speedily to bring the mixture to the heart. And this seems to be a circumstance of great moment; for had the chyle entered the blood at an artery, or at a distant vein, the fluid, composed of the old and the new materials, must have performed a considerable part of the circulation, before it received that churning in the lungs, which is, probably, necessary for the intimate and perfect union of the old blood with the recent chyle. Who could have dreamt of a communication between the cavity of the intestines and the left great vein of the neck? Who could have suspected that this communication should be the medium through which all nourishment is derived to the body; or this the place, where, by a side inlet, the important junction is formed between the blood and the material which feeds it?

We postponed the consideration of digestion, lest it should interrupt us in tracing the course of the food to the blood; but, in treating of the alimentary system, so principal a part of the process cannot be omitted.

Of the gastric juice, the immediate agent by which that change which food undergoes in our stomachs is effected, we shall take our account from the numerous, careful, and varied experiments of the Abbé Spallanzani*.

1. It is not a simple diluent, but a real solvent. A quarter of an ounce of beef had scarcely touched the stomach of a crow, when the solution began.

2. It has not the nature of saliva; it has not the nature of bile; but is distinct from both. By experiments out of the body it appears, that neither of these secretions acts upon alimentary substances, in the same manner as the gastric juice acts.

3. Digestion is not putrefaction: for the digesting fluid resists putrefaction most pertinaciously; nay, not only checks its farther progress, but restores putrid substances.

4. It is not a fermentative process: for the solution begins at the surface, and proceeds towards the centre, contrary to the order in which fermentation acts and spreads.

5. It is not the digestion of heat for the cold maw of a cod or sturgeon will dissolve the shells of crabs or lobsters, harder than the sides of the stomach which contains them.

In a word, animal digestion carries about it the marks of being a power and a process completely sui generis: is distinct from every other; at least from every chymical process with which we are acquainted. And the most wonderful thing about it is its appropriation; its subserviency to the particular economy of each animal. The gastric juice of an owl, falcon, or kite, will not touch grain; no, not even to finish the macerated and half-digested pulse which is left in the crops of the sparrows that the bird devours. In poultry, the trituration of the gizzard, and the gastric juice, conspire in the work of digestion. The gastric juice will not dissolve the grain whilst it is whole. Entire grains of barley, enclosed in tubes or spherules, are not affected by it. But if the same grain be by any means broken or ground, the gastric juice immediately lays hold of it. Here then is wanted, and here we find, a combination of mechanism and chymistry. For the preparatory grinding, the gizzard lends its mill. And as all millwork should be strong, its structure is so, beyond that of any other muscle belonging to the animal. The internal coat also, or lining of the gizzard, is, for the same purpose, hard and cartilaginous. But, forasmuch as this is not the sort of animal substance suited for the reception of glands, or for secretion, the gastric juice, in this family, is not supplied, as in membranous stomachs, by the stomach itself, but by the gullet, in which the feeding glands are placed, and from which it trickles down into the stomach.

Lazarus Spallanzani, whom Paley so often quotes, long devoted himself to the study of animal chymistry; but his success in the research, although very considerable, was by no means equal to the talents he possessed; he had, moreover, the fate shared by all those who have experimented upon living animals, to wade through many very disagreeable examinations, to perpetrate many cruelties, and some of them such as must make the most callous shudder.

The history of Spallanzani is not destitute of romance. He was originally intended for the law, for which he studied at Modena; but on his arrival at Bologna, the talents of his cousin, the celebrated Laura Bassani give him a distaste for his profession. He attended her public

lectures, and acquired a taste for experimental philosophy. He, in consequence, abandoned the law, and was made professor of Greek, at Reggio, afterwards filling the chair of natural philosophy at Modena, and finally at Padua.

He was one of Paley's greatest chemical contemporaries, for he was born in 1729, and died in 1798, only seven years before our author. His character was that of many men of genius: with great mental powers he had an insatiable thirst for knowledge, dabbled in many sciences, had a restlessness and curiosity which carried him over most parts of Europe and even Asia, visited most men of science, and was a member of many distinguished philosophical societies.-Thomson.-Dict. Hist.

In sheep, the gastric fluid has no effect in digesting plants, unless they have been previously masticated. It only produces a slight maceration; and nearly such as common water would produce, in a degree of heat somewhat exceeding the medium temperature of the atmosphere. But provided that the plant has been reduced to pieces by chewing, the gastric juice then proceeds with it, first, by softening its substance: next, by destroying its natural consistency; and lastly, by dissolving it so completely, as not even to spare the toughest and most stringy parts, such as the nerves of the leaves.

So far our accurate and indefatigable Abbé.-Dr. Stevens, of Edinburgh, in 1777, found, by experiments tried with perforated balls, that the gastric juice of the sheep and the ox speedily dissolved vegetables, but made no impression upon beef, mutton, and other animal bodies. Dr. Hunter discovered a property of this fluid, of a most curious kind; viz. that in the stomachs of animals which feed upon flesh, irresistibly as this fluid acts upon animal substances, it is only upon the dead substance that it operates at all. The living fibre suffers no injury from lying in contact with it. Worms and insects are found alive in the stomachs of such animals. The coats of the human stomach, in a healthy state, are insensible to its presence; yet in cases of sudden death (wherein the gastric juice, not having been weakened by disease, retains its activity), it has been known to eat a hole through the bowel which contains it. How nice is this discrimination of action, yet how necessary!+

But to return to our hydraulics.

III. The gall-bladder is a very remarkable contrivance. It is the reservoir of a canal. It does not form the channel itself, i. e. the direct communication between the liver and the intestine, which is by another passage, viz. the ductus hepaticus, continued under the name of the ductus communis; but it lies adjacent to this channel, joining it by a duct of its own, the ductus cysticus; by which structure it is enabled, as occasion may require, to add its contents to, and increase, the flow of bile into the duodenum. And the position of the gall-bladder is such as to apply this structure to the best advantage. In its natural situation, it touches the exterior surface of of the stomach, and consequently is compressed by the distention of that vessel: the effect of which compression is to force out from the bag, and send into the duodenum, an extraordinary quantity of bile, to meet the extraordinary demand which the repletion of the stomach by food is about to occasion. Cheselden describes the gall-bladder as seated against the duodenum, and thereby liable to have its fluid pressed out, by the passage of the aliment through that cavity; which likewise will have the effect of causing it to be received into the intestine, at a right time, and in a due proportion.

There may be other purposes answered by this contrivance; and it is probable that there are. The contents of the gall-bladder are not exactly of the same kind as what passes from the liver through the direct passage. It is possible that the gall may be changed, and for some purposes meliorated, by keeping.

The entrance of the gall-duct into the duodenum furnishes another observation.

When

ever either smaller tubes are inserted into larger tubes, or tubes into vessels and cavities, such

* Phil. Trans. vol. lxii. p. 447.

+ So corrosive is the action of the gastric juice upon the dead coats of the stomach, that there have been several cases in which innocent persons have been accused of poisoning those who have died suddenly, whose stomachs were found corroded.

The importance of this mystic fluid to all animals, has rendered the most eminent of chemists anxious to investigate its properties. Vauquelin, Sennebier, Hunter, Stevens, Brugnatelli, and others have, however, in vain laboured for that purpose; and our knowledge of its chemical composition is not now much more accurate than in the days of Paley and the Abbé Spalanzani. The result of their labours does not afford any "explauation," to use the words of Dr. Henry, "of the solvent power which it exerts on all animal and vegetable substances. Even out of the body it appears to retard the putrefaction of animal substances, and to reduce them to a state somewhat similar to that in which they are found after having been for some time in the stomach."

The appearance of this fluid would excite little suspicion of its powers. That of carnivorous animals, according to Brugnatelli, has "an acid and resinous odour, is very bitter, but is not watery "-contains an acid, a resin, an animal substance, and a small quantity of common salt. That of herbivorous animals, on the other hand, is of a watery nature: its taste is bitter and saltish, contains ammonia, an animal extract, and a considerable proportion of common salt. But all these ingredients of the gastric juice afford no explanation to the chemist of the powers possessed by this almost universal solvent -he would have rather expected to have found in its composition powerful alkalies and acids, something more like the re-agents he is wont to employ in his laboratory, than a bitter solution resembling gum-water in appearance. -Elements of Exp. Chem. vol. ii. p. 330; Thomson's System, vol. iv. 8 594; Hunter, Phil. Trans. vol. 62; Baillie's Morbid Anatomy, p. 75.

Keill's Anat. p. 64.

Keill (from Malpighius), p. 63.

G

Anat. P. 164.

receiving tubes, vessels, or cavities, being subject to muscular constriction, we always find a contrivance to prevent regurgitation. In some cases, valves are used; in other cases, amongst which is that now before us, a different expedient is resorted to, which may be thus described: The gall-duct enters the duodenum obliquely; after it has pierced the first coat, it runs near two fingers' breadth between the coats before it opens into the cavity of the intestine*. The same contrivance is used in another part, where there is exactly the same occasion for it, viz. in the insertion of the ureters in the bladder. These enter the bladder near its neck, running obliquely for the space of an inch between its coats †. It is, in both cases, sufficiently evident, that this structure has a necessary mechanical tendency to resist regurgitation; for whatever force acts in such a direction as to urge the fluid back into the orifices of the tubes, must, at the same time, stretch the coats of the vessels, and thereby compress that part of the tube which is included between them.

IV. Amongst the vessels of the human body, the pipe which conveys the saliva from the place where it is made, to the place where it is wanted, deserves to be reckoned amongst the most intelligible pieces of mechanism with which we are acquainted. The saliva, we all know, is used in the mouth; but much of it is manufactured on the outside of the cheek, by the parotid gland, which lies between the ear and the angle of the lower jaw. In order to carry the secreted juice to its destination, there is laid from the gland on the outside, a pipe, about the thickness of a wheat straw, and about three fingers' breadth in length; which, after riding over the masseter muscle, bores for itself a hole through the very middle of the cheek; enters by that hole, which is a complete perforation of the buccinator muscle, into the mouth; and there discharges its fluid very copiously.

The

V. Another exquisite structure, differing indeed from the four preceding instances, in that it does not relate to the conveyance of fluids, but still belonging, like these, to the class of pipes or conduits of the body, is seen in the larynx. We all know that there go down the throat two pipes, one leading to the stomach, the other to the lungs; the one being the passage for the food, the other for the breath and voice: we know also that both these passages open into the bottom of the mouth; the gullet, necessarily, for the conveyance of food; and the wind-pipe, for speech and the modulation of sound, not much less so: therefore the difficulty was, the passages being so contiguous, to prevent the food, especially the liquids, which we swallow into the stomach, from entering the wind-pipe, i. e. the road to the lungs; the consequence of which error, when it does happen, is perceived by the convulsive throes that are instantly produced. This business, which is very nice, is managed in this manner. gullet (the passage for food) opens into the mouth like the cone or upper part of a funnel, the capacity of which forms indeed the bottom of the mouth. Into the side of this funnel, at the part which lies the lowest, enters the wind-pipe, by a chink or slit, with a lid or flap, like a little tongue, accurately fitted to the orifice. The solids or liquids which we swallow, pass over this lid or flap, as they descend by the funnel into the gullet. Both the weight of the food, and the action of the muscles concerned in swallowing, contribute to keep the lid close down upon the aperture, whilst anything is passing; whereas, by means of its natural cartilaginous spring, it raises itself a little, as soon as the food is passed, thereby allowing a free inlet and outlet for the respiration of air by the lungs. Such is its structure: And we may here remark the almost complete success of the expedient, viz. how seldom it fails of its purpose, compared with the number of instances in which it fulfils it. Reflect how frequently we swallow, how constantly we breathe. In a city-feast, for example, what deglutition, what anhelation! yet does this little cartilage, the epiglottis, so effectually interpose its office, so securely guard the entrance of the wind-pipe, that whilst morsel after morsel, draught after draught, are coursing one another over it, an accident of a crumb or a drop slipping into this passage (which nevertheless must be opened for the breath every second of time), excites in the whole company, not only alarm by its danger, but surprise by its novelty. Not two guests are choked in a century.

There is no room for pretending that the action of the parts may have gradually formed the epiglottis : I do not mean in the same individual, but in a succession of generations. Not

enly the action of the parts has no such tendency, but the animal could not live, nor consequently the parts act, either without it, or with it in a half-formed state. The species was not to wait for the gradual formation or expansion of a part which was, from the first, necessary to the life of the individual.

Not only is the larynx curious, but the whole wind-pipe possesses a structure adapted to its peculiar office. It is made up (as any one may perceive by putting his fingers to his throat) of stout cartilaginous ringlets, placed at small and equal distances from one another. Now this is not the case with any other of the numerous conduits of the body. The use of these cartilages is to keep the passage for the air constantly open; which they do mechanically. A pipe with soft membranous coats, liable to collapse and close when empty, would not have answered here; although this be the general vascular structure, and a structure which serves very well for those tubes which are kept in a state of perpetual distention by the fluid they enclose, or which afford a passage to solid and protruding substances.

Nevertheless (which is another particularity well worthy of notice), these rings are not complete, that is, are not cartilaginous and stiff all round; but their hinder part, which is contiguous to the gullet, is membranous and soft, easily yielding to the distentions of that organ occasioned by the descent of solid food. The same rings are also bevelled off at the upper and lower edges, the better to close upon one another, when the trachea is compressed or shortened.

The constitution of the trachea may suggest likewise another reflection. The membrane which lines its inside, is, perhaps, the most sensible, irritable membrane of the body. It rejects the touch of a crumb of bread, or a drop of water, with a spasm which convulses the whole frame; yet, left to itself, and its proper office, the intromission of air alone, nothing can be so quiet. It does not even make itself felt; a man does not know that he has a trachea. This capacity of perceiving with such acuteness, this impatience of offence, yet perfect rest and ease when let alone, are properties, one would have thought, not likely to reside in the same subject. It is to the junction, however, of these almost inconsistent qualities, in this, as well as in some other delicate parts of the body, that we owe our safety and our comfort;—our safety to their sensibility, our comfort to their repose.

The larynx, or rather the whole wind-pipe taken together (for the larynx is only the upper part of the wind-pipe), besides its other uses, is also a musical instrument, that is to say, it is mechanism expressly adapted to the modulation of sound; for it has been found upon trial, that, by relaxing or tightening the tendinous bands at the extremity of the wind-pipe, and blowing in at the other end, all the cries and notes might be produced of which the living animal was capable. It can be sounded, just as a pipe or flute is sounded.

Birds, says Bonnet, have, at the lower end of the wind-pipe, a conformation like the reed of a hautboy, for the modulation of their notes. A tuneful bird is a ventriloquist. The seat of the song is in the breast.

The use of the lungs in the system has been said to be obscure; one use, however, is plain, though, in some sense, external to the system, and that is, the formation, in conjunction with the larynx, of voice and speech. They are, to animal utterance, what the bellows are to the organ*.

The apparatus, the various organs, employed for the formation of the human voice, afford a most interesting subject for investigation. The trachea, or wind-pipe; the larynx, at its summit; the glottis, a small oval cleft, with its appendages; the epiglottis, or valve, and the various regulating ligaments, are all subservient to the production of that modulation of the human voice which is the chief charm of life, and bond of social intercourse. The sound, which may be called the basis of the voice, is formed by the vibrations of the glottis, as the air is forced through it from the lungs. The sound thus formed passes into the cavity of the mouth and nostrils, where, by its reflections and resoundings, the modulations are formed, which constitute our articulate voice: and on these reflections the agreeableness of the voice entirely depends. If the Great Artificer of this apparatus had not designed our happiness,

had not aimed at our gratification, why should there be this intricacy to secure a pleasing modulation of the voice? -a melody surpassing in sweetness and effect any that can be produced from an instrument by the most skilful musician. Yet how little is required to make the voice dissonant, and displeasing-close but the nostrils; let the tongue be disproportioned; let the palate be defective, and all the harmony is gone. But how rarely is the voice unpleasing; so rare is it, that the occurrence is remarkable as a seldom-occurring exception. At the same time, to secure this agreeableness of sound, movements the most delicate are required to be faultless. M. Dodart has shewn that to perform all the tones and semi-tones of a common voice, the minute diameter of the glottis, which does not exceed th of an inch, must be capable of 9632 different degrees of closure, and many of them being

For the sake of method, we have considered animal bodies under three divisions; their bones, their muscles, and their vessels: and we have stated our observations upon these parts separately. But this is to diminish the strength of the argument. The wisdom of the Creator is seen, not in their separate but their collective action; in their mutual subserviency and dependance; in their contributing together to one effect and one use. It has been said, that a man cannot lift his hand to his head, without finding enough to convince him of the existence of a God. And it is well said; for he has only to reflect, familiar as this action is, and simple as it seems to be, how many things are requisite for the performing of it: how many things which we understand, to say nothing of many more, probably, which we do not; viz. first, a long, hard, strong cylinder, in order to give to the arm its firmness and tension; but which, being rigid, and in its substance inflexible, can only turn upon joints: secondly, therefore, joints for this purpose; one at the shoulder to raise the arm, another at the elbow to bend it; these joints continually fed with a soft mucilage to make the parts slip easily upon one another, and held together by strong braces, to keep them in their position: then, thirdly, strings and wires, i. e. muscles and tendons, artificially inserted for the purpose of drawing the bones in the directions in which the joints allow them to move. Hitherto we seem to understand the mechanism pretty well; and, understanding this, we possess enough for our conclusion: nevertheless, we have hitherto only a machine standing still; a dead organization,—an apparatus. To put the system in a state of activity; to set it at work; a farther provision is necessary, viz. a communication with the brain by means of nerves. We know the existence of this communication, because we can see the communicating threads, and can trace them to the brain its necessity we also know, because if the thread be cut, if the communication be intercepted, the muscle becomes paralytic: but beyond this, we know little; the organization being too minute and subtile for our inspection.

To what has been enumerated, as officiating in the single act of a man's raising his hand to his head, must be added likewise, all that is necessary, and all that contributes to the growth, nourishment, and sustentation, of the limb, the repair of its waste, the preservation of its health such as the circulation of the blood through every part of it; its lymphatics, exhalants, absorbents; its excretions and integuments. All these share in the result; join in the effect and how all these, or any of them, come together without a designing, disposing intelligence, it is impossible to conceive.

CHAPTER XI.

OF THE ANIMAL STRUCTURE REGARDED AS A MASS.

It

CONTEMPLATING an animal body in its collective capacity, we cannot forget to notice, what a number of instruments are brought together, and often within how small a compass. is a cluster of contrivances. In a canary bird, for instance, and in the single ounce of matter which composes its body (but which seems to be all employed), we have instruments for eating, for digesting, for nourishment, for breathing, for generation, for running, for flying, for seeing, for hearing, for smelling; each appropriate, each entirely different from all the

rest.

The human, or indeed the animal frame, considered as a mass or assemblage, exhibits in

very unequal, sometimes the open fissure cannot exceed the 200th part of an inch.

Speech, an articulate voice, is one cause of man's rapid advance in civilisation, as it is also a token of his high prerogative over other created beings. It is not owing, says Mr. Lawrence, as some have imagined, to any defect

in their organs, that animals are denied the faculty of speech. Several animals may be taught to pronounce words, and even repeat sentences; but to make them conceive the ideas which these words express, is beyond the power of art: they articulate and repeat like an echo or machine.