nute branches, which traverse every part of the lungs. The black blood, therefore, carried to the lungs by the pulmonary artery, is divided into an infinite number of minute streamlets, which are conducted, in every direction, through the lungs, by the innumerable hair-like branches of the pulmonary artery.

The lungs are made up of a countless number of small cells, through among which the little streamlets of black blood are, of course, conveyed, and every time we draw in our breath, these cells become filled with air, and the air which they then contain comes in contact with the little vessels containing the minute streamlets of black blood, and, acting through the delicate coats of these, it operates those changes in the blood which it was sent to the lungs for the sole purpose of undergoing.

What the whole of these changes are is not thoroughly understood. But this much is certain-that, whereas the blood enters the lungs of a black colour, and in a condition unfit to effect the nutrition of the body, it no sooner becomes exposed to the influence of the air in the cells of the lungs, than it loses its black colour, acquires the brilliant hue of vermilion, and becomes at once endowed with all the properties necessary to the nutrition of the body, and to the production or secretion of the several juices, such as the gastric, the pancreatic, &c.

The black blood, then, having been exposed in the air-cells of the lungs, to the action of the air, and having been by it purified, re-impregnated with nutritious particles, and every way requalified to fulfil its appointed offices in the body, is collected into four veins, called the pulmonary veins, by which it is brought back to the left side of the heart, from which it first started. And thus the lesser circle of circulation has been accomplished, and the whole circulation of the blood completed.

Allow me to recur, for a moment, to the metaphor of the bees. I like it—it is a little fanciful, perhaps, but nevertheless appropriate, and not inelegant.

Consider the lungs, then, as a bed of sweet flowers upon which a swarm of bees (the little black streamlets of blood) have settled. These bees, having laden their thighs with honey, quit the flowers, and, taking their flight through the garden gate, (the heart,) pursue their way by various routes, (the arteries,) in order to deposit their little burdens, and distribute them equally throughout the honey-comb, that is, the body. Having done this, they take wing once more, and returning in the same direction, but by different routes, (the veins,) they re-enter the garden-gate, (the heart,) and again settle themselves down upon the flower-bed, (the lungs,) in order to collect a fresh supply of honey, that is, of nutritive properties. Observe-a stream of arterial blood is a bee laden with honey; a stream of venous blood a bee despoiled of its honey.

Now if, as I hope, you have understood my former letters, you will recollect that the old body is constantly being dissolved and carried away and emptied into the venous blood by the lymphatic absorbents, and that the new materials afforded by new food are also emptied into the venous blood at the same place, (viz. just before it enters the right side of the heart on its way to the lungs,) by the lacteal ab

sorbents. The black blood, therefore, when it reaches the lungs, has, mixed up with it, a portion of the old body in a fluid state called lymph, and also a certain quantity of fresh nutriment, also in a fluid state, called chyle. But the fresh nutriment, that is, the chyle, has not yet become blood; it is merely mechanically commingled with the blood. The effect, therefore, which the air exerts on the blood in the lungs, is not merely to revivify old blood, but likewise to convert the chyle into blood. This conversion of chyle into blood is called sanguification.

There is another important office fulfilled by respiration-viz. the expulsion from the body of such portions of the lymph as are no longer fit to remain in it, in the shape of that watery vapour which we denominate breath. The mouth, therefore, is a portal through which you receive the materials for a new body, and also through which you blow away the worn-out materials of the old. Every time you breathe, you blow away a little bit of your nose, (would it were a larger piece!) a little bit of your ears, a fragment of your eyes, a particle of your brain, an atom of your heart, in short, a part of your whole person. If you chance to be walking in the fields, a portion mounting through the air assists in forming the clouds, and again descending in showers of rain, contributes its share towards the formation of the multitudinous ocean. Another portion falls upon the green herbage of the meadow, and constitutes a part of the nourishment upon which that herbage subsists. Thus, not only is "flesh grass," but grass also

is flesh.

I shall now endeavour to exhibit the principal actions concerned in the nutrition of the body, by tracing a portion of food through all the necessary changes, until it has ceased to be food, and has become an integrant part of yourself.

Let us suppose you to be in the act of despatching a hearty meal, consisting of animal food and various kinds of vegetables. You first introduce it into your mouth-(" first catch your hare," says Mrs. Glass) with your teeth you masticate it--by means of your tongue you roll it about your mouth. This rolling about brings it in contact with the several excretory ducts of the salivary glands, which open on the internal surface of the mouth, as we have before seen. These ducts, by virtue of their SENSIBILITY, become aware of the presence of a stimulus (the food). The stimulation which the food in the mouth exerts upon the ducts, is propagated along them to the arteries which, as we have before seen, form the salivary glands, by being coiled up into those little knots so called. The arteries, thus stimulated, are excited to increased action. They bring a greater quantity of blood to the glands, and those parts of the arteries which constitute the glands, being also excited to greater action and having an increased supply of blood, separate from that blood an increased quantity of that juice or secretion, which is called saliva. And this increased secretion of saliva is the first of that series of actions by which the nutrition of the body is effected; and in this, the very first stage, you see exemplified those three important properties of which I have said so much in my two last letters-STIMULATION, SENSIBILITY, and CONTRACTILITY. For it is by virtue of

their contractility that the arteries supplying the salivary glands with blood, and also constituting the gland itself, are capable of acting, that is, of contracting, and so of supplying the gland with blood, from which blood the saliva has to be secreted. It is by virtue of the stimulating property of the food that their contractility is roused into action; and it is by virtue of their sensibility that they are aware that a stimulus is acting upon them.

The nutritious bolus, then, having been thoroughly masticated and rolled about the mouth until it has been well mixed up with saliva, is, by a very complicated movement, mounted upon the back of the tongue, and by it jerked into the throat, by which it is propelled downward into the stomach. Its presence in the stomach stimulates that organ as it stimulated the glands of the mouth, and a copious secretion of gastric juice (that is, stomach-juice) is brought about in the same way as a copious secretion of saliva was effected by its presence in the mouth. But as there are neither teeth nor tongue in the stomach, the food, when there, cannot be so readily and at once mixed up and kneaded, as it were, with the gastric juice, as it was, by means of those instruments, comminuted and commingled with the saliva in the mouth. It is not, therefore, subjected all at once to the action of the gastric juice, but gradually, layer after layer. While the food is lying, in mass, in the upper part of the stomach, the gastric juice poured out from the sides of the stomach, above and around it, falls upon the surface of the nutritious mass. When this upper surface or layer has been sufficiently acted on by the gastric juicewhen, by virtue of the inherent properties of this juice, it has been converted into a semifluid peculiar to itself, and called chyme, it floats off and away from the rest towards the lower part of the stomach, where it is united to the upper extremity of the bowels. The upper layer of the alimentary mass having been thus converted into chyme by the action of the gastric juice, and sent away from the remainder, the next layer becomes exposed to the action of this juice, and having, like the first, become converted into chyme, swims away after it to the pylorus, that is, the lower extremity of the stomach. Thus, layer after layer, the whole mass eventually becomes changed from the nature of food into chyme; gastric juice, during the whole time this change is going on, being poured out from the internal surface of the stomach upon the surface of the alimentary mass. The whole quantity is usually converted into chyme in about three hours.

Now mark. Whatever has been the nature and kind of the food which you have eaten-however heterogeneous the several viands may be--they must all be reduced to this unique homogeneous semifluid, called chyme-they must all lose their own several natures and take upon themselves the one sole nature of chyme, and so become chyme itself before they can leave the stomach and enter the bowels, in order there to undergo the next necessary change.

Now if you have eaten any matters at your meal which are what is called difficult of digestion, that is, which are not easily reduced to chyme by the action of the gastric juice, when these matters become exposed to the action of the gastric juice they will necessarily require to be so exposed for a longer time than is natural because of the diffi

culty which the gastric juice experiences in reducing them to chyme. It will be therefore a longer time before these float off from the surface of the alimentary mass, so as to leave the next layer exposed to the action of the gastric juice; and the under layers, or portions of food, which are waiting for their turn to be exposed, will be kept so waiting for a longer than the wonted space of time. The consequence of this is, that they are kept waiting, untouched by the gastric juice, until they begin to undergo those changes common to all vegetable and animal matter when placed in a warm, moist, and confined situation, viz. fermentation; the vegetable matter undergoing the acid fermentation and the animal the putrefactive. For it must be remembered, that the food in the stomach still continues to be food, still remains unaltered, still continues, therefore, to be obedient to the common laws of fermentation and putrefaction, until it has its nature and identity destroyed, and a new nature and identity bestowed upon it by virtue of the action which the gastric juice exerts upon it. It ferments and putrefies, therefore, in the stomach (if not acted upon by the gastric juice) as quickly as it would do, on a sultry summer's day, in a small pantry, with its windows and door kept shut. And this shows you the reason why such matters as undergo putrefaction with the greatest rapidity, as some fish and fresh pork, do not well agree with weak stomachs; for that which putrefies most rapidly in the pantry will do so in the stomach also.

While, therefore, the indigestible matters are slowly submitting to the action of the gastric juice, the good and wholesome portion of the food is actually putrefying, and can, therefore, afford no more nutriment than if you had dined on putrid carrion. During the process of fermentation and putrefaction, moreover, as all the world knows, a number of fetid gases are given out. These poisonous gases distend the stomach, weaken its energies, oppress its sensibility, enfeeble its contractility, diminish the secretion of gastric juice, and, in a word, disturb, interrupt, and wholly overturn the whole process of assimilation in the stomach, and there is tumbled into the bowels, instead of a bland, smooth, homogeneous, healthy chyme, a filthy, fermenting, yeasty mess, smoking with offensive gases, and consisting of little else than sour vegetables and putrid meat. For the sensibility of the pyloric valve, of which I am to speak directly, is overcome by the oppressive influence and expansive nature of the gases which are distending the stomach. Is it possible, I ask, that healthy chyle and sound blood can be formed out of such a villainous compound of nastiness as this?

As soon as this precious hodge-podge reaches the bowels, it will generally be expelled by them with violence; and this is the way in which bowel complaints are so often produced by salmon and fresh pork, when eaten by persons whose stomachs are too weak to furnish a sufficient quantity of gastric juice to reduce them to chyme before they have had time to run into putrefaction; and the wind, which such persons discharge by the mouth after eating, consists of the offensive gases above-mentioned. Strong, healthy stomachs, pour out their gastric juice so rapidly and abundantly, that the whole meal is reduced to chyme before the process of putrefaction has had time to begin. Now let us proceed.

The food, having been properly acted upon by the gastric juice of the stomach, is now no longer food, but a bland, smooth, homogeneous semi-fluid called chyme, which, quitting the upper part of the stomach, flows downward to the lower extremity-that part where the stomach is joined to the bowels. This junction of the lower extremity of the stomach with the upper extremity of the bowels is called the pylorus; and the pylorus is furnished with a peculiar valve which accurately closes the communication between the stomach and bowels at all times, excepting when chyme is in the act of passing out of the stomach into the bowels. This valve is endowed with a singular and most beautiful kind of eclectic sensibility, which enables it to know, by the feel, whether the matters which come in contact with it be pure chyme or not; and nothing can enter the bowels from the stomach without coming in contact with it.

Now let us suppose that a portion of food has been reduced to chyme, has flowed down to the lower extremity of the stomach, and has presented itself at the pyloric valve for admission through it into the bowels; and let us suppose that there is, swimming in the chyme, a particle of food which has not yet been sufficiently acted upon by the gastric juice. I will tell you what happens. As soon as the pyloric valve feels the presence of the smooth and bland chyme it instantly opens and allows it to pass through; but no sooner does the particle of food, which has not yet been reduced to chyme, attempt to pass, than the valve instantly closes the aperture, and refuses to suffer it to go through. And this particle of food must return to the upper part of the stomach, and be again submitted to the agency of the gastric juice before it can be permitted to escape from the stomach into the bowels. Is not this a beautiful exemplification of the importance of the sensibility of our organs?-and said I not true, when I called it our guardian angel? For what is the sensibility of the pyloric valve, by which it is enabled to distinguish between perfect and imperfect chyme?-what is it, I say, but a watchman, a sentinel, posted at the entrance into the bowels, in order to watch over their safety, to see that nothing be allowed to enter them which is likely to disturb or irritate them; to take care that nothing injurious, nothing offensive, nothing which may be in any way hostile to their safety, nothing, in fact, which has no business there, be permitted to trespass within the sacred precincts of organs so important to the health and welfare of the whole being, of which they form so vital a part?

That imperfectly chymified food cannot enter the bowels without injury to them is sufficiently proved by the very existence of this valve. For surely it is foolish to suppose that nature, who does nothing in vain, would have been at the pains of establishing so beautiful, so wonderful, so inestimable a contrivance, if the office which it fulfils were not, in the last degree, essential.

What mischief, therefore, do those persons inflict upon themselves -what a wide door for the admission of all sorts of evils do those persons throw open, who, perpetually stimulating the pyloric valve by the unnatural stimuli of ardent spirit, and highly-seasoned sauces, enfeeble, wear out, and eventually destroy its sensibility; so that whatever the caprice of the palate throws into the stomach, is tumbled, right or wrong, assimilated or unassimilated, good, bad, and indif