this simple fact refute the notions of digestion being mechanical trituration; or solution by heat (for the animal is cold-blooded); or the effect of fermentation, or putrefaction, or coction!

The slow and languid motion of the blood in cold-blooded animals, has enabled us to demonstrate in them the circulation, which in man can only be proved by argument.

Physiologists have been much perplexed to find out a common centre in the nervous system, in which all sensations may meet, and from which all acts of volition may emanate; a central apartment for the superintendent of the human panopticon; or. in its imposing Latin name, a sensorium commune. That there must be such a point they are well convinced, having satisfied themselves that the human mind is simple and indivisible, and therefore capable of dwelling only in one place. The pineal gland, the corpus callosum, the pons Varolii, and other parts, have been successively suggested. Now, there are many orders of animals with sensation and volition, who have none of these parts. And this assumed unity of the sentient principle becomes very doubtful, when we see other animals, possessed of nervous systems, which, after being cut in two, form again two perfect animals. Is the immaterial principle divided by the knife, as well as the body?

The heart has been regarded by many physiologists as the prime mover in the animal machine ;—the origin of vital motion in the embryo, the chief agent in forming and maintaining the fabric, and the main-spring for keeping the whole machinery in action. There are whole classes of living beings, and some of complicated structure, which have no heart.

Some have regarded the spleen as a spunge; soaking up the blood when the stomach is empty, and allowing it to be squeezed out again by the pressure of this bag when distended. In many animals the spleen is neither cellular, nor so situated as to be compressible by the stomach. This is the case, generally speaking, with birds and reptiles.

The office of conveying away fluids from the stomach has been assigned to it, making it a kind of waste-pipe to prevent the liquid contents of the digestive cistern from rising above a certain level. But it exists in reptiles and fishes, where neither the figure of the stomach, nor the known habits of the animals, in respect to food and digestion, admit of this explanation. In the camel, which retains the water in its stomach, and in the horse, where it passes very rapidly into the cæcum, the spleen is as large as in other animals. In beasts of prey, which hardly

drink at all, it is as large and cellular as in the herbivorous ruminant animals. Its size and its cells are particularly conspicuous in the latter: yet the fluids which they swallow, go into the paunch, and not into the true digestive stomach.

Although arguments from analogy are of great service in physiology, and other departments of natural history, although they throw light on obscure points, and give an interest to many discussions, their employment requires caution, and they should rather be resorted to for illustration than be relied on for direct proof. Organs corresponding in situation and name are not always constructed alike; hence a part is sometimes employed in one class of animals for a different purpose from that which the instrument of the same name and of analogous position in the body executes in another. The gizzards of the gallinæ have a prodigious triturating power; and those, who first ascertained by experiment the extent of their power, were disposed to infer that digestion is effected in man by mechanical attrition. Now, the gizzard, although the corresponding part to our stomach, is in structure and action the instrument of mastication; and, as birds have no teeth, it is the only instrument for dividing the hard grain on which they feed. Further inquiry shows, that even in this stomach, which is covered by a thick insensible cuticle, capable of bearing the friction of grain and siliceous pebbles, digestion is really effected, as in the stomach of man, by solution; the solvent juice being secreted by the large collection of glands at the cardiac end of the oesophagus, and having an operation similar to that of the gastric fluid of quadrupeds.

It has been argued, that the arteries of the mammalia must have a contractile power, because, in some worms without a heart, these vessels carry on the circulation alone. The whole economy is too different in the two instances to admit of inferences from analogy; the circulating apparatus, in particular, is formed on plans altogether different in the two cases; and the structure and actions of the vessels of worms, are, in fact, very little known.

Because the vesiculæ seminales in some animals do not communicate with the vasa deferentia, and therefore cannot receive the fluid secreted in the testicles, it has been inferred that they do not serve the purpose of reservoirs for the seminal secretion in man, where, however, they have so free a communication with the vasa deferentia, that any fluids pass into and even distend the former, before they go on into the urethra. The organic arrangement is different in the two instances; and this difference

leads us to expect a modification in the function, instead of authorising us to infer that the same office is executed in exactly the same manner in both cases. If we met with animals, in whom the cystic duct opened into the small intestines separately from the hepatic, shall we therefore infer that the human gallbladder is not a receptacle for the hepatic bile?

Again, animals may be compared to each other. Each organ must be examined in all the gradations of living beings; its modifications compared and surveyed in relation to the varieties of other parts, before a just notion of its functions can be formed. This kind of examination of the animal kingdom, leads to what may be called general anatomy, the basis of general physiology; the objects of which are to determine the organization, and unfold the vital laws of the whole system of living beings.

In the physical sciences we have the power of insulating the various objects of our research; of analysing them into their component elements, of subtracting these successively, and thus determining beforehand all the conditions of the problem we may be studying. It would be desirable to employ the same proceeding in natural history; and it is resorted to, when the objects are sufficiently simple. But they are for the most part too complicated, and connected too closely by mutual influences. We cannot analyse an animal of the higher orders, and observe the simple result of each organ by itself; for, if we destroy one part, the motion of the whole machine is stopped. The phenomena come before us under conditions not regulated by our own choice; and in a state of complication requiring close attention and careful discrimination to search out and determine the precise share of each component part.

In this difficulty, comparative observations afford some assistance. The animals of inferior classes are so many subjects of experiment ready prepared for us; where any organ may be observed under every variety of simplicity and complication in its own structure of existence alone, or in combination with others,

LECTURE IV.

Nature of Life;-Methodical Arrangement of living Beings; Specics, Varieties, Genera, Orders, &c.-Progressive Simplification of Organization, and of Functions.-Intellectual Functions of the Brain, in the natural and disordered State, explained on the same Principles as the Offices of other Organs.

THE notion of life is too complicated, embraces too many particulars, to admit of a short definition. It varies in the different

kinds of animals, as their structure and functions vary; so that a description drawn from one would not be applicable to others differently situated in the animal series. If we include in the description those circumstances only, which are common to the whole animal kingdom, we must direct our view to beings of the most simple structure, where the phenomenon is reduced to its essential features, and these are not obscured or confused by accessary circumstances.

The distinguishing characters of living beings will be found in their texture or organization; in their component elements; in their form; in their peculiar manifestations or phenomena; and in the limits, that is, in the origin and termination of their vital existence.

Their body is composed of solids and fluids; the former arranged in fibres and laminæ, so as to intercept spaces, which are occupied by the latter. The solids give the form to the body, and are contractile. The fluids are generally in motion.

The component elements, of which nitrogen is a principal one, united in numbers of three, four, or more, easily pass into new combinations; and are, for the most part, readily convertible into fluid or gas.

Such a kind of composition, and such an arrangement of the constituent parts, is called organization; and, as the vital phenomena are only such motions as are consistent with these material arrangements, life, so far as our experience goes (and we have no other guide in these matters), is necessarily connected with organization. Life presupposes organization, as the movements of a watch presuppose the wheels, levers, and other mechanism of the instrument.

The organization assumes certain definite forms in each kind of animals; not merely in the external arrangement of the whole, but in each part, and in all the details of each. On this depends the kind of motion which each part can exercise; the share which it is capable of contributing to the general vital movement; which latter, or, in short, life, is the result of the mutual actions and re-actions of all parts.

Living bodies exhibit a constant internal motion, in which we observe an uninterrupted admission and assimilation of new, and a correspondent separation and expulsion of old particles. The form remains the same, the component particles are continually changing. While this motion lasts, the body is said to be alive; when it has irrecoverably ceased, to be dead. The organic struc

F

ture then yields to the chemical affinities of the surrounding agents, and is speedily destroyed.

All living beings have, in the first place, formed part of a body like their own; have been attached to a parent before the period of their independent existence. The new animal, while thus connected, is called a germ : its separation constitutes generation or birth. After this it increases in size according to certain fixed laws for each species and each part.

The duration of existence is limited in all animals: after a longer or shorter period the vital movements are arrested, and their cessation or death seems to occur as a necessary consequence of life.

Thus, then, absorption, assimilation, exhalation, generation, and growth, are functions common to all living beings; birth and death the universal limits of their existence; a reticular con tractile tissue, with fluids in its interstices, the general essence of their structure; substances easily convertible into the state of liquid or gas, and combinations readily changing, the basis of their chemical composition. Fixed forms, perpetuated by generation, distinguish their species, determine the combination of secondary functions peculiar to each, and assign to them their respective situations in the system of the universe.

After forming this general notion of living beings, we proceed to examine the animal kingdom in detail. The first glance discovers to us an infinite variety of forms; diversities so numerous, that the attempt to observe and register the whole seems almost hopeless. We find, however, that these forms, at first view so infinitely various, admit of being classed together, of being formed into groups, each of which is distinguished by certain essential characters. In the latter all the animals comprehended in each group agree; while they differ from each other in particulars of minor importance.

I have already mentioned that a fixed eternal form belongs to each animal, and that it is continued by generation. Certain forms, the same as those existing in the world at the present moment, have existed from time immemorial. Such, at least, is the result of the separate and combined proofs furnished by our own observation and experience respecting the laws of the animal kingdom, by the voice of tradition and of history, by the remains of antiquity, and by every kind of collateral evidence.

All the animals belonging to one of these forms constitute what zoologists call a SPECIES. This resemblance must not be