Order in Physiology.-The Physiology of Animal and Vegetable Life, being a subject of great extent, must be methodically treated; and first, it is necessary to determine what principle of arrangement is to be adopted, in order to exhibit, in a connected form, the complete phenomena of these kinds of existence. There are several modes in which such phenomena have been methodized; and it will be convenient briefly to consider some of these, as exhibiting a general view of the whole subject.

The phenomena of animal and vegetable life may be described as Mechanical Phenomena, Chemical Phenomena, Electrical Phenomena, and the peculiar Phenomena of Excitability-the first three orders being common to all departments of nature. A great part of many of the most important actions of the perfect animal body are purely mechanical or purely chemical, or partly chemical or partly mechanical; while such actions are, in their remaining part, the result of a peculiar excitability. In the circulation of the blood, for example, in man, and in the animals resembling man, the blood is propelled onwards by mechanical forces, while these mechanical forces are called into activity in obedience to the laws of excitability. In the function of respiration the air enters the lungs in conformity with the laws of that part of mechanical science termed Pneumatics. The change which the blood undergoes by the contact of this air is a chemical change, or a change closely analogous to a chemical change; while these laws of pneumatics, and the chemical laws, are brought into operation by the agency of an organic excitability. The fluids contained in the leaves of plants in contact with atmospheric air, by the influ

ORGANIC NATURE.-No. II.

C

34

INERT MATTER DISTINGUISHED FROM ORGANIC.

ence of light, undergo a chemical change, or a change exactly analogous to a chemical change; while the leaf presents its upper surface to the light, under the direction of a peculiar excitability.

Excitability, however, can hardly be defined; and in the present state of physiology; it is more a negative than a positive term. All the properties of an organic tissue, whether from the animal or from the vegetable kingdom, which are neither mechanical nor chemical, fall under excitability. Thus, excitability is that which renders animal and vegetable tissues susceptible of certain phenomena, different from the phenomena produced by the same causes on inert matter. For example, with inert matter, the form and textures of a leaf may be exactly imitated; but such an artificial leaf will be destitute of the susceptibility to turn towards the light in sunshine.

Under these several heads all the phenomena of plants and animals might probably be arranged; but the arrangement would be far from convenient.

It belongs to the arrangement of the phenomena of organic life to point out what distinction exists between an organic body and inert matter; and the extreme divisibility of inert matter supplies the readiest ground of distinction. The divisibility of inert matter is either infinite, or, at least, such that no limit can be assigned to it-the minutest portion still retaining all the properties of the original mass. An organic body, on the contrary, is destroyed by division. Again, it seems a universal law, that living bodies alone can give origin to other living beings, either by a partial division of themselves, or by the process of generation; whereas the origin of inorganic substances is always quite independent of any pre-existing substance of a similar kind. Finally, the actions of organic substances, having attained their acme of intensity, gradually decay, and at length, from causes which are inherent in each individual, cease altogether, when the substance becomes at once amenable to the operations of merely chemical and mechanical agents. Such is not the case, however, with inorganic substances, which maintain the same state unalterably, and for any length of time, provided no external agents are brought to operate upon them.

But, from the very earliest times, it has been perceived that a kind of agreement exists between plants and animals; and that, in certain respects, both possess a common nature. In the fifth century before the Christian era, Empedocles taught that seeds are the true eggs of plants; and that plants, like animals, exhibit difference of sex, and a degree of sensibility. Setting out with the idea of this common nature of plants and animals, philosophers naturally next sought to discover some prominent mark of distinction between the two kinds of organic existences. Since the time of Aristotle, in the fourth century before the Christian era, the search after such a distinction has been often renewed; yet, strange to say, almost every distinction hitherto fixed upon, though sufficiently obvious when confined to the higher orders of plants and animals, has been found to fail when applied to discriminate those organic beings lying on the confines of the two kingdoms. The distinction pointed out by Aristotle has been revived in recent times, though hardly with the expected success. This distinction proceeds on the ground that animals receive their nutriment into an internal cavity before it is absorbed into the substance of the body; that plants, on the contrary, absorb their nourishment by the external surface. Animals, in short, have a mouth and stomach; while plants feed by the spongioles of their radicles, and by their leaves.

While this distinction to a very great extent holds good, it cannot be affirmed that it has supplied an adequate test in doubtful cases.

The most recent test suggested for distinguishing whether an organic existence of

PLANTS DISTINGUISHED FROM ANIMALS.

35

doubtful aspect belongs to the vegetable or to the animal kingdom, is of a chemical character. Starch is a constituent of vegetable tissues; and by the blue colour which iodine imparts to starch, even when present in the most minute proportion, it can be detected, wherever it exists, with the greatest facility. This substance, starch, then, being supposed not to exist in the animal kingdom, promised to solve the long-studied problem, or, at least, to be the only test of distinction which, until very lately, could hold its ground; but the recent researches of some German physiologists have demonstrated the existence of particles of an amylaceous nature in some of the lower animals; and even in the brain and spinal cord of man a substance, termed cellulose, hitherto presumed to be proper to vegetables, has been discovered.

It is not to be concluded, however, because so great a difficulty occurs in discriminating from each other those plants and animals which stand on the confines of the two kingdoms, that the laws governing the vegetable economy are identical with those governing the animal economy.

On this important point, we will cite the following passage from the recent work of one of the most distinguished of living physiologists (Valentin's Physiology, by Brinton) :"The constant physical and chemical changes which accompany life depend upon various exchanges, which are produced by the work of the different parts of the body—the extrusion of what is useless; the assimilation of what is received; and the restoration of the organs, by which all these operations are effected. The whole of the vegetable or general organic functions on which nutrition and generation depend, are repeated in every living body. It has often been supposed that all their particulars correspond in the two organic kingdoms; that there is a digestion, a respiration, a perspiration, and an excretion, in plants as well as animals. But a more accurate examination teaches that this is not the case. Vegetables possess no tissues which allow of the same kind of nutritive absorption, of distribution of juices, or of secretion, that we meet with in, at least, the higher animals. They have no large cavities in which considerable quantities of food can be collected and dissolved by special fluid secretions. They possess no point midway in the movement of their juices, and no mechanism, other than that of a casual and secondary apparatus for the inhaustion or the expulsion of the respiratory gases. They are devoid of the changeable epithelial coverings, which play an important part in many of the animal excretory organs. In one word, the general organic functions are introduced into the two living kingdoms of nature, and probably into their subordinate divisions, by two different ways. This difference leads at once to the conclusion, that the structure of the animal is not a simple repetition of that of the plant, with the addition of a series of new apparatus. The nature of the tissues, the mode of their actions and change-the form, division, and destiny of the organs-all these rather teach us that animals of any development are constructed upon an altogether different plan."

Whatever in the above quotation may appear obscure to those to whom physiological ideas are new, will be cleared up, we trust, by what we are about to say on the prominent distinctions between those organic existences which are unequivocally animals, and those which are unequivocally plants, with reference to a basis for the arrangement of the phenomena of vegetable and animal life.

In physiology, the term function is of continual occurrence. What, then, does function signify? Function is the use of a part or organ. The function of the eye is sight; that of the ear, hearing; that of the lungs, the purification of the blood by ventilation; that of the stomach, digestion; that of the liver, to secrete bile. In plants-that of the spongioles of the radicles, to absorb from the soil; that of the leaves, to decompose the

carbonic acid of the atmosphere, so as to appropriate the carbon for the uses of the plant; that of the anther, to impregnate the ovule, by means of its secretion, the pollen; that of the ovary, to mature the ovule into a seed.

As the functions in all the higher animals and the higher plants are numerous, there is room for method in the arrangement of them. Various methods have been suggested; and, in accordance with some one or other of these arrangements, it has been common to methodize the various topics belonging to physiology.

The kinds of function common to plants and animals, are properly termed vegetative functions—the same which are called vegetable or general organic functions in the quotation from Valentin. The kinds of function, not so obviously possessed by plants, so as to seem peculiar to animals, are named the animal functions.

b

The vegetative functions are the functions of maintenance; the animal functions are the relative functions, or the functions of relation. The vegetative functions end in the organism of the individual, or, at most, in the organism of the species; the functions of relation establish relations between the animal and the world without.

n

If we follow the food, in one of the higher animals, from the mouth to its incorporation with the previously existing tissues of the body, the waste of which it is its office to supply, we shall discover what are the more immediate d vegetative functions-the same which, by other names, are known as the functions of maintenance; the functions of nutrition; the assimilative functions, or functions of assimilation; and the functions of organic life.

-f

The food-let it be a piece of meat, or bread -is reduced to a pulp by the movements of the e teeth, and the admixture of the saliva, secreted by the salivary glands; it is then swallowed by a somewhat complex muscular action. It is moved about in the stomach by the contraction of its muscular fibres; and, being mixed with the gastric juice, a peculiar fluid secreted by the lining membrane of the stomach, it passes into chyme this chyme is then, in successive portions, transmitted, by muscular contraction, into the highest part of the intestinal tube, termed the duodenum, which is a kind of second stomach, where the partially assimilated food is first mixed with the bile, and then with the secretion derived from the sweetbread, or pancreas. The mass is now ready to afford chyle, the immediate nourishment of the blood, to the absorbent vessels, termed lacteals, the extremities of which abut on the lining membrane of the higher parts of the intestinal tube, while the residue is sent downwards by what is termed the peristaltic action of the tube, for evacuation. The chyle, taken up on a very wonderful plan by the lacteal tubes,

DIGESTIVE APPARATUS OF MAN.

a, œsophagus; b, pancreas; c, stomach; d, spleen; e, colon; f, small intestines; g, rectum; h, anus; i, appendix of cæcum; k, cæcum;, large intestines; m, gall bladder and ducts; n, liver; o, pylorus and stomach.

is transmitted through the singular small organs termed the mesenteric glands, whence, after important changes, it is again collected by what are named the efferent lacteal tubes; these by degrees unite together into a trunk, which joins the lymphatic vessels coming from the pelvis and the lower parts of the body, to form

the thoracic duct
a, commencing
in the abdomen,
dividing opposite
the middle of the
dorsal vertebræ

into two branches,
which soon reunite,
passing behind the
arch of the aorta and
subclavian
artery,
and making its turn
at b, where it re-
ceives several lymph-
atic trunks, termi-
nates at the point of
junction of the in-

CHYLE VESSELS.

ternal jugular and subclavian veins on the left side of the neck, and into which it pours its contents. The chyle, being thus mixed with the venous blood, is carried with it to the right side of the heart; and, by the motion of the heart, is thoroughly mingled with that blood; from the right side of the heart the blood, reinforced by the chyle, is transmitted to the lungs, where, by exposure to the air, the venous blood is converted into arterial; the arterial blood, so rendered fit for the nutrition of the body, being sent forth from the left side of the heart, is conveyed by the aorta, the great arterial trunk, and its branches, to the capillary bloodvessels, which pervade all the sensible parts of the body. From these capillary blood-vessels, the several component textures of the living frame attract the new matter, of which they stand in need; while that which is already reduced to the state of THE COURSE AND TERMINATION debris, re-enters the blood of the capillary system, and returns OF THE THORACIC DUCTafter Wilson. with the blood, now become venous, to the right side of the c, the aorta; d, the superior heart. The blood, having become impure by the admixture cava; e, the greater vena azygos, in which, in some of the debris of the tissues, and from other causes, is purified, mammals, the duct ter- partly by the lungs, by which a superfluity of carbon is thrown off, while, by the slow combustion which it sustains,

minates.

animal heat is developed; and partly by the kidney, of which last organ the particular office plainly is to keep the blood free from the various chemical products generated during the successive decompositions which the textures and their first debris undergo.