repairing and augmenting them."* Yet, when we come to examine into the subject more closely, we find that all these terms, so expressive of a specific being and distinct reality-this ESSENCE that VIVIFIES and ANIMATES, has neither being, nor essence, nor vivification, nor animation, nor reality of any kind; that the whole of these expressions are metaphysical; and that the word VITAL PRINCIPLE is not designed to express a distinct being, but is merely an abridged formula, denoting the TOTALITY OF POWERS ALONE which animate living bodies, and distinguish them from inert matter, the TOTALITY OF PROPERTIES and LAWS which govern the animal economy.† So that we have here not only the employment of terms that have no meaning, but properties and laws, powers and principles, without any source,-a superstructure without a foundation,-effects without a cause.

But what is this curious and delicate instrument itself?-this machine that so nicely responds to the impressions communicated to it, and visibly envelopes so invisible a constituent?

It is not my intention in this series of popular study to enter into any minute history of the animal frame, but shall confine myself to those general views of it which are requisite to show by what means it is operated upon by the delicate powers we have just contemplated, and the more curious phenomena which result from such an impulse.

The animal frame, then, is a combination of living solids and fluids, duly harmonized, and equally contributory to each other's perfection. The principle of life, whatever it consists of, exists equally in both; in some kinds in a greater, in others in a less degree. In the fluids, Mr. Hunter has traced it down to their first and lowest stage of existence, for he has traced it in the chyle; and there are evident proofs of its accompanying several of those which are eliminated from the body; in the blood it is found, as we have already had occasion to notice, in a high degree of activity, and probably in a still higher in the nervous fluid.

In the solids it varies equally. There are some in which it can scarcely be traced at all, excepting from their increasing growth, as the cellular membrane, and the bones; in others, we find a perpetual internal activity, or susceptibility to external impressions. But it is in those irritable threads or fibres which constitute the general substance of the muscles or flesh of an animal, that the principle of life exerts itself in its most extraordinary manner, and which it more immediately, therefore, falls within the scope of the present lecture to investigate.

The muscle of an animal is a bundle of these irritable fibres, or soft, red, cylindrical, and nearly inelastic threads, formed out of a substance which the chemists, from the use to which it is applied, denominate fibrine; and which, when examined microscopically, are seen to divide and subdivide, as far as the power of glasses will carry the eye, into minuter bundles of fibrils, or still smaller threads, parallel to each other, and bound together by a delicate cellular web-work, obviously of a different nature. They are uniformly accompanied through their course by a number of very minute nerves, which are chords or tubes that originate from the brain, and branch out in every direction, either immediately from the brain itself, or from some part of the spinal marrow, which is a continuation of this organ; by which means a perpetual communication is kept up between the sensorium and the remotest part of the body, as we shall have farther occasion to notice hereafter. Upon the

* "Personne aujourd'hui ne conteste l'EXISTENCE D'UN PRINCIPE DE VIE qui soumet les êtres qui en jouissent à un ordre de lois differentes de celles auxquelles obeissent les êtres inanimes, force à laquelle on pourroit assigner, comme principaux caractères, de soustraire les corps qu'ELLE ANIME, à l'empire absolu des affinités chimiques, auxquelles ils auroient tant de tendance à ceder, en virtu de la multiplicité de leurs élémens; et de maintenir leur température à un dégré presque égal, quelle que soit d'ailleurs celle de l'atmosphère, Son ESSENCE n'est point de conserver l'aggregation des molécules constitutives, mais d'attirer d'autres molécules qui, s'assimilant aux organes qu'ELLE VIVIFIE, remplacent celle qu'entrainent les pertes journalières, et sont employées à les nourrir et à les accroître."-Nouveaux Eléméns de Physiologie, tom. i. p. 81. Paris, 8vo. 1804.

"Le mot de PRINCIPE VITAL, force vitale, &c. n'exprime point un être existant par lui-même, et indépendamment des actions par lesquelles il se manifeste: il ne faut l'employer que comme une formule abrégée dont on se sert pour désigner l'ensemble des forces qui animent les corps vivans et les distinguent de la matière inerte:-l'ensemble des propriétés et des loix qui régissent l'économie animale."-Ib. p. 80. » Series 1, Lecture xv.

On the Blood, p. 91.

application of any irritating or stimulating power, these fibres immediately contract in their length, and upon the cessation of such power return to their former state of relaxation: and it is chiefly by this curious contrivance that the animal system is enabled to fulfil all its functions. The stimuli by which the fibres, whether of motion or of sensation, are roused into action, are perhaps innumerable in the whole; but a few general classes may easily be devised to comprise all those by which they are ordinarily affected. And while by an admirable diversity of construction, some sets of fibres are only affected by some sets of stimuli, other sets are only affected by others; and in, this manner all the organs are compelled, as it were, to execute the different offices intrusted to thein, and no one interferes with that of another. Thus the fibres of the external senses are affected by external objects; they contract and give notice of the presence and degree of power of such objects to the brain, through the medium of the nerves, which, as I have just observed, always accompany them, and which either terminate in or arise from that organ: but while the irritative and sensitive fibres of the ear are excited only by the stimulus of sound, and have no impression produced upon them by that of light, those of the eye are excited only by the stimulus of light, and remain uninfluenced by that of sound: and so of the other organs of external sense. And hence we obtain a knowledge of one set or class of stimuli, which from their acting upon the organs of sense, are called sensitive stimuli, and the motions to which they give rise sensitive motions.

Again, the very substances naturally introduced into many of the muscular organs of the body, and especially the hollow muscles, are sufficient to excite them to a due performance of their functions: thus, the lungs are excited to the act of respiration by the stimulus of the air we breathe, the stomach to that of digestion by the stimulus of the food introduced into it; so the heart and blood-vessels are excited by the stimulus of the blood; and the vessels that carry off the recremental materials by the different stimuli which these materials contain in themselves. We hence obtain another class of stimuli, which are denominated stimuli of simple irritation; and the motions they produce, simple irritative motions, or motions of irritation.

But the sensory, or brain, which thus receives notice generally, or is impressed upon by the different actions that are perpetually taking place all over the system, through the medium of its own ramifications, or nerves, that uniformly acccompany the irritable fibres, in many instances originates motions, and thus proves a stimulus in itself. All voluntary motions are of this kind; the will, which is a faculty of the sensorium, being the exciting cause, and thus giving birth to a third class of stimuli, and of a very extensive range, which are called stimuli of volition. While habit or association becomes, in a variety of instances, a sufficient impulse to other motions, and thus constitutes a fourth class; which are hence named associate stimuli, or stimuli of association.

But though the muscular fibre is, perhaps, more irritable than any other part of the system, the principle of irritability and a fibrous structure are by no means necessarily connected; for, while the cellular membrane is fibrous but has no irritability whatever, the skin is not fibrous but is highly irritable. Hence solids and fluids are equally necessary to the perfection of the living system. Food, air, and the ethereal gases, caloric, oxygen, and the medium of electricity, are the stimuli by which it is chiefly excited to action; and, by their combination, contribute in some degree to the matter of the system itself; but of the mysterious power that developes the organs and applies the stimuli, that harmonizes the action and constitutes the life, we know nothing.

We see clearly, however, that the moving powers are, for the most part, the muscles; and it is a subject of perpetual astonishment to the physiologist to observe the prodigious force which these vital cords are made capable of exerting, and the infinite variety of purposes to which they thus become subservient. And were it not that the whole universe swarms with proofs of intelligence and design-were it not that there exists, to adopt the beautiful words of the poet—

Books in the running brooks,

Sermons in stones, and good in every thing

this, perhaps, might be the part of creation which we could best select in proof of the wisdom of the Creator.

It was formerly too much the custom to regard the animal frame as a mere mechanical machine; whence, in that spirit of absurdity with which the wisest of mankind are occasionally afflicted, Descartes affected to believe that brutes are as destitute of consciousness as a block of wood, and that it is exactly the same sort of necessity which drives a dog forward in pursuit of a hare, that compels the different pipes of an organ to give forth different tones upon a pressure of the fingers against its different keys. It is not every one, however, in modern times who has adopted the mechanical theory that has carried it to this extremity of absurdity; but all of them are still carrying it too far who reason concerning the principal motions of the body as mere mechanical motions, and the powers which the muscles exert as mere mechanical powers; in which the bones are the levers, the joints the fulcra, and the muscles the moving cords; for it so happens that all the effects for which the whole of this complicated machinery is absolutely necessary out of the body, are in many instances performed by a single part of it within the body, namely, by the moving cords or muscles alone, without either bones or joints, levers or fulcra. I do not mean to contend that there is no kind of resemblance or conformity of principle between the laws of animate and inanimate mechanics, for I well know that in a variety of points the two systems very closely concur; but I am obliged to contend that they are still two distinct systems, and that in the one case the living power exercises an influence which finds no sort of similitude in the other.

It is, indeed, curious to observe the difference of result which has flowed from the calculations of the different promoters of this theory; and which alone, were there nothing else to oppose them, would be sufficient to prove the fallacy of their reasoning. Among those who have adopted this mode of explanation, and have pursued it with most acuteness, and may be regarded as the fathers of the school, I may be allowed to mention Borelli and Keil; but while the former, in order to account for the circulation of the blood in man, calculated the force with which the heart contracts to be equal to not less than a hundred and eighty thousand pounds weight at every contraction, the latter could not estimate it at more than eight ounces.

In like manner Borelli, in applying the same theory to the power with which the human stomach triturates, or, as we now call it, digests its food, calculated it, in conjunction with the assistance it receives from the auxiliary muscles, which he conceived to divide the labour about equally with itself, as equal to two hundred and sixty-one thousand one hundred and eighty-six pounds; and Pitcairn has made it very little less, since he estimates the moiety contributed by the stomach alone at one hundred and seventeen thousand and eighty-eight pounds; which gives to these organs jointly a force more than equal to that of twenty mill-stones! "Had he," says Dr. Munro, "assigned five ounces as the weight of the stomach, he had been nearer the truth."*

The fallacy of this theory, however, and especially as it applies to the stomach, has been completely exposed in our own day, by the well-ascertained fact, that though the muscular coat of the stomach in most animals bears some part in the process of digestion, this important operation is almost entirely performed by a powerful chemical solvent secreted by the stomach itself for this very purpose, and hence denominated the gastric juice; and which answers all the purposes of the most violent muscular pressure we can conceive, and with a curious simplicity of contrivance.

The laws of physical force will certainly better apply to the action of the heart and arteries than to that of the stomach, and in some measure assist us

* Comp. Anat. prof. p. viii. note.

in accounting for the circulation of the blood; but the moment we reflect that one-half of this very circulation, that I mean which depends upon the veins, and which has for the most part to contend against the attraction of gravitation, instead of being able to avail itself of its assistance, is produced without any muscular propulsion that we are able to discover, and that even the arteries do not, when uninfluenced by pressure, appear to change their diameter in a state of health,* we are necessarily driven to the conclusion, that there is in animal statics, as well as in animal mechanics, a something distinct and independent, and which the laws of physical force are altogether incompetent to explain. Dr. Young, in his excellent Croonian lecture, read before the Royal Society in 1809,† has endeavoured to revive the mechanical theory; but he is still compelled to admit a variety of phenomena in the animal machine, and especially in the circulatory system, which are altogether unaccountable upon any of the known principles of common hydraulics, and which can never fail to reduce us to the same result.

So far, therefore, as we at present know, the circulation of the blood is performed by a double projectile power; one moiety being dependent on the action of the living principle in the heart, and perhaps the arteries; and the other moiety on the common law of hydraulics, or the vacuum produced in the heart by that very contraction or systole which has just propelled the blood returned from the lungs into the arterial system. Whence the heart itself becomes alternately a forcing and a suction pump; being the former in respect to the arteries, and the latter in respect to the veins.‡

Upon a moderate estimate, the common labourer may be said to employ a force capable of raising a weight of ten pounds to the height of ten feet in a second, and continued for ten hours a day. A moderate horizontal weight for a strong porter, walking at the rate of three miles an hour, is 200 pounds: the chairman walks four miles an hour, and carries 150 pounds. The daily work of a horse is equal to that of five or six men upon a plane; but from his horizontal figure in drawing up a steep ascent, it does not exceed the power of three or four men. In working windmills, twenty-five square feet of the sails is equivalent to the work of a single labourer; whence a full-sized. mill, provided it could be made to work eight hours a day, would be equivalent to the daily labour of thirty-four men. A steam engine of the best construction, with a thirty inch cylinder, has the force of forty horses; and as it acts without intermission, will perform the work of 120 horses, or of 600 men; every square inch of the piston being equivalent to the power of a labourer.

There are many muscles given to us which the common customs and habits of life seldom render it necessary to exert, and which in consequence grow stiff and immoveable. Tumblers and buffoons are well aware of this fact; and it is principally by a cultivation of these neglected muscles that they are able to assume those outrageous postures and grimaces, and exhibit those feats of agility, which so often amuse or surprise us.

The same muscles of different persons, however, though of the same length and thickness, and, so far as we are able to trace, composed of the same number of fibres, are by no means uniformly possessed of the same degree of power; and we here meet with an express deviation from the law of physical mechanics; as we do also in the curious fact, that whatever be the power they possess, they grow stronger in proportion to their being used, provided they are well used, and not exhausted by violence or over-exertion. I have calculated the average weight carried by a stout porter in this metropolis at 200 pounds; but we are told there are porters in Turkey, who by accustoming themselves to this kind of burden from an early period, are able to carry from 700 to 900 pounds, though they walk at a slower rate, and only carry the burden a short distance. "The weakest man can lift with his hands about 125 pounds, a strong man 400. Topham, a carpenter, men

*See Lect. vIII p. 91, as also the Author's Study of Medicine, vol. fi. p. 16. Edit. 2d, 1825. On the Functions of the Heart and Arteries, Phil. Trans. 1809, p. 1.

See Study of Med, vol. ii. p. 19. Ed. 2d.

tioned by Desaguliers, could lift 800 pounds. He rolled up a strong pewter dish with his fingers. He lifted with his teeth and knees a table six feet long, with a half hundred weight at the end. He bent a poker, three inches in circumference, to a right angle, by striking it upon his left forearm; another he bent and unbent about his neck, and snapped a hempen rope two inches in circumference. A few years ago there was a person at Oxford who could hold his arm extended for half a minute, with half a hundred weight hanging on his little finger."* We are also told by Desaguliers of a man who, by bending his body into an arch, and having a harness fitted to his hips, was capable of sustaining a cannon weighing two or three thousand pounds. And not many winters ago, the celebrated Belzoni, when first entering on public life, exhibited himself to the theatres of this metropolis, and by a similar kind of harnessing was capable of supporting, even in an upright position, a pyramid of ten or twelve men surmounted by two or three children, whose aggregate weight could not be much less than 2000 pounds; with which weight he walked repeatedly towards the front of the stage.

The prodigious powers thus exerted by human muscles will lead us to behold with less surprise the proofs of far superior powers exerted by the muscles of other animals, though it will by no means lead us to the means of accounting for such facts.

The elephant, which may be contemplated as a huge concentration of animal excellencies, is capable of carrying with ease a burden of between three and four thousand pounds. With its stupendous trunk (which has been calculated by Cuvier to consist of upwards of thirty thousand distinct muscles) it snaps off the stoutest branches from the stoutest trees, and tears up the trees themselves with its tusks. How accumulated the power that is lodged in the muscles of the lion! With a single stroke of his paw he breaks the backbone of a horse, and runs off with a buffalo in his jaws at full speed: he crushes the bones between his teeth, and swallows them as a part of his food. Nor is it necessary, in the mystery of the animal economy, that the muscles should always have the benefit of a bony lever. The tail of the whale is merely muscular and ligamentous; and yet this is the instrument of its chief and most powerful attack; and, possessed of this instrument, to adopt the language of an old and accurate observer,†"a long-boat he valueth no more than dust, for he can beat it all in shatters at a blow." The skeleton of the shark is entirely cartilaginous, and totally destitute of proper bone; yet is it the most dreadful tyrant of the ocean: it devours with its cartilaginous jaws whatever falls in its way; and in one of its species, the squalus carcharias, or white shark, which is often found thirty feet long, and of not less than four thousand pounds weight, has been known to swallow a man whole at a mouthful.

The sepia octopodia, or eight-armed cuttlefish-the polypus of Aristotle -is found occasionally of an enormous size in the Mediterranean and Indian seas, its arms being at times nine fathoms in length, and so prodigious in their muscular power, that when lashed round a man, or even a Newfoundland dog, there is great difficulty in extricating themselves; and hence the Indians never venture out without hatchets in their boats, to cut off the animal's holders, should he attempt to fasten on them, and drag them under water.

But this subject would require a large volume, instead of occupying the close of a single lecture. Let us turn from the great to the diminutive. How confounding to the skill of man is the muscular arrangement of the insect class! Minute as is their form, there are innumerable tribes that unite in themselves all the powers of motion that characterize the whole of the other classes; and are able, as their own will directs, to walk, run, leap, swim, or fly, with as much facility as quadrupeds, birds, and fishes exercise these faculties separately. But such a combination of functions demands a more complicated combination of motive powers; and what † Frederick Martens. See Shaw, II. H. 489.

· Young's Lect. on Nat. Phil. i. 129.