-which may not be corrected, until we have ascertained by the touch, the flatness of the real object.

937. This production of ideas, by the agency of sensations, is a process altogether mental, and dependent upon the laws of Mind. We find that some of these perceptions or elementary notions are intuitive: that is, they are prior to all experience, and are as necessarily connected with the sensation which produces them, as reflex movements are with the impression that excites them. This seems to be the case, for example, with regard to erect vision. There is no reason whatever to think, that either infants or any of the lower animals see objects in an inverted position, until they have corrected their notion by the touch; for there is no reason why the inverted picture on the retina should give rise to the idea of the inversion of the object. The picture is so received by the mind, as to convey to us an idea of the position of external objects, which harmonizes with the ideas we derive through the touch; and whilst we are in such complete ignorance of the manner in which the mind becomes conscious of the sensation at all, we need not feel any difficulty about the mode in which this conformity is effected. But in Man, as already stated, the attaching definite ideas to certain groups of lines, colors, &c., with respect to the objects they represent, is a subsequent process, in which experience and memory are essentially concerned; as we see particularly well, in cases presently to be referred to, in which the sense of sight has been acquired comparatively late in life, and in which the mode of using it, and of connecting the sensations received through it with those received through the touch, has had to be learned, by a long-continued training. The elementary notions thus formed,-which may, by long habit, present themselves as immediately and unquestionably, as if they were intuitive,—are termed acquired perceptions.

938. It is probable that, among the lower animals, the proportion of intuitive perceptions is much greater than in Man; whilst, on the other hand, his power of acquiring perceptions is much greater than theirs. So that, whilst the young of the lower animals very soon becomes possessed of all the knowledge which is necessary for the acquirement of its food, the construction of its habitation, &c., its range is very limited, and it is incapable of attaching any ideas to a great variety of objects, of which the Human mind takes cognizance. This correspondence between the acquired perceptions of Man, and the intuitive perceptions of many of the lower animals, is strikingly evident in regard to the power of measuring distance. This is acquired very gradually by the Human infant, or by a person who has first obtained the faculty of sight later in life; but it is obviously possessed by many of the lower animals, to whose maintenance it is essential, immediately upon their entrance into the world. Thus, a Fly-catcher, immediately after its exit from the egg, has been known to peck at and capture an insect,— an action which requires a very exact appreciation of distance, as well as a power of precisely regulating the muscular movements in accordance with it.

2. Of the Sense of Touch.

939. By the sense of Touch is usually understood that modification of the common sensibility of the body, of which the surface of the skin is the especial seat, but which exists also in some of its internal reflexions. In some animals, as in Man, nearly the whole exterior of the body is endowed with it, in no inconsiderable degree; whilst in others, as the greater number of Mammalia, most Birds, Reptiles, and Fishes, and a large proportion of the Invertebrata, the greater part of the body is so covered with hairs, scales, bony or horny plates, shells of various kinds, complete horny envelopes, &c., as to be nearly insensible; and the faculty is restricted to particular portions of the surface, or to organs projecting from it, which often possess a peculiarly high degree of this endowment. Even in Man, the acuteness of the sensibility of the cutaneous surface varies greatly in different parts, being greatest at the extremities of the fingers and in the lips, and least in the skin of the trunk, arm, and thigh. Thus the two points of a pair of compasses (rendered blunt by bits of cork) can be separately distinguished by the point of the middle finger, when approximated so closely as 1-3d of a line; whilst they require to be opened so widely as 30 lines from each other, to be separately distinguished, when pressed upon the skin over the spine, or upon that of the middle of the arm or thigh.

940. The impressions that produce the sense of touch are received through the sensory papillae, with which the surface of the true Skin is beset,-more or less closely, according to the part of it that is examined. These papillæ are minute elevations, which enclose loops of capillary vessels (Fig. 157), and branches of the sensory nerves. With regard Fig. 157.

to the precise course of the latter, there is some uncertainty; but it is probable, from analogy, that the representation given of them by Gerber (Fig. 158) is in the main correct; and that each loop of the Sensory nerve is connected with one or more vesicular foci, on some change in which the formation of the sensory impression is immediately dependent (§ 382). It is peculiar to the sense of Touch, and to that of Taste (which is closely related to it) that the impression must be made by the contact of the object itself with the sensory surface, and not through any intermediate agency. The only exception to this is in regard to the sense of Temperature, which seems to be in many respects different from ordinary touch; here the proximity of the warm

or cold body is sufficient, the impressions being made after the manner of those of odors, sounds, &c. It is worth remarking, with reference to the question of the special nature of the sensory fibres, which are the

channel of these impressions, that no mechanical irritation of the nerves of common sensation ever seems to excite sensations of heat or cold; these being apparently almost as distinct from the sense of contact as they are from that of light or sound.

941. The only idea communicated to our minds, when this sense is exercised in its simplest form, is that of resistance; and we cannot acquire a notion of the size or shape of an object, or of the nature of its surface, through this sense alone, unless we move the object over our own sensory organ, or pass the latter over the former. By the various degrees of resistance which we then encounter, we form our estimate of the hardness or softness of the body. By the impressions made upon our sensory papillæ, when they are passed over its surface, we form our idea of its smoothness or roughness. But it is through the muscular sense which renders us cognizant of the relative position of the fingers, the amount of movement the hand has performed in passing over the object, and of other impressions of like nature, that we acquire our notions of the size and figure of the object; and hence we perceive that the sense of touch, without the power of giving motion to the tactile organ, would have been of comparatively little use. It is chiefly in the variety of movements of which the hand of Man is capable,-thus conducive as they are, not merely to his prehensile powers, but to the exercise of his sensory endowments, that it is superior to that of any other animal; and it cannot be doubted that this affords us a very important means of acquiring information in regard to the external world, and especially of correcting many vague and fallacious notions which we should derive from the sense of Sight, if used alone. On the other hand, it must be evident that our knowledge would have but a very limited range, if this sense were the only medium through which we could acquire ideas. Of this we have the clearest evidence in the very imperfect development of the mental powers in those unfortunate persons who have suffered under the deprivation of sight and hearing from their birth, and who have been consequently cut off from the most direct means of profiting by the knowledge possessed by their fellowbeings, through want of power to use the organs of speech. It is only

where such individuals have fallen under the care of judicious and persevering instructors, that their mental powers have been called into their due activity, or that any ideas have been awakened, beyond those immediately connected with the gratification of the animal wants, or with painful or pleasurable sensations. Thus a mind quite capable of being aroused to activity and enjoyment, may remain in a condition nearly allied to that of idiocy, simply for want of the sensations requisite to produce ideas of a higher and more abstract character than those derived through the sense of Touch, Taste, and Smell.

942. For the exercise of the sense of Temperature, the integrity of the sensory apparatus contained in the Skin appears to be requisite; for it has been ascertained by the recent experiments of Prof. Weber, that if the integuments be removed, the application of hot or cold bodies only causes pain, their elevation or depression of temperature not being perceived; and the same is the case when hot or cold bodies are applied to the nerve-trunks. It is worthy of note that there are many cases on record in which the sense of Temperature has been lost, while the ordinary tactile sense remained; and the former is sometimes preserved when there is a complete loss of every other kind of sensibility. So again we find that the subjective sensations of temperature (that is, sensations which originate from changes in the body itself, not from external impressions) are frequently excited quite independently of the tactual sensations; a person being sensible of heat or of chilliness in some part of his body, without any real alteration of its temperature, and without any corresponding affection of the tactual sensations. It is curious that the intensity of the sensation of temperature should depend, not merely upon the relative degree of heat to which the part is exposed (§ 933), but also upon the extent of the surface over which it is applied; a weaker impression made on a larger surface seeming more powerful than a stronger impression made on a small surface. Thus, if the forefinger of one hand be immersed in water at 104°, and the whole of the other hand be plunged in water at 102°, the cooler water will be thought the warmer; whence the well-known fact that water in which a finger can be held without discomfort, will produce a scalding sensation when the entire hand is immersed in it.

3. Of the Sense of Taste.

943. The sense of Taste, like that of Touch, is excited by the direct contact of particular substances with certain parts of the body; but it is of a much more refined nature than touch, inasmuch as it communicates to us a knowledge of properties which that sense would not reveal to us. All substances, however, do not make an impression on the organ of Taste. Some have a strong savor, others a slight one, and others are altogether insipid. The cause of these differences is not altogether understood; but it may be remarked, that, in general, bodies which cannot be dissolved in water, alcohol, &c., and which thus cannot be presented to the gustative papillæ in a state of solution, have no This sense has for its chief purpose to direct animals in their choice of food; hence its organ is always placed at the entrance to the

taste.

digestive canal. In higher animals, the Tongue is the principal seat of it; but other parts of the mouth are also capable of receiving the impression of certain savors. The mucous membrane which covers the tongue is copiously supplied with papillæ, of various forms and sizes. Those of simplest structure closely resemble the cutaneous papillæ; but there are others, which resemble clusters of such papillæ, each being composed of a fasciculus of looped capillaries (Fig. 159), with a bundle

Capillary network of fungiform papilla of the Tongue.

of nerve-fibres, whose precise mode of termination it has not yet been found possible to ascertain. These fungiform papillæ, which are covered with a very thin epithelium, are probably the special instruments of the sense of taste; for the exercise of which it seems probable that the sapid substances must penetrate, in solution, to the interior of the papilla. When these papillæ are called into action by the contact of substances having a strong savor, they not unfrequently become very turgid, by a distension of their vessels analogous to that which occurs in erection; and they rise up from the surface of the mucous membrane, so as to produce a decided roughness of its surface. The conical papillæ, on the other hand, are furnished with thick epithelial investments, which are sometimes prolonged into filamentous appendages; and, looking to their higher development among other animals, and the offices to which they are there subservient, it seems probable that their functions are purely mechanical, and that they serve especially to cleanse the teeth from adhering particles. The nerve-fibres can be seen to form distinct loops in their interior, at some distance from the

apex.

944. There has been much discrepancy of opinion as to the nerve which is specially concerned in the sense of Taste. The tongue of Man is supplied by two sensory nerves: the lingual branch of the Fifth pair; and the Glosso-pharyngeal. The former chiefly supplies the upper surface of the front of the tongue, and is copiously distributed to the papillæ near the tip. The latter is mostly distributed upon the mucous surface of the Fauces, and upon the back of the tongue; but it sends a branch forwards, beneath the lateral margin on each side, which supplies the edges and inferior surface of the tip of the tongue, and inosculates with the preceding. There is reason to believe, from experiment, that the gustative sensibility of the tongue is not destroyed by section of either of these nerves; though the operation produces a total or partial loss of sensibility over certain parts of the surface. There seems good reason to conclude, that the lingual branch of the Fifth pair is the