referred, not to the point injured, but to the surface to which these nerves are distributed. Hence the wellknown fact that, for some time after the amputation of a limb, the patient feels pains, which he refers to the fingers or toes that have been removed; this continues until the irritation of the cut extremities of the nervous trunks has subsided.

931. It would seem probable that, among the lower tribes of Animals, there exists no other kind of sensibility, than that termed general or common; which pervades, in a greater or less degree, nearly every part of the bodies of the higher. It is by this, that we feel those impressions, made upon our bodies by the objects around us, which produce the various modifications of pain, the sense of contact or resistance, the sense of variations of temperature, and others of a similar character. From what was formerly stated (§ 403) of the dependence of the impressibility of the sensory nerves, upon the activity of the circulation in the neighborhood of their extremities, it is obvious that no parts destitute of blood-vessels can receive such impressions, or (in common language) can possess sensibility. Accordingly we find that the hair, nails, teeth, cartilages, and other parts that are altogether extra-vascular, are themselves destitute of sensibility; although certain parts connected with them, such as the bulb of the hair, or the vascular membrane lining the pulp-cavity of the tooth, may be acutely sensitive. Again, in tendons, ligaments, fibro-cartilages, bones, &c., whose substance contains very few vessels, there is but a very low amount of sensibility. On the other hand, the skin and other parts, which are peculiarly adapted to receive such impressions, are extremely vascular; and it is interesting to observe, that some of the tissues just mentioned become acutely sensible, when new vessels form in them in consequence of diseased action. It does not necessarily follow, however, that parts should be sensible in a degree proportional to the amount of blood they may contain; for this blood may be sent to them for other purposes, and they may contain but a small number of sensory nerves. Thus, although it is a condition necessary to the action of Muscles, that they should be copiously supplied with blood (§ 359), they are by no means acutely sensible; and, in like manner, Glands, which receive a large amount of blood for their peculiar purposes, are far from possessing a high degree of sensibility.

932. But besides the general or common sensibility, which is diffused over the greater part of the body, in most animals, there are certain parts, which are endowed with the property of receiving impressions of a peculiar or special kind, such as sounds or odors, that would have no influence on the rest; and the sensations which these excite, being of a kind very different from those already mentioned, arouse ideas in our minds, which we should never have gained without them. Thus, although we can acquire a knowledge of the shape and position

of objects by the touch, we could form no notion of their color without sight, of their sounds without hearing, or of their odors without smell. The nerves which convey these special impressions, as already mentioned, are not able to receive those of a common kind; thus the eye, however well fitted for seeing, would not feel the touch of the finger, if it were not supplied by branches from the Fifth pair, as well as by the Optic. Nor can the different nerves of special sensation be affected by impressions that are adapted to operate on others; thus the ear cannot distinguish the slightest difference between a luminous and a dark object; nor could the eye distinguish a sounding body from a silent one, except when the vibrations can be seen. But Electricity possesses the remarkable power, when transmitted along the several nerves of special sense, of exciting the sensations peculiar to each; and thus, by proper management, this single agent may be made to produce flashes of light, distinct sounds, a phosphoric odor, a peculiar taste, and a pricking feeling, in the same individual, at one time. Each kind of sensation may also be excited, however, by mechanical irritation of the nerve which is subservient to it. The feeling of pain may be induced by impressions made upon the nerves of special sense, as well as upon those of feeling, if these impressions be too violent or excessive. Thus the dazzling of the eye by a strong light, and still more, the action of a moderate light in an irritable state of the retina,-sudden loud sounds, or even sounds of moderate intensity but of peculiar harshness,-powerful odors, or even such as are agreeable in moderation, produce feelings of uneasiness, which may be properly called painful, even though they are different from those excited through the nerves of common sensation.

933. As a general rule, it may be stated, that the violent excitement of any sensation is disagreeable; even when the same sensation, experienced in a moderate degree, may be a source of extreme pleasure. But the question of degree is relative rather than absolute: that is, a sensation may be felt as extremely violent by one individual, whilst another, who is more accustomed to sensations of the same kind, is not disagreeably affected by it. Thus our sensations of heat and cold are entirely governed by the previous condition of the parts affected; as is shown by the well-known experiment of putting one hand in hot water, the other in cold, and then transferring them both to tepid water,which will seem cool to the one hand and warm to the other. The same is the case in regard to light and sound, smell and taste. A person going out of a totally dark room, into one moderately bright, is for the time painfully impressed by the light, but soon becomes habituated to it; whilst another, who enters it from a room brilliantly illuminated, will consider it dark and gloomy.

934. The intensity with which sensations are felt, therefore, depends upon the degree of change which they produce in the sensorium. The more frequent the recurrence of any particular sensation, the more does the system become adapted to it, and the less change does it produce. It is, therefore, perceived in a less and less degree, and at last it ceases to excite attention. The stoppage of a constantly-recurring sensation, however, will produce a change, which makes as strong an

impression on the system as its first commencement; thus there are persons, who have become so habituated to the sound of a waterfall or even of a forge-hammer, that they cannot sleep anywhere but in its vicinity; and it is well known that, when a person has gone to sleep under the influence of some continuous or frequently-recurring sound (such as the voice of a reader, the dropping of water, the tread of a sentinel, &c.), the cessation of the sound will cause his awaking.

935. The acuteness of particular sensations is influenced in a remarkable degree by the attention they receive from the mind. If the mind be entirely inactive, as in profound sleep, no sensation whatever is produced by very feeble impressions; on the other hand, when the mind is from any cause strongly directed upon them, impressions very feeble in themselves produce sensations of even painful acuteness. It is in this manner, that the habit of attending to sensations of any particular class increases their vividness; so that they are at once perceived by an individual on the watch for them, when they do not excite the observation of others. We may even, by a strong effort, direct the mind into one particular channel, so as to receive only those sensations which have reference to it, and to be unconscious quoad all others. Thus, the application of the mind to some particular train of thought may prevent our being conscious of anything that is going on around or within us, the conversation of friends,-the striking of the clock,— the calls of hunger, &c. This abstraction may be altogether voluntary; and the possession of the power of thus withdrawing the mind at will from the influence of external disturbing causes, and of fixing it upon any particular train of ideas, is an extremely valuable one. But it may also be involuntary, and may be a source of inconvenience from its tendency to recur at improper times,-producing the habitual state which is known as absence of mind or reverie.

936. It is desirable that we should make a distinction, between the sensations themselves, and the ideas which are the immediate results of those sensations, when they are perceived by the mind. These ideas relate to the cause of the sensation, or the object by which the impression is made. Thus, the formation of the picture of an object, upon the retina, produces a certain impression upon the optic nerve; which, being conveyed to the sensorium, excites a corresponding sensation, with which, in all ordinary cases, we immediately connect an idea of the nature of the object. So closely, indeed, is this idea usually related to the sensation, that we are not in the habit of making a distinction between them. Thus, I may say at this moment, "I see a book on the table before me;" the fact being, that I am conscious of a certain picture, which conveys to my mind the ideas of a book and of a table, and of their relative positions; these ideas being (in Man) the result of experience and associations,-in fact, originating in the immediate application of the knowledge we have previously acquired, that a certain object, whose picture we see, is a book, another object a table, and so on. We are liable to be deceived on this assumption; as when, by a clever imitation, a picture on a plane surface is made to represent an object in relief, so perfectly as at once to excite the idea of the latter,