before it has experienced them. This judgment is ex- History. erted on the relations between the objects that excite these impressions, and the actual state of the body; and it is the intuitive knowledge of these relations that determines, in all their infinitely diversified shades, the pleasure or the pain which the animal experiences from the objects that surround it.

History. the manner most favourable to the end which it proposes. A speculative metaphysician, accustomed to wander over the field of abstraction, to enlarge the sphere of his intellectual notions, to transform sensible objects into ideas, this author could never persuade himself that a being could not proportion and adapt its organs to the operations they are to perform, without possessing a knowledge of these operations, and having already exercised a judgment with respect to them. It is from this that he confounds the principle of life with the thinking soul, which being incessantly present in every part of the body, directs and disposes them according to its own views, and to the end that it proposes in the continual developement of the actions it is to conduct.

The formation, the structure, duration and movements of the body, do not belong peculiarly to it, as it is only a passive subject on which the soul impresses and realises the idea of the phenomena that she has conceived. Every thing is derived from the union of the body with the active foreseeing principle, which governs, according to special laws, those phenomena which are more particularly vital, and which are most independent of the will. The immediate action of this latter faculty does not require the assistance of any other substance. The intervention of an intermediate principle would be there superfluous; and Stahl rejects that of the animal spirits, which had been introduced to explain the mechanism of vitality, and which, by overcharging the science, embarrasses it with a useless hypothesis.

Two faculties are sufficient for the soul to act upon the body, and to preserve it in a living state, viz. those of sense and motion. By the former the animal learns to know the properties of the objects by which he is surrounded, or in which he is interested, and to estimate the relations that subsist between these objects and himself; the latter produces the motion of the whole machine, and determines all the changes of situation which it has to undergo in its whole, and in its parts.

The faculty of sensation has two modifications, relative to the two kinds of knowledge which the soul may receive by means of that function. The first of these resides in the organs of sense, and is adapted to external objects; the second establishes its seat in the interior or gans, and refers to objects that are within, or ideas. Sometimes the moving power enveloped in the muscular system is displayed by the sensible actions that regulate the position of the body with respect to the objects of the universe, of which it makes a part; sometimes concentrated within these organs, it excites intestinal motions, which maintain among their constituent parts, those relations, and that equilibrium, which are necessary to preserve the healthy state, consistence, and tone of each organ. The muscular apparatus is subservient to the exercise of the senses; and the different motions which it impresses on the body, for the purpose of transporting it towards, or to a distance from, certain objects, are always determined by the convenience or inconvenience which the body, by means of the senses, experiences from those objects. The tonic motion, determined by the confused ideas of the principle of life, is displayed in the most hidden organic parts, in the most perfect repose and profound silence of the voluntary movements. The soul gives to its organs the disposition that is favourable to the sensations it wishes to receive, by virtue of the judgment that it exerts respecting these sensations,

Stahl regards the excretions as the means employed by nature to counteract the natural tendency of the body towards putrefaction. He believes that the animal humours are exceedingly disposed to thicken, and that the circulation of the blood is the means made use of by nature to keep up their original fluidity. One of the causes that most favour the tendency of the humours to putrefaction, is plethora, to which nature opposes, sometimes the motion of the solids that distribute the blood; sometimes the hemorrhagic fluxes which unload the vascular system. These latter opinions are the principal foundations of what has been called the humoral pathology, which prevailed so long in most of our medical schools, and which, with certain modifications, is still maintained in many parts of the continent.

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The favourable impulse given to physical science in Of Haller general, by the philosophical writings of Bacon and Newton, extended itself at length to physiology; and physiological writers became convinced that it was better to collect and arrange the facts that related to the economy of living beings, than to frame hypothetical systems concerning them. The honour of forming a rational digest of the phenomena of the animal economy was reserved for Haller, who perceived the importance of assembling under one view, the experiments, facts, and observations of preceding writers, and of substituting them in the place of hypothetical reasonings. He traced the plan of the immense edifice that he designed to construct in his First Lines of Physiology, and executed it on a grand and extensive scale, in his Elements, in which he has brought together into a body of doctrine, as complete as could be expected in his time, all the materials of the science. He perceived the inconvenience of a too strict application of the laws of mechanical philosophy to the living system. He admitted an active force, which he considered as peculiar to the animal body, viz. irritability, which contains the reason or the experimental cause of muscular motion. He maintained that irritability should never be confounded with sensibility, and that the irritable fibre differs as much from the sensible fibre, as the function of motion from that of sensation. Lastly, in his Opera Minora, he lays down many new and important points of doctrine respecting the structure of our organs, and the mechanism of our functions; and he relates a number of experiments made on living animals, for the purpose of drawing from nature the secret of those phenomena which she appears most desirous to conceal. We owe to Haller some curious researches respecting the formation of bone, and the production of callus, as well as some important elucidations of the manner in which the embryo contained in the egg is developed, and passes through the successive stages of its organization. He has left us many experiments and details respecting the structure of the heart, the circulation of the blood, and the pulsation of the arteries; on the mechanism of the ribs, and the action of the intercostal muscles during respiration; on the differences between the sensible and irritable organs; on the action of the brain and nerves, &c.

The

The latter half of the 18th century is remarkable for many able physiologists, who will be admired by posterity, cither for the acuteness of their genius, or the important improvements that they have made in the science. We may mention the names of Bordeu, La Caze, Bonnet, Vicq d'Azyr, Bichat, Dumas, and Cuvier in France; of Fontana and Spallanzani in Italy, and of Whytt, Cullen, Brown, and Darwin in Britain. We cannot pretend to enumerate all the opinions and discoveries of these celebrated men, but must content ourselves with giving a sketch of the three rival systems of Cullen, Brown, and Darwin, and a brief outline of the opinions of Bichat.

The physiological system of Cullen was founded chiefly on that of Hoffman. He placed the principle of the whole animal economy in the movement of the vital solids, regulated by the fundamental laws of the nervous system. This notion of the vital solids, according to him, originates in the nerves, and being almost always united in the sensorium, is easily transmitted from one nervous part to another, as long as the medullary substance of the nerves continues in its natural state of life and continuity. The contraction of the moving fibres connected with the sensible organs through the medium of the brain, is the direct effect of a movement that commences with those objects. It is on the contractility inherent in the moving fibres, excited by their own extension, by the application of various stimuli, and often by the immediate influence of the animal or nervous powers, that all the physical actions of a living being depend. He regards this contractile force as distinct from all those which are possessed by the simple solid, and the inorganic elastic parts of the body.

Of the theory of Brown, we have given a sufficient detail under his life, and need not repeat it here.

It is not easy to give a compendious view of the system of Dr Darwin, that shall be intelligible to those who have not examined his celebrated work, the Zoo. nomia; but we shall endeavour to give as brief and perspicuous an account of it as possible. It is necessary first to notice the descriptions given of the terms to be employed, which are as follows.

The immediate organs of sense are, by Dr Darwin, asserted to consist, like the muscles, of moving fibres. The contractions of the muscles and of the organs of sense, are comprehended under what are called fibrous motions, in contradistinction to the sensorial motions, or the changes which occasionally take place in the sensorium. By this latter term is understood, not only the medulla of the brain and nerves, but also at the same time that living principle or spirit of animation, which resides throughout the body, and which we perceive only in its effects. An idea is defined to be a motion of the fibres of some immediate organ of sense; and hence is frequently termed also a sensual motion. Perception comprehends both the fibrous motion or idea, and the attention to it. When the pain or pleasure arising from this motion and this attention produces other fibrous motion, it is termed sensation; thus limiting this term to an active sense. Ideas, not immediately excited by external objects, but which recur without them, are termed either, 1. Ideas of recollection, as when we will to repeat the alphabet backwards; or 2. Ideas of suggestion, as when we repeat it forwards, A suggesting B, B suggesting C, &c. from habit.

After mentioning a number of experiments to prove History. the fibrous motions of the organs of sense, Dr Darwin proceeds to lay down the following laws of animal causation.

1. The fibres which constitute the muscles, and organs of sense, possess a power of contraction. The circumstances attending the exertion of this power of contraction constitute the laws of animal motion, as the circumstances attending the exertion of the power of attraction constitute the laws of inanimate matter.

2. The spirit of animation is the immediate cause of the contraction of animal fibres. It resides in the brain and nerves, and is liable to general or partial diminution or accumulation.

3. The stimulus of bodies external to the moving organ is the remote cause of the original contractions of animal fibres.

4. A certain quantity of stimulus produces irritation, which is an exertion of the spirit of animation exciting the fibres to contraction.

5. A certain quantity of contraction of animal fibres, if it be perceived at all, produces pleasure; a greater or less quantity of contraction, if it be perceived at all, produces pain. These constitute sensation.

6. A certain quantity of sensation produces desire or aversion. These constitute volition.

7. All animal motions which have occurred at the same time or in immediate succession, become so connected, that when one of them is reproduced, others have a tendency to accompany or succeed it. When fibrous contractions succeed or accompany other fibrous contractions, the connection is termed association; when fibrous contractions succeed sensorial motions, the connection is termed causation; when fibrous and sensorial motions reciprocally introduce each other, it is termed catenation of animal motions. All these connections are said to be produced by habit; that is, by frequent repetition. These laws of animal causation are, according to our author, evinced by numerous facts, which occur in our daily exertions, and are employed by him to explain the diseases and decay of the animal system.

The four sensorial powers, upon which all the actions or motions depend, are thus characterized :

Irritation is an exertion or change of some extreme part of the sensorium, presiding in the muscles or organs of sense, in consequence of the appulses of external bodies.

Sensation, is an exertion or change of the central parts of the sensorium, or the whole of it, beginning in some of those extreme parts of it which reside in the muscles or organs of sense.

Volition is an exertion or change of the central parts of the sensorium, or of the whole of it, terminating in some of those extreme parts of it which reside in the muscles or organs of sense.

Association is an exertion or change of some extreme part of the sensorium, residing in the muscles or organs of sense, in consequence of some antecedent or attendant fibrous contractions.

To these four faculties correspond so many classes of fibrous contractions, named irritative, sensitive, voluntary, and associate. But all muscular motions, and all ideas, are originally irritative, and become causable by sensation and volition from habit, i. e. because pleasure or pain, or desire or aversion, have accompanied them; 3 M 2 those

History. those ideas or muscular motions which have been frequently excited together, ever afterwards have a tendency to accompany each other.

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Opinions and arrangement of Bichat.

Of these motions the associate seem most to have excited Dr Darwin's attention. He divides them into three kinds, irritative associations, as when any part of the extracted heart of a frog is irritated by puncture, the whole heart contracts regularly sensitive associations, or the trains or tribes of motions established by pain or pleasure; and the voluntary associations, or those produced by volition.

:

The activity of this power of volition is supposed to form the great difference between man and the brute creation; the means of producing pleasure and avoiding pain given to man by this power being denied to

brutes.

Corresponding to these four classes of motions, there are four classes of ideas; irritative, preceded by irritation; sensitive, preceded by the sensation of pleasure or pain; voluntary, preceded by voluntary exertion; and associate, preceded by other ideas or muscular motions.

It has been observed in Hudibras, that

A rhetorician's rules

Serve nothing but to name his tools."

So we find that a considerable part of Darwin's works is taken up in establishing the new meaning which he attaches to terms well understood and long adopted.

We cannot enter more fully at present into the opinions of the Zoonomia, but we shall have occasion to notice some of them in the succeeding part of this article.

Bichat's system, which has made so much noise on the continent, is chiefly founded on the division of life into two kinds, organic and animal; the former of which is common to all organized beings, while the latter, as its name imports, is peculiar to animals. Each of these two kinds of life may be considered as composed of two orders of functions, which succeed each other in an inverse order. The first of these series in animal life commences with external objects, and proceeds towards the brain; the second begins in the brain, and is thence

ORGANS

1. Locomotive,

propagated to the organs of motion and voice. In the History. first order of functions, the animal is passive; in the second he is active. External objects act on the body through the medium of the first; by the second, the body reacts on the external objects.

Two kinds of motion take place in organic life. In the first the formation of the body is constantly going on; in the second there is a constant decomposition : hence the elements of the body are continually changing, while the organization continues the same. Organic life is accommodated to the continual circulation of matter. The one order of functions assimilates to the nature of the animal, the nutritious particles received into the system; the other rejects what is useless, or is so much altered as to become noxious. The assimilating order of functions consists of digestion, circulation, respiration, and nutrition; all of which processes the matter received into the body must undergo, before it can become a part of the animal substance. When it has for some time constituted a part of the body, it is taken up by absorption, conveyed into the circulation, and thrown out thence, by cutaneous or pulmonary exhalation, or by some other emunctories. Hence, the second order of organic functions, or disassimilating functions, consist of absorption, circulation, exhalation, secretion, and excretion. The brain is the centre of animal life; the heart of organic life.

Bichat considers the proper balance of life to be preserved by the proportion which exists between the action of surrounding bodies, and the reaction of the system. This reaction is greatest in youth, hence the principle of life is at that time in excess. It is least in old age, and then the vital principle is defective. The measure of life is therefore the difference which exists between the efforts of external powers to overturn life, and the internal resistance to support it. The excess of the former shows the weakness of life; that of the latter indicates its strength.

The following table exhibits Bichat's distribution of the organs, or, as he calls them, appareils, belonging to animal and organic life, and to generation, which is common to both.

1. The mouth.

History.

We have now taken such a view of the progressive state of physiological science, as we deemed consistent with the nature and extent of this article. It has taught us that the prevailing spirit of every age has had a marked influence on the productions both of art and science that have appeared during that period; and that physiology has always been impressed with the character of the science that was most prevalent at any particular period. While the doctrines of Aristotle prevailed in the schools, physiology never extended beyond the bounds that had been set to it by Galen; and the belief in occult qualities universally prevailed. When a taste for metaphysical speculations began to gain ground, this science was given over to the most abstract subtilities and absurd fictions. When Des Cartes had reformed the principles of the ancient philosophy, the study of the animal economy, like all the other branches of physics, was fettered by the trammels of the Cartesian doctrines. After the genius of philosophers was directed to chemistry, physiology also took a chemical turn, which it quitted only to take a new direction pointed out to it by the taste for mathematics and mechanical philosophy, which prevailed among all the literary at the end of the 17th and beginning of the 18th century; and now that the study of chemistry is become so general, we see that physiologists are for reducing the functions of the animal economy to the analytical and synthetical operations of the laboratory, and converting the living body into a furnace where a constant combustion is going on while life remains.

WE are now to enter on the phenomena of life, and the functions of organized beings; and here we must premise, that in our illustration of these phenomena and functions we shall occasionally refer to every class of

living creatures; it being our object rather to give a comparative view of physiology in general, than to confine our remarks to the human economy in particular. Indeed much of the physiology of man has already been given under ANATOMY and MEDICINE; and of that of the inferior animals, we have treated of the physiology of the order Cete under CETOLOGY; of that of Reptiles under ERPETOLOGY; of that of Fishes under ICHTHYOLOGY; of that of Serpents under OPHIOLOGY.

CHAP. I. Of the General Phenomena of Life.

WHEN we take a general view of the objects of na-General ture, we see that they differ from each other in many idea of life, important particulars, and we soon find that they may be conveniently divided into two great classes; one capable of growth, nourishment, and reproduction; the other not susceptible of these changes. We perceive that all those substances which are found in the bowels of the earth, and many of those which appear upon its surface, continue for an indefinite time in the same circumstances, until they are acted on by each other, when they undergo certain changes which entirely alter their nature and former properties.

ble in water, and possessing little activity when apSulphur, in its natural state, is a solid substance inso- luble plied to the human skin; but if it be subjected to the action of heat, in contact with atmospheric air, or any other gas containing oxygen, it becomes a fluid, very miscible with water, and of a most corrosive quality, namely sulphuric acid. The hydrogenous gas found in the upper part of mines, would remain for ever uncombined with the oxygenous gas which forms part of the atmosphere in which it floats, were it not subjected to the action of caloric, or electricity in a very concentrat

Extension,

General ed state; but as soon as either of these agents comes in Phenomena contact with the mechanical mixture of gases, a comof Life. bination takes place, attended with a tremendous explosion, and the hydrogenous gas disappears.

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Distinction between organized and inor. ganic matter.

We find that all the bodies to which we give the name of minerals, possess no power in themselves which can enable them to resist the operation of external aeach individual of them is composed of a small gents; number of principles, and their texture appears to be made up of independent particles. Every other body in nature, comprehending the almost infinite variety of plants and animals, though under certain circumstances subject to the same changes which take place among minerals, have, when these circumstances do not exist, an innate property by which they are enabled to resist the production of these changes. They do indeed undergo certain alterations, but by these their original habit and essential properties are not changed. From the time that a plant springs from the seed, till it ceases to vegetate, it is perpetually receiving an accession of new matter, and giving out a part of its former composition but the new matter is assimilated to it, and becomes a part of the plant; the identity of the plant is preserved, though its component parts are perpetually changing. The same in a still higher degree takes place in animals. The individuals of this latter class, comprehending plants and animals, possess peculiar structure, very different from that of the former. Their texture is fibrous, and the fibres arranged and interwoven, so as to form parts called organs, by means of which they carry on certain operations or functions necessary for their preservation, or for the reproduction of the species. Hence these have been called organized bodies, while the others have been denominated brute or inorganic matter. See NATURAL History, No 7.

The component principles of organized beings are much more numerous in each individual than those of

inorganic matter, though their absolute number in the former class is smaller than in the latter. In order to present, under a compendious point of view, the distinguishing characteristics of these two classes of beings, we shall give the following table.

Gravity,
Hardness,

Elasticity,
Fluidity,
Impenetrability,
Divisibility,
Expansibility,
Permeability,
Figurability.

Regularity, Individuality, Spontaneity, Sensibility, Contractility, Irritability, Expansibility, Corruptibility.

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The differences that are found to prevail between Effects of organized beings and inorganic matter, have always life. been attributed to something of a superior nature, called vitality or life. This term life forms one of those simple ideas which it is difficult to define, and as all understand the meaning of the expression, a definition is the less necessary; but if it be required, it cannot be expressed more accurately than in the language of Bichat, who calls life the sum of those functions which resist death. In short, life is best described by the effects produced on a body while it resides in it, contrasted with those appearances which take place in the same body when life is no longer present.

In

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to chemical

One of the most general effects of the presence of life Resistance is, as we have said, the resistance which living beings change. are by it enabled to oppose to the operation of external agents; and this is most remarkably seen with respect to temperature. Every living being possesses, in a greater or less degree, the power of preserving nearly an uniform temperature, which is always a few degrees greater than that of the medium in which it lives. plants, this power seems to exist but in a low degree. Some of the lower animals which inhabit the air, particularly insects, possess it much more completely. The great heat generated in a hive of bees is a familiar illustration of this. In birds this property is very remarkable, the heat of their bodies being greater than that of any other species of animals. The heat of fishes, worms, and of most reptiles, very little exceeds the temperature of the medium in which they reside; but when the water in which fishes live is frozen, they are capable of resisting, for a long time, the consequences of the diminished temperature. The power which many animals possess of resisting high degrees of heat without any considerable increase of their own temperature, seems still more remarkable, and probably led to the fable of the salamander, which was supposed able to endure the heat of fire, and even extinguish it, when thrown in for that purpose.

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Life seems to pervade almost every part of a living Degrees of being. In animals, every part, except the cuticle, hair, vitality. and nails, exhibits marks of vitality; but it seems to be distributed