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things; and very many of the animal species are powerfully affected, and exhibit some marked peculiarity of manner which enables them to be cited as a prognostication of the coming storm. A general and pervading uneasiness of the system would, under the reflex influence that we are now considering, cause a general agitation and flutter of the system, ending in no specific movement; but the other endowments of the animal are usually brought into play in the circumstances, and it acts according to the best of its judgment in endeavouring to get away from the evil influence. In this it
may or may not succeed; but the efforts that are made are a proof of the presence of a stimulus to action whenever the feelings of organic life assume an uncongenial cast. Quadrupeds, birds, reptiles; all seem to feel the influence of atmospheric changes, and in all of them some action or other follows
up the sensation—those actions being usually something more than the mere reflex influence upon the corresponding muscular parts, which is a necessary part of every sensation.
We can thus see an innumerable variety of causes under this one sense tending to stimulate the movements of animals through the mechanism that joins sense and motion into one whole.
2. Sensations of the Alimentary Canal.— The peculiar process of the digestion of the food, and its absorption along the surface of the intestine, appears to yield a state of sensation or feeling over and above the feelings of organic life. There is an approach to specialty in this department of the organism: a set of nerves would seem to be expressly designed for conveying to the general consciousness certain impressions derived from the changes going on over the mucous membrane of the alimentary canal, through the presence or absence of nutritive material.
The contact of the food with the coat of the stomach causes a secretion of gastric juice, and the dissolved food begins to be absorbed into the blood at the very commencement of the process of digestion; and all along the intestines a double transudation appears to take place, certain matters being constantly given out of the mucous membrane, and others taken in. There is a sensitiveness developed by this action, and two distinct results are produced by it. By a reflex action from the ganglionic system (nerves which do not enter the conscious brain), the vermicular motion of the bowels is sustained; this being an exact parallel to the case of an ordinary sensation, in the cardinal peculiarity of the union of force with stimulus. In the second place, impressions are conveyed to the conscious system of a cast corresponding with the nature of the processes at the time, sometimes pleasant and luxurious, sometimes indifferent, and at other times painful or oppressive.
If we inquire to what muscles the stimulus of the alimentary states is reflected in the first instance, in order to complete the circle of sensation,
answer is—the diaphragm and the muscles of the abdomen. These form the set immediately enclosing and controlling the digestive viscera, and all analogy would point to them as the recipients of the reflected influence. There is a considerable likelihood that the healthy and vigorous action of the digestive processes communicates a vigour of tone to the abdominal and respiratory muscles, and that a perverted action of the secretions paralyses the corresponding muscular parts.
The mass or amount of sensation derived from the alimentary canal is
very great, and it forms a considerable fraction of the total happiness or misery of all animal tribes. Nature has evidently associated the circles and ganglia of digestion in intimate and powerful bonds of union with the general centre of the nervous system; so that the impressions of the digestive states, besides their proper reflex actions on the digestive muscles, rouse up extensive operations over the active organs at large. This secondary and wide-ranging influence (whose explanation belongs to the higher functions of instinct) may be seen in many forms and ways. The tendency to repose after a full meal, the fretfulness of a hungry man, the setting of the wits to work upon the pursuit of food as a primary object of life, are instances of the powerful alliance between the digestive circles and the other regions of activity. Of the pains and agonies of existence, none are more horrible or more unhinging to the general system than some of the perverted states of the stomach.
3. Sensations of Taste. The organ of Taste is situated at the entrance of the alimentary canal, and serves as a means of discriminating the substances proper to be taken as food. In so doing it contributes a certain quota to the pleasures of existence.
The structure of the organ is a mucous membrane spread over the tongue and palate, and secreting a liquid to combine with a portion of the food. The act of combination between the liquid and the food affects the nerves of taste embedded in the membrane.
The circle of sensation is completed by the action of the muscles of the mouth, tongue, and lower jaw; an agreeable sensation stimulates the processes of mastication and swallowing; a repulsive sensation causes an opposite action, such as to expel the obnoxious mouthful.
There is evidently some harmony between agreeableness to the taste and agreeableness to the stomach and to the organic system; hence what passes the ordeal of the palate is usually suitable to the real wants of the individual. This arrangement gives to animals the power of instinctive discrimination of food. In the inferior orders of creatures the sense of taste seems very powerful and predominant, and the instinctive capacity arising from it is proportionally great. The human subject, less frequently repelled by the tastes of substances found in nature, can indulge in a greater variety of articles of food, and in consequence may sin more largely against the wellbeing of the system than creatures of more narrow and exclusive likings can do.
We thus find in the mechanism of the sense of taste the origin of a class of practical judgments of the greatest importance in the guidance of animal life. This is a true case of instinct, and in it we can see a very large and comprehensive end effected by great simplicity in the means.
4. Sensations of Smell.—The sense of Smell is placed at the portal of the lungs, to test the quality of the inspired air, and give timely warning of a noxious atmosphere. The organ consists of a membraneous expansion covering the convoluted cavities
of the nose, and connected by nervous fibres with a central ganglion of a conspicuous character. The circle is completed by nerves returning to the muscles of the nostrils in the first instance, and, in the second place, to the muscles moving the chest. An agreeable flavour stimulates one set of movements, tending to increase the inhalation; a flavour of an unsuitable kind has the contrary effect of checking the inhalation and promoting expiratory movements. It thus happens that the sensitiveness to smell may be the means of sensibly exalting or depressing the function that more than any other connects itself with the vital energy of the system-namely, the purification of the blood by the lungs.
The circle of the sense of smell, acting by itself, has no other effect beyond this of modifying the breathing. But this circle is brought into connection with other circles, and originates through these a wider range of activity. It is impossible to get rid of unpleasant smells by merely retarding the process of inhalation, and employing strong expiratory efforts; the effect still continues to irritate the system, and must at last rouse up the movement of flight or some other activity at the command of the creature of a kind to rid it of the evil.
The sensations of smell appear in various ways to be instrumental in setting on general activity. Animals that pursue their prey have in many cases the power of detecting it through their scent; and the far-darting odour of the creatures preyed on seems to have the power both of stimulating the lungs, and through them the vital energy or animal spirits, and also of inflaming the entire nervous system with an uncontrollable energy of pursuit. The carnivorous creature has all its bloodthirstiness fired by the smell of its accustomed victims, and with this are wakened up the whole destructive energies of its nature. Hunger and the flavour of meat are sufficient to spread an irritation over the active system of this class of animals.
Nature has thus based extensive endowments on the sense of smell. The detection of prey and of the means of subsistence is given by this sense acting within its own circle; and when once a victim comes within the scent, there is produced by it a stimulus proceeding to other circles, and causing the other movements that bring the prey within reach, and end in its being finally devoured. Smell, therefore, like taste, is of itself a knowledge-conferring faculty, and is a commencing link in some of the more complicated instinctive operations.
5. Sensations of Touch.—The sense of Touch is situated all over the surface of the animal body, and is conceived as residing in the skin. The true sense of touch, as distinguished from the sensibility to shocks or pressure, consists in discerning a substance in contact with the body, as made of separate parts, and having extension in space. None of the foregoing senses can give any feeling of the solidity or dimensions of bodies; indeed they can hardly be said to recognise of themselves the external existence of matter.
The ganglion of the sense of touch requires to be much more complicated than the ganglion of taste or smell. The power of discriminating different points in a surface implies a series of independent nerve-fibres distributed in the skin, and having each a distinct connection with the muscles of the part; the general ganglion must, in fact, be a mass of smaller ganglia, with outgoing threads connecting all of them with the corresponding muscular apparatus. This constitutes a higher order of nervous organisation than would appear to belong to the four first senses, and it may be expected to yield a more complex kind of instinctive action.
It is actually found that the movements responsive to the sense of touch are more various and remarkable than the responses of the above-named senses. In the human hand, for example, an object laid on the palm, and touching the five fingers, stimulates all the muscles necessary for clenching the fist; and even the paw of an inferior animal is led into a variety of movements by the touch of any solid body.
Touch is highly developed in the tongue, and enters into the sense of taste, acting as a guide to mastication and deglutition.
The muscular feelings of force and resistance are inevitably mixed up with the sense of touch, but are nevertheless perfectly distinct. Touch is also the medium of many indescribable electric or magnetic - stimuli, especially in the contact of living beings; every individual creature being a huge machine for generating this species of influence.
The whole of the action of animals upon the outer world is through the sense of touch and the moving organs. The material things coming in contact with the body stimulate a constant activity and an enterprising turn; whence arises a great development of the mechanical capacities, and a variety of durable impressions of outward things, a sort of germ
of natural knowledge in its lowest form. An animal comes to feel in the first grapple with solid masses that the sensation changes with every movement and turn that it takes, and a renewed stimulus is thus given to groping and manipulation.
6. Sensations of Hearing.—The sense of Hearing is lodged in a very refined and delicate organ of touch. Sounds being a series of mechanical blows or pressures, they require for their reception a surface affected by pressure. The nerves of hearing are spread out on a membraneous surface in the inner ear, which surface floats in the liquid contents of the chamber. The vibrations of sound strike first a tight membrane, next a series of little bones, and lastly the liquid of the inner ear. This liquid, when compressed itself, compresses the nerve, and gives the sensation of sound.
The responsive action that completes the auditory nervous circle is directed to the small muscles of the ear, whose connection leads them to tighten or relax the membrane of the tympanum, according as the sound is agreeable or the contrary. Such is the delicacy of the hearing organ in the higher animals that sounds differing in the smallest peculiarity may be perfectly discriminated. This discrimination is at the basis of much knowledge of the world, and of great variety of action, particularly in the vocal organs, these being more especially connected with the organ of hearing. The communication established between the ear and the voice is one of the higher arrangements of the nervous system, and from it proceeds the whole development of the vocal powers of the animal.
Hearing, like touch, is a sense giving a feeling of expansion and volume, and also of direction, but not in a very accurate way. This sense is also remarkable for the pleasures that may be imparted through its instrumentality.
7. Sensations of Sight.—Sight is in many respects the highest and most commanding of the senses. It reveals the outspread creation with a degree of fidelity that closer examination can but rarely impugn. The impressions that it leaves behind it are largely involved in the operations of intellect as well as in the highest class of emotions.
The organ of sense is an optical lense formed so as to project a picture of outward things upon the back of the eyeball, where lie outspread the filaments of the nerve of vision. The pictorial impression thus produced is conveyed inwards in fragments, each along a distinct fibre of the optic nerve, which are thus kept asunder on their way to the optio ganglion. Each fibre must be conceived, as in the case of touch, to have an independent connection with the muscles of the eyeball in the responsive action, and to stimulate a position appropriate to its own place in the retina. The attempt of the muscles to adjust the eyeball to the picture is the returning action of the circle. It is impossible to fix the eye upon all the points thus conveying a stimulus to the optic ganglion ; what is done in actual vision is to fix the eye upon one point at a time, and to run it backwards and forwards across the field of view.
The muscles of the eyeballs are themselves very sensitive, and their feelings mix with the sensation of light in the various processes of vision. The feeling of distance from the eye is muscular; likewise the feelings of lateral dimensions and superficial area ; and the feeling of solidity from all combined.
The recognition of the forms and appearances of the outer world, and the guidance of the movements of the individual, are the great practical endowments conferred immediately by the organ of sight. . As the higher senses never exist without possessing the power of fixing and retaining their impressions to some degree—a power which, surprising as it is, seems to be a constant attribute of the nervous system-every animal gifted with eyes has the power of recognising and identifying the place of its own habitation and all its familiar haunts. Hence vision is in all cases the means of making the creature at home somewhere in the wide world.
The nervous circle of vision is, even by itself apart, a complicated and versatile mechanism : moreover, its connections with the nervous system at large, and with all the other energies of the framework, are wide and deep. In those connections we have to search for a great number of the instinctive and other capacities of animal life. Th intimate alliance above noticed between the ear and the voice is paralleled in a grand scale in the present case; the eye and its sensations are deeply associated with the action of the body as a whole, and with all those exertions and manipulations that engross the entire system. Locomotion and pursuit are closely controlled by sight; the same is true of every kind of mechanical process operated by the moving organs at large. In our subsequent expositions we shall have to revert to the visual mechanism, and its alliances with the active circles generally.
Muscular System. In this preparatory survey of the elementary mechanism of sense and activity, a few words require to be said on the muscular system in addition to the notice taken of it as a part of every circle of sensation. We do not assert too much when we denominate it the essential instrument of action, emotion, and thought, throughout the entire animal system.
The muscles are subject to a great variety of states, and yield as many varieties of feelings to the general consciousness. They may be tense or relaxed; they may move rapidly or slowly, continuously or interruptedly, irregularly or rythmically. Some of their movements are luxurious in the extreme, others are painful or disagreeable ; and this distinction determines a preference in the turn of activity.