These faculties communicate to man and animals knowledge of their own internal sensations, and also of the external world: their object is to know existence, and to perceive qualities and relations. They consist of three genera: the first includes the Five Senses; the second, those powers which take cognizance of external objects and their relations, named Knowing or Perceptive Faculties; and the third, the faculties which trace abstract relations, and reason, or reflect.
BY means of the Five Senses, man and animals are brought into communication with the external world.
Each sense has two organs; but a single impression is received by the mind from affections of them. Various theories have been formed to account for this circumstance. Drs. GALL and SPURZHEIM are of opinion, that only one of the organs of a sense is active at the same time, and that they alternately act and rest. Thus, if we look through spectacles having one glass yellow and another blue, external objects will not appear green, as has been reported by philosophers, and believed by the public; but, if the glasses are equally thick, and equally transparent, they will be seen blue or yellow, according as we look fixedly with the one eye or the other. If one of the glasses is thinner or more transparent than the other, it will give its color to the objects perceived. Another explanation may be found in the fact, that the mind has no consciousness either of the existence of the organs of sense, or of the functions performed by them. Hence, the perceptions of the mind are always directed to the objects which make the impressions, and not to the instruments by means of which they are experienced; and the mental affection partakes of the unity of the object exciting it, and not of the duplicity of the organs through which the impression is transmitted.
The functions of every sense depend on its peculiar organization; and hence no preceding exercise or habit is necessary in order to acquire the special power of any sense. If the organization be perfect, the functions are perfect also; and, if the former be diseased, the latter are deranged, notwithstanding all preceding exercise. Each sense is subject to its own positive laws. For example, we see according to the laws of the refraction of light; and hence a straight rod, half plunged in water, appears crooked, although touch proves that, in this situation, it continues straight. This is a kind of rectification; but it must not be confounded with the doctrine which maintains, that one sense acquires its functions by means of another. Touch may show that a rod, which is plunged in water, and looks crooked, is straight; but the eyes will see it crooked as before. The rectifications thus effected by the senses are mutual, and not the prerogative of one sense. In this view, the eyes may rectify the sense of touch. If, without our knowledge, a piece of thin paper be placed between one of our fingers and the thumb, we may not feel but we may see it. Even smell and taste may rectify the senses of seeing and touch. Thus many fluids look like water; and it would be impossible to discover them to be different by the sense of touch; but it is easy to do so by smell and taste.
It is difficult to point out accurately the precise limits of the functions of the senses, because, in every act of perception, their instrumentality is combined with that of the internal faculties. The senses themselves do not form ideas. For example, when an impression is made upon the hand, the organs of touch there situated receive it, and transmit it to the brain; and a faculty of mind, through the instrumentality of another organ, perceives the object. Hence, previous to every perception, there must be an antecedent impression on the organs of sense; and the whole functions of these organs consist in receiving and transmitting this impression to the organs of the internal faculties. The organs of sense, in a state of health, never produce the impressions which result from the activity, except when excited by an external cause. Hence, whatever perceptions or impressions, received from external objects, can be recalled by an act of volition, cannot depend exclusively upon the senses; because we cannot excite them by an act of volition. On the other hand, whatever impression we are unable to recall by an act of the Will, must depend on the senses alone; for we are able to produce at pleasure ideas formed by our internal intellectual faculties. There is reason to conjecture that particular parts of the brain receive impressions transmitted by the external senses, and that it is by their instrumentality that the gourmand, for instance, recalls the flavor of a particular wine or the savor of a favorite dish. He cannot reproduce the part of the sensation which depends on the activity of the nerves of taste; but he can recall all that is mental in the perception or that depends on the activity of any part of the brain.
After these general considerations, which apply to all the external senses, a few words may be added on the specific functions of each sense in particular.
DR. SPURZHEIM inferred from pathological facts, that the nerves of motion must be distinct from the nerves of feeling; and subsequent experiments have proved his inference to be well founded. The sense of feeling is continued, not only over the whole external surface of the body, but even over the intestinal canal. It gives rise to the sensations of pain and pleasure; of the variations of temperature; and of dryness and moisture. These cannot be recalled by the will; and I therefore consider them as depending on the sense alone. The impressions made upon this sense serve as the means of exciting in the mind perceptions of figure, of roughness and smoothness, and numerous other classes of ideas; but the power of experiencing these perceptions, is in proportion to the perfection of certain internal faculties, and of the sense of touch jointly, and not in proportion to the perfection of this sense alone.
THE functions of this sense are, to produce sensations of taste alone; and these cannot be recalled by the will. We may judge the qualities of external bodies by means of the impressions made on this sense; but to form ideas of such qualities is the province of the internal faculties.
BY means of smell, the external world acts upon man and animals from a distance. Odorous particles are conveyed from bodies, and inform sentient beings of the existence of the substance from which they emanate. The functions of smell are confined to the producing of agreeable or disagreeable sensations, when the organ is so affected. These cannot be reproduced by an effort of the will. Various ideas are formed of the qualities of external bodies, by the impressions which they make upon this sense; but these ideas are formed by the internal faculties of the mind.
IN new-born children this sense is not yet active; but it improves by degrees, and in proportion as the vigor of the organ increases. Its proper function is the production of the impressions called Sounds; yet it assists a great number of internal faculties. The auditory nerve has a more intimate connexion with the organs of moral sentiments than with those of the intellectual faculties.
THIS fifth and last of the senses, is the second of those which inform man and animals of remote objects, by means of an intermedium; and which, in this instance, is Light. This sense has been said to acquire its functions by touch or by habit. But vision depends on the organization of the eye, and is weak or energetic, as the organization is imperfect or perfect. Some animals come into the world with perfect eyes; and these see distinctly from the first. The young chicken is guided, immediately on escaping from the shell, by the sense of sight; and the sparrow, on taking its first flight from the nest, does not strike its head against a wall, or mistake the root of a tree for its branches; and yet, previously to their first attempts, these animals can have no experience of distance. On the other hand, animals which come into the world with eyes in an imperfect state, distinguish size, form, and distance, only by degrees. This last is the case with new-born children. During the first six weeks after birth, their eyes are almost insensible to light; and it is only by degrees that they become fit to perform their natural functions. When the organs, however, are matured, children see, without the aid of habit or education, in the same manner, and as accurately, as the greatest philosopher. They eye only receives, modifies, and transmits the impressions of light; and internal faculties form conceptions of the figure, color, distance, and other attributes of external objects; the power of forming these conceptions is in proportion to the perfection of the eyes and the organs of the internal faculties jointly.
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