until the last branches are inconceivably small, and ramify throughout every portion of the body and indeed, so small are they, that their existence is not detected by the knife or eye, but only known by the necessity of the blood passing through them. This is the system of the arteries, which carry the blood from the heart, and constitute only half of the general system of circulation. Next, the veins return the blood. At first these are exceedingly small, like the last arteries, and the little vessels join together, forming larger tubes, till they are united in two, which open into the right side of the heart.

After the blood is returned to the heart from the lungs, it is ready for the use of the system; to supply the wants, the waste, the exhaustion and the growth of the body. It is sent through the arteries, the large and the small, and in these minute capillaries the work of formation is supposed to be carried on. I say supposed, for it is only a theory rendered probable by the certainty of its being done somewhere, and the impossibility of discovering it in the larger vessels, which are subject to the cognizance of the eye and perhaps the theory is as much supported by the impossibility of denying anything that may be asserted of the capillaries. For if we cannot assert, and still less demonstrate, anything positive of these secret recesses, who can deny anything, and confidently say that this is not done there? Hence, physiologists who broach the doctrine of the formation in these capillaries, may so do without fear of refutation, at least, if not of contradiction. But this is

a matter of little consequence, so long as we know the fact, that this blood, which is the same in all the arteries of the body, and nearly the same in all animals whatever, and however various may be the food from which it is originally formed: and this blood is the storehouse for all the different materials of the body, and out of this is formed any part that is wanting; and with unerring certainty, in state of ordinary health, every part has its due supply. In the bone the earthy-lime and the gelatine are deposited to make this rigid part of the frame. In the nerve this substance is created with all the proper nervous, exquisite sensibility to pain and pleasure, and the power of convey

ing the will to govern and set in motion the several organs in which it is deposited. In the muscle is deposited a fibrous substance, possessed of power of contraction with great strength, and subject to the control of the will, except, the heart and some of the other involuntary muscles, which seem to be endowed with a discretionary power to act of their own accord, without our direction. In other places only fat is wanted to fill out the frame and enable the other organs to act without interruption.

In short, every part and kind of organ, whatever may be its composition or its use and relations, is supplied from this wonderfully versatile operator, the circulatory system, with a precision and faithfulness which would seem almost to indicate a thinking principle in every particle of matter, enabling it to seek out the respective wants of the organs and determine the appropriate relief, and perform such a variety of operations, as nothing but the constant watch of that all-wise Providence who at first created it could carry on without failure or interruption, and without interference one with another.*

After these operations, the peculiar nutritious and vitalizing power of the blood is lost, and it can no longer give life and vigor to the parts of the body: its color is changed from the scarlet to the purple; it contains more carbon. If its course be stopped, as by tying a cord about the arm or finger, or by strangulation, the finger or arm below, or the face above is swollen, and changed to a livid hue; and if this interruption is continued, so that the fresh blood of the arteries is not sent to these parts, nor the venous blood carried off, the part is palsied and dies: for though the amount of blood is not diminished, yet the quality will not allow it to maintain life.

It is now carried through the veins to the heart, and thus completes the circulation of the blood. It then goes again through the lungs, and again through the body; and this continual circulatory process is carried on while life

* Some Atheists, in place of a constant Providence, believe in laws of matter, inherent in all its particles, and adequate to all the changes through which they go. Pursuing this doctrine to its ultimate application, it is resolvable into nothing Less than the admission of an intelligent, thinking principle in each particle.

lasts; the particles constantly changing, and the whole mass, though never identical, is forever the same.

The proper juice of plants is that changed state of the sap after it has been exposed to the air and light in the leaf, and is returning from it to form the different secretions.'* It is now fitted for the nourishment and the growth of the plant, and is carried to the various parts by a process more simple than the distributing vasculer system of the animal body.

A new set of tubes lead from the sap-cells through the foot-stalk of the leaf to the bark, and through this downward over all the branches, trunk and roots, corresponding to the arteries and the extreme vessels in man.

This bark is the workhouse, in which the formation of the new wood and of the various peculiar principles of the plant takes place. The animal grows by interstitial deposition, by the addition of new particles between those already deposited. Thus the liver grows, not by the addition of layers on the outer surface, but by internal increase; so the finger, or any limb; so every organ, and indeed the whole frame. But the plant grows by the addition of new layers of wood on the outer surface of those already formed. This manner of growth gives the trunk of the pine and other trees, when cut through, the appearance of concentric rings. One of these rings is added annually, and is formed by the inner bark, or rather by the vessels and glands within this organ: for the organs by which the secretion is formed are probably glands; and the secreted fluids themselves are deposited in cells in different parts of the plant, particularly in the bark and the roots.' 'And hence these parts, the bark and roots, receive different properties from the matters which are thus lodged in them.' All the gums, resins, balsams, spices and coloring matters are formed from this proper juice by means of these little organs, which carry on their mysterious opera

* Nuttall.

*

tions like the formation of the parts in the animal body, never to be unveiled to the eye of man.

After having served the purposes of nutrition and growth, and of the secretions, Dr. Darwin thinks the superabundant sap is carried, by means of transverse vessels, which constitute the silver grains, to the ascending vessels in the wood, and there unites with the sap, again to be carried to the leaves and remodified by the air, again to go down through the bark, and thus finish the round of vegetable circulation.

These descriptions of the respiratory apparatus apply to the perfect animals and the trees; but in both races there are exceptions, which are destitute of lungs and leaves. The insects have no lungs; they admit no air into their bodies through their mouths; but they have innumerable little apertures through their skins called stigmata. By these they admit air, which comes directly in contact with the blood as it circulates in the vessels. Thus the skin answers the double purpose of protecting the animal from external matter, and purifying the blood.

The leafless plants find a substitute for these organs in the bark, which has its air cells, into which it absorbs the air, and produces the same effect that is ordinarily had in the leaves.

CAUSES OF THE CIRCULATION OF THE BLOOD.

There is no difficulty of accounting for this in the heart and arteries. Physiologists are agreed that the contraction of the heart is the efficient cause, and the elasticity of the arteries keeps it up through their course. They assume that this power of the heart, or the contractile power of the extreme small vessels, forces the blood through these to the veins; and as these minute organs are beyond ourinvestigation, we cannot deny anything that may be asserted concerning their action. But when the blood reaches the veins, there arises a difficulty. They have given us theory upon theory, till at last they have been driven to the poor and uncertain aid of the muscles, which some consider as the great cause of the venous circulation.

For who that considers that the blood goes through these vessels just as well in the sleep, when all the muscles are quiescent, as when we are in motion, can believe the irregular and accidental actions of these organs are the permanent causes of the motion of the blood, which must be constant and regular? Let it be, that we know nothing about this, and we shall be in no worse condition respecting these than we are respecting the circulation of the sap. To show how far all the philosophical investigations have fallen short of a discovery and satisfactory explanation of this mystery, it is sufficient to state a few of their prominent theories. Some have believed that the sap ascends by the pressure of the atmospheric air on the water in the earth; and others, by the evaporation from the leaves, not thinking that even if the vessels were rigid tubes, this gravitation would force it to only thirty-two feet, and neither of these causes would carry the blood through the leaves or bark. Grew thought it ascended by its specific levity; but the sap is specifically heavier than water. Malpighi supposed it ascended by the contraction and expansion of the air in the air-vessels of the tree; but there are no such air-vessels. Others have attributed this to fermentation to heat. (This last cause is supported by the great names of Knight and Davy, whose authority ought to command faith in any opinion on this subject. But they only found that the heat was an accessory, not a prime cause. The only effect of heat is expansion, and this expansion will produce motion.) An increase of heat will expand, and hence set any liquid in motion. But this takes place only in changes of temperature; and Mr. Knight found that the sap flowed faster in the spring and autumn, when the weather was variable. But what moves the sap in the summer, or in the hot-house, where the temperature remains the same? Beside, we find that

If we heat ourand so also mowhen the winds

this same agent increases our circulation. selves suddenly, our heart beats quickly tion, which some think propels the sap act upon the plants. When we run, the pulse rises; and if we do this in summer, both heat and motion combine, and increase the accidental effect on the circulatory system.