The only reason we have for asserting in any case that any property belongs to any substance, is the certainty or universality with which we find the substance and the property in question accompanying each other. Thus we say that gold is yellow, ductile, soluble in nitro-muriatic acid, because we have always found gold, when pure, to be so. We assert that living muscular fibres are irritable, living nervous fibres sensible, for the same reason. The evidence of the two propositions presents itself to my mind as unmarked by the faintest shade of difference.

Having found by experience that every thing we see has some cause of its existence, we are induced to ascribe the constant concomitance of a substance and its properties to some necessary connection between them: but, however strong the feeling may be, which leads us to believe in some more close bond, we can only trace, in this notion of necessary connection, the fact of certainty or universality of concurrence. Nothing more than this can be meant, when a necessary connection is asserted between the properties of sensibility and irritability, and the structures of living muscular and nervous fibres.

This language does not explain how the thing takes place: it is merely a mode of stating the fact. To say that irritability is a property of living muscular fibres, is merely equivalent to the assertion, that such fibres have in all cases possessed the power of contraction. What then is the cause of irritability? I do not know, and cannot conjecture.

In physiology, as in the physical sciences, we quickly reach the boundaries of knowledge, whenever we attempt to penetrate the first causes of the phenomena. The most we can accomcircumstances, its immediate effects. When we speak of all the qualities of a body, or all its properties, we mean nothing more, and we mean nothing less. Certain substances are conceived by us, and certain changes that take place in them, which, we believe, will be uniformly the same, as often as the substances of which we speak exist in circumstances that are exactly the same.

"The powers, properties, or qualities of a substance, are not to be regarded, then, as anything superadded to the substance, or distinct from it. They are only the substance itself, considered in relation to various changes, that take place, when it exists in peculiar circumstances,"

We cannot be surprised that the author of the Physiological Lectures should have poured forth the full vials of his wrath on doctrines at once completely subverting all his airy structures of subtle fluids, mobile matters, &c. &c. considered as causes of vital actions, and so simple and logical, that any attempt at direct opposition by reasoning would be utterly hopeless. He therefore boldly affirms that "if they mean to insinuate that we have no knowledge of cause or effect beyond that which results from mere observation, they publish at the same time a libel on the human understanding, a prohibition to rational inquiry, and a most severe satire on themselves." P. 91. Unless the author should show, on some future occasion, what he has not even attempted on the present; viz. what it is that the words cause and effect denote in addition to relative invariable antecedence and consequence, this volley of hard words will only recoil on his own head.

plish is to make gradual conquests from the territories of ignorance and doubt; and to leave under their dominion those objects only, which our reason has not reached, or is not able to reach. The great end of observation and experiment is to discover among the various phenomena, those which are the most general. When these are well ascertained, they serve as principles, from which other facts may be deduced. The Newtonian theory of gravitation is a most splendid example. The only object of uncertainty, which then remains, is the first cause of a small number of facts. The phenomena succeed each other, like the generations of men, in an order which we observe, but of which we can neither determine nor conceive the commencement. We follow the links of an endless chain; and, by holding fast to it, we may ascend from one link to another; but the point of suspension is not within the reach of our feeble powers.

To call life a property of organization would be unmeaning; it would be nonsense. The primary or elementary animal structures are endued with vital properties; their combinations compose the animal organs, in which, by means of the vital properties of the component elementary structures, the animal functions are carried on. The state of the animal, in which the continuance of these processes is evidenced by obvious external signs, is called life.

The striking differences between living and inorganic bodies, and the strong contrast of their respective properties, naturally excited curiosity respecting the causes of this diversity, and endeavours to show the mode in which it was effected. Here we quit the path of observation, and wander into the regions of imagination and conjecture. It is the poetic ground of physiology; but the union is unnatural, and, like other unnatural unions, unproductive. The fiction spoils the science, and the admixture of science is fatal to inspiration. The fictitious beings of poetry are generally interesting in themselves, and are brought forward to answer some useful purpose; but the genii and spirits of physiology are awkward and clumsy, and do nothing at last, which could not be accomplished just as well without them: they literally incumber us with their help.

For those, who think it impossible that the living organic structures should have vital properties without some extrinsic aid;—although they require no such assistance for the equally wonderful affinities of chemistry, for gravity, elasticity, or the other properties of matter, a great variety of explanations, suited to all tastes and comprehensions, has been provided.

Some are contented with stating that the properties of life arise from a vital principle. This explanation has the merit of simplicity, whatever we may think of its profoundness: and it has the advantage of being transferable and equally applicable to any other subject. Some hold that an immaterial principle, and others, that a material, but invisible and very subtle agent is superadded to the obvious structure of the body, and enables it to exhibit vital phenomena. The former explanation will be of use to those who are conversant with immaterial beings, and who understand how they are connected with and act on matter. But I know no description of persons likely to benefit by the latter. For subtle matter is still matter; and if this fine stuff can possess vital properties, surely they may reside in a fabric which differs only in being a little coarser.

Mr. HUNTER has a good substantial sort of living principle; he seems to have had no taste for immaterial agents, or for subtle matters. His materia vitæ is something tangible; he describes it as a substance like that of the brain, diffused all over the body, and entering into the composition of every part. He conceives even the blood to have its share. We may smile at these fancies, without any disrespect to a name that we all revere, without any insensibility to the merits of a surgeon and physiologist, whose genius and labours have reflected honour on our profession and our country. If the father of poetry sometimes falls asleep, a physiologist may be allowed to dream a little; but they who are awake, need not shut their eyes, and endeavour to follow his example, need not exhibit another instance of the perverted taste, which led the disciples of an ancient philosopher to drink spinach-juice, that they might look pale like their master.

PLATO made the vital principle to be an emanation of the anima mundi, or soul of the world; an explanation, no doubt,

That the author of the Physiological Lectures should have published two books, principally for the purpose of explaining, illustrating, and confirming Mr. Hunter's" Theory of Life," without showing us in either what that theory was, without a single citation or reference to identify this doctrine, thus boldly baptised with the name of Hunter, as the literary offspring of its alleged parent, appears strange and suspicious. It is easily explained; for this Hunterian theory of life, which its real author so stoutly maintains to be not only probable and rational, but also verifiable, is no where to be found in the published writings of Mr. Hunter; and does not even resemble the speculations on the same subject, which occur in the posthumous work on the Blood, Inflammation, &c. part i. chap i. sec. 5, on the living Principle of the Blood. In perusing the writings of Mr. Hunter, we should always remember his unfortunate want of early education, the difficulty he felt in conveying his notions clearly by words, and the mutilation which his thoughts must have suffered in passing though the press, both from the causes just mentioned, and from the revision and correction to which some of his writings were subjected.

quite satisfactory to those who know what the soul of the world is, and how other souls emanate from it.

The Brahmins of the East hold a similar notion: but they make the soul after death pass on into other bodies or into animals, according to its behaviour; admitting, however, that those of the good are immediately re-absorbed into the Divinity. Some of the Greeks adopted a distinct vital, sensitive, and rational principle in man.

These are merely specimens; a few articles, as patterns. selected from a vast assortment. If you do not like either of them, there are plenty more to choose from. As these and a hundred other such hypothesis are all supported by equally good proof; which is neither more nor less, in each instance, than the thorough conviction of the inventor; and, as they are inconsistent with each other, and, therefore, mutually destructive, we need not trouble ourselves further until their respective advocates can agree together in selecting some one for their patronage, and discarding the rest. For of these, as of the numerous religions in the world, only one can be true.

What is comparative anatomy? The expression is rather vague and indefinite. You naturally inquire what is compared? What is the object of comparison? The structure of animals may be compared to that of man. To lay down the laws of the animal economy from facts furnished by the human subject only, would be like writing the natural history of our species from observing the inhabitants of a single town or village.

Repeated observations and multiplied experiments on the various tribes of animated nature have cleared up many obscure and doubtful phenomena in the economy of man: a continuation of this method will place physiology on the solid basis of experience, and build up science on ground hitherto occupied by fancy and conjecture.

The physiologist, who is conversant with natural history in general, is fortified against uncertain opinions, and the showy but flimsy textures of verbal sophistry. An hypothesis, which to others appears perfectly adequate to the object in view, is not convincing to him. He rises above the particular object to which it is accommodated, in order to appreciate its value; as we ascend an eminence to gain a commanding view of a district, to distinguish its features, to ascertain the number and bearings of its parts, and their relations to the surrounding country.

There are three points of view, in which comparative anatomy has an important bearing on human physiology.

In the infancy of science, physiology, such as it was, owed its origin to zootomy, which was practised by physicians and naturalists eighteen centuries before human dissections began. The Anatomia Partium Corporis humani of MONDINI, written in the beginning of the fourteenth century, was the first compendium of human anatomy composed from actual dissection. It is easy to show that even the osteology of GALEN was not drawn from the human skeleton: and many parts of the body still bear names derived from animals, which names are in some instances not correctly applicable to the human structure; for example, the epithets right and left as applied to the cavities of the heart.

Although human anatomy, after its first scientific development by BERENGAR of Carpi, was so quickly brought to a high pitch of perfection by the great triumvirate, VESALIUS, FALLOPIUS, and EUSTACHIUS, yet the most important discoveries, those of greatest weight in physiology, considered as the basis of medicine, were made in animals. No period has been so fruitful in these discoveries, nor so distinguished in the literary history of our science, as the seventeenth century, in which the anatomy of brutes was most zealously cultivated, and most of the great anatomical facts were found out, which, by unveiling the hidden springs and movements of the animal machine, have furnished the principles, on which rational pathology and practical medicine have been established.

These comparative researches render the most important service by affording a criterion in doubtful cases for determining the uses of parts; which, as the main object of this fundamental medical science, has been well chosen by GALEN for the title of his classical work on physiology. Hence HALLER observes that the situation, figure, and size of parts ought to be learned from man; their uses and motions must be drawn from animals. I shall adduce a few particulars for the purpose of exemplifying the preceding remarks.

A serpent swallows an animal larger than itself, which fills its œsophagus, as well as stomach, and of which the digestion occupies several days or even weeks. We open the reptile during this process, and find that part of the animal which remained in the œsophagus, sound and natural, while the portion which had descended into the stomach, though still retaining its figure, is semi-liquefied, reduced into so soft a state, as to break down under the slightest pressure. How effectually does