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the weight of the body is more diminished by the same quantity of sweat than of mere perspiration.

Sanctorius, whose experiments of measuring the weight of the body were made in the warm climate of Italy, ascertained that in that region eight pounds of food received by the mouth were, by the different insensible secretions, reduced to three; making the proportion of insensible exhalation as five to eight. In cold climates, however, it has been determined that it does not amount to more than two-thirds of this proportion; and of either quantity it has lately been very satisfactorily established, that more than half this secretion has been thrown forth from the surface of the lungs; which I estimated in a previous lecture, and from the experiments and calculations of Lavoisier, as discharging not less than eleven ounces of solid carbon or charcoal in every four-and-twenty hours.*

Plants transpire precisely in the same way, and to a much greater extent, through the medium of their leaves; which, while they form a great part of their cuticle, may, as I have observed on a former occasion,t be also contemplated as their lungs. Hales calculated that a sun-flower, three feet high, transmits in twelve hours one pound four ounces of fluid by avoirdupois weight. Bishop Watson put an inverted glass vessel, of the capacity of twenty cubic inches, on grass which had been cut during a very intense heat of the sun, and after many weeks had passed without rain; in two minutes it was filled with vapour, which trickled with drops down its sides. He collected these on a piece of muslin, carefully weighed, and repeated the experiment for several days between twelve and three o'clock; and estimated, as the result of his experiment, that an acre of grass land transpires in twenty-four hours not less than 6,400 quarts of water. Dalton, for dew and rain together, makes the mean of England and Wales 36 inches, thus amounting, in a year, to 28 cubic miles of water. Grew, in 1711, calculated the number of acres in South Britain at 46,800,000, and allowed a million to Holland. I Smith, for England alone, gives 734 millions in the present day.

But the same general surface in animals and vegetables that thus largely secretes delicate fluids, largely also imbibes them by the corresponding system of absorbent vessels, opening with their spongy mouths or ducts in every direction. Hales ascertained that the above sun-flower, which threw off not less than twenty ounces of fuid in twelve hours, suspended its evaporation as soon as the dew fell, and absorbed two or three ounces of the dew instead. And among animals, and especially among mankind, the manisest operations of medicines and other foreign substances, merely diffused through the air, or simply applied to the skin; of various vapours, as those of mercury, turpentine, and saffron; of various baths, as of tobacco, bitter-apple, opium, cantharides, arsenic, and other poisons, producing the most fatal effects, and altogether absorbed by the skin, are decisive and incontrovertible proofs of such an action. It is hence the bradypus, or sloth, supports itself without drinking, perhaps, at any time, and the ostrich and camel for very long periods, though the latter is also possessed of a natural reservoir. And hence the chief impletion of the human body, in many cases of abdominal dropsy; since persons labouring under this disease have often been observed to fli with rapidity during the most rigid abstinence from duinks of every kind.

Along with the common odour of insensible perspiration, discharged from the human surface, we often meet with other odours of a much stronger kind, produced by particular diseases or particular modes of life, and which are distinctly perceptible. Thus the food of garlic yields a perspiration possessing a garlic smell; that of pease a leguminous smell; coarse oils and fat a rancid smell, which is the cause of this peculiar odour among the inhabitants of Greenland; and acids a smell of acidity. Among glass-blowers, from the large quantity of sea-salt that enters into the materials of their manufacture, the sweat is sometimes so highly impregnated, that the salt thev

• Series 1. Lecture xiii.
Phil Trans for 1811, p. 265

| Series 1. Lecture ix.
D) Phil. Mag. xix. 197. Young's Nat. Phil. il. 369

employ, and imbibe by the skin and lungs, has been seen to collect in crystals upon their faces.

Hence, too, the various smells that are emitted from the surface of other animals, and especially that of musk, which is one of the most common. We trace this issuing generally from the bodies of many of the ape species, and especially the simia jacchus; still more profusely from the opossum, and occasionally from hedgehogs, water-rats, hares, serpents, and crocodiles. The odour of civet is the production of the civet-cat alone, the viverra zibetha, and viverra civetta of Linnæus ; though we meet with faint traces of it in some varieties of the domestic cat, the felis catta of the same writer. Genuine castor is, in like manner, a secretion of the castor fiber; but the sus Tajassu, and various other species of swine, yield a smell that makes an approach towards it.

Among insects, however, these odours are considerably more varied, as well as considerably more pleasant; for the musk-scent of the cerambix moschatus, the apis fragrans, and the tipula moschifera, is far more delicate than that of the musk quadrupeds; while the cerambix suaveolens, and several species of the ichneumon, yield the sweetest perfume of the rose; and the petiolated sphex a balsamic ether highly fragrant, but peculiar to itself. Yet insects, like other classes of animals, furnish instances of disagreeable and even disgusting scents, as well as of those that are fragrant. Thus, several species of the melitæ breathe an essence of garlic or onions; the staphilinus brunipes has a stench intolerably fetid, though combined with the perfume of spices; while the caterpillars of almost all the hymenoptera, and the larves of various other orders, emit an exhalation in many instances excessively pungent. The carabus crepitans, and sclopeta of Fabricius, pour forth a similar vapour, accompanied with a strange crackling sound.

The odorous secretions belonging to the 'vegetable tribes are well known to be still more variable ; sometimes poured forth from the leaves of the plant, as in the bay, sweet-briar, and heliotrope ; sometimes from the trunk, as in the pines and junipers; but more generally from the corol. It is from the minute family of the jungermannia, nearly related to the mosses, and often scarcely visible to the eye, that we derive the chief sense of that delightful fragrance perceptible after a shower, and especially at even-tide :* and from the florets of the elegant anthoxanthum odoratuin, or spring-grass, that we are chiefly furnished with the sweet and fragrant scent of new-mown hay. But occasionally the odours thus secreted are as intolerable as any that are emitted from the animal world; of which the ferula usafotida, or asafetida plant, and the stapelia hirsuta, or carrion-flower, are sufficient examples.

To the same secernent powers, moreover, of animals and vegetables, existing in particular organs rather than extended through the system generally, we are indebted for a variety of very valuable materials in trade and diet, as gums, resins, wax, fat, oils, spermaceti. And to the same cause we owe, also, the production of a multiplicity of poisons and other deleterious substances : such, for instance, as the poison of venomous serpents, which is found to consist of a genuine gum, and is the only gum known to be secreted by animal organs; the electric gas of the gymnotus electricus and raia torpedo ; the pungent sting of the stinging-nettle, urtica urens, and of the bee, both which are produced from a structure of a similar kind; for every aculeus or stinging point of the nettle is a minute and highly irritable duct, that leads to a minute and highly irritable bulb, filled with a minute drop of very acrid fluid: and hence, whenever any substance presses against any of the aculei or stinging points of the plant, the impression is communicated to the bulb, which instantaneously contracts, and throws forth the minute drop of acrid fluid through the ducts upon the substance that touches them.

As the secernent system thus evidently allots particular organs for the becretion of particular materials, the absorbent system is in like manner only capable of imbibing and introducing into the general frame particular mate

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* Hooker's Monography of British Jungerm.

rials in particular parts of it. Thus, opium and alkohol, the juice of aconite, and essential oil of laurel or bitter almonds, produce little or no effect upon the absorbents of the skin, but a very considerable effect upon the coating of the stomach. In like manner, carbonic acid gas invigorates rather than injures, when applied to the absorbents of the stomach, but instantly destroys life when applied to those of the lungs; while the aroma of the toxicaria Macasariensis, or Boa upas, of which we have heard so much of late years, proves equally a poison, whether received by the skin, the stomach, or the lungs.

So, also, substances that are poisonous to one tribe of animals are medi. cinal to a second, and even highly nutritive to a third. Thus, swine are poisoned by pepper-seeds, which to man are a serviceable and grateful spice; while henbane-roots, which destroy mankind, prove a wholesome diet to swine. In like manner, aloes, which to our own kind is a useful medicine, is a rank venom to dogs and foxes; and the horse, which is poisoned by the phellandrum aquaticum, or water-hemlock, and corrosive sublimate, will take a drachm of arsenic daily, and improve hereby both in his coat and condition.

It has already appeared, that the secernent vessels of any part of the system, in order to accomplish a beneficial purpose, as, for example, that of restoring a destroyed or injured portion of an organ, may change their action, and secrete a material of a new nature and character. An equal change is not unfrequently produced under a morbid habit, and the secretion will then be of a deleterious instead of being of a healthy and sanative kind. And hence, under the influence of definite causes, the origin of such mischieyous and fatal secretions, in some instances thrown forth generally, and in others only from particular organs, as the matter of small-pox, measles, putrid fevers of various kinds, cancer, and hydrophobia, or the poisonous saliva of mad dogs,

But the field opens before us to an unbounded extent, and we should lose ourselves in the subject if we were to proceed much farther. It is obvious, that in organic, as in inorganic nature, every thing is accurately arranged upon a principle of mutual adaptation, and regulated by an harmonious antagonism, a system of opposite yet accordant powers, that balance each other with most marvellous nicety; that increase and diminution, life and death, proceed with equal pace; that foods are poisons, and poisons foods; and, finally, that there is good enough in the world, if rightly improved, to make us happy in our respective stations so long as they are allotted to us, and evil enough to wean us from them by the time the grant of life is usually recalled.

LECTURE XV.

ON THE EXTERNAL SENSES OF ANIMALS.

The subject of study for the present lecture is the organs of external sense in animals : their origin, structure, position, and powers; and the diversities they exhibit in different kinds and species.

The external senses vary in their number: in all the more perfect animals they are five; and consist in the faculties of sight, smell, hearing, taste, and touch.

It is by these conveyances that the mind or sensory receives a knowledge of whatever is passing within or without the system; and the knowledge it thus gets

possession of is called perception. The different kinds of perception, therefore, are as numerous as the different channels through which they are received, and they produce an effect upon

the sensory, which usually remains for a long time after the exciting cause has ceased to operate. This effect, for want of a better term, we call empressions; and the particular facts, or things impressed, and of which the impressions retain, as it were, the print or picture, ideas.

The sensory has the power of suffering this effect or these ideas to remain latent or unobserved, and of calling them into observation at its option : it is the active exercise of this power that constitutes thought.

The same constitution, moreover, by which the mind is enabled to take a review of any introduced impression, or to exercise its thought upon any introduced idea, empowers it to combine such impressions or ideas into every possible modification and variety. And hence arises an entirely new source of knowledge, far more 'exalted in its nature, and infinitely more extensive in its range : hence memory and the mental passions; hence reason, judgment, consciousness, and imagination, which have been correctly and ele. gantly termed the internal senses, in contradistinction to those by which we obtain a knowledge of things exterior to the sensorial region.

Thus far we can proceed safely, and feel our way before us ; but clouds and darkness hang over all beyond, and a gulf unfathomable to the plummet of mortals. Of the sensory, or mind itself, we know nothing; we have no chemical test that can reach its essence, no glasses that can trace its mode of union with the brain, no abstract principles that can determine the laws of its control. We see, however, enough to convince us that its powers are of a very different description from those of the body, and Revelation informs us that its nature is so too. Let us receive the information with gratitude, and never lose sight of the duties it involves.

But this subject would lead us astray even at our outset : it is important, and it is enticing; and the very shades in which much of it is wrapped up prove an additional incitement to our curiosity. It shall form the basis of some subsequent investigation,* but our present concern is with the external senses alone.

These, for the most part, issue from the brain, which, in all the more perfect animals, is an organ approaching to an oval figure; and consists of three distinct parts: the cerebrum, or brain properly so called; the cerebel, or little brain, and the oblongated marrow. The first constitutes the largest and uppermost part; the second lies below and behind; the third, level with the second, and in front of it-it appears to issue equally out of the two other parts, and gives birth to the spinal marrow, which may hence be regarded as a continuation of the brain, extended through the whole chain of the spine or back-bone.

From this general organ arises a certain number of long, whitish, pulpy strings or bundles of fibres, capable of being divided and subdivided into minuter bundles of filaments or still smaller fibres, as far as the power of glasses can carry the eye. These strings are denominated nerves; and by their different ramifications convey different kinds or modifications of sensation to different parts of the body, keep up a perpetual communication with its remotest organs, and give activity to the muscles. They have been supposed by earlier physiologists to be tubular or hollow, and a few experiments have been tried to establish this doctrine in the present day, but none that have proved satisfactory.

As the brain consists of three general divisions, it might, at first sight, be supposed that each of them is allotted to some distinct and ascertainable purpose: as, for example, that of forming the seat of intellect, or thinking; the seat of the local senses of sight, sound, taste, and smell; and the seat of general feeling or motivity. But the experiments of anaiomists upon this abstruse subject, numerous and diversified as they have been of late years, and, unhappily, upon living as well as upon dead animals, have arrived at nothing conclusive in respect to it: and have rather given rise to contending than to concurrent opinions. So that we are nearly or altogether unac.

Series ni. Lectures i. ii. ii. iv.

quainted with the reason of this conformation, and of the respective share which each division takes in producing the general effect.

The nerves uniformly issue in pairs, one for each side of the body, and the number of the pairs is thirty-nine ; of which nine rise immediately from the great divisions of the brain, under which we have just contemplated it, and are chiefly appropriated to the four local senses; and thirty from the spinal marrow, through different apertures in the bone that encases it, and are altogether distributed over the body, to produce the fifth or general sense of couch and feeling, as also irritability to the muscles.

That these nervous or pulpy fibres are the organs by which the various sensations are produced or maintained is demonstrable from the following facts: If we divide, or tie, or merely compress, a nerve of any kind, the muscle with which it communicates becomes almost instantly palsied; but upon untying or removing the compression, the muscle recovers its feeling and mobility. If the compression be made on any particular portion of the brain, that part of the body becomes motionless which derives nerves from the portion compressed. And if the cerebrum, cerebel, or oblongated marrow be irritated, excruciating pain or convulsions, or both, take place all over the body, though chiefly where the irritation is applied to the last of these three parts.

The matter of sensation, or nervous fluid, as for want of a more precise knowledge upon this subject we must still continue to call it, is probably as homogeneous in its first formation as the fluid of the blood; but, like the blood, it appears to be changed by particular actions, either of particular parts of the brain, or of the particular nervous fibres themselves, into fluids of very different properties, and producing very different results. And it is probably in consequence of such changes alone that it is capable of exciting one set of organs to communicate to the brain the sensation of sound alone, another set that of sight alone, and so of the rest. While branches from the spinal marrow, or fountain-nerve of touch, are diffused over every portion of the body, sometimes in conjunction with the local nerves, as in the organs of local sense, and sometimes alone, as in every other part of the system.

Such an idea leads us naturally to a very curious and recondite subject, which has never, that I know of, been attended to by physiologists, and will at the same time throw no small degree of light upon it :- I mean the production of other senses and sensorial powers than are common to the more perfect animals, or such a modification of some one of them as may give the semblance of an additional sense.

What, for example, is that wonderful power by which migratory birds and fishes are capable of steering with the precision of the expertest mariner from climate to climate, and from coast to coast; and which, if possessed by man, might, perhaps, render superfluous the use of the magnet, and consider. ably infringe upon the science of logarithms? Whence comes it that the fieldfare and red-wing, that pass their summers in Norway, or the wild-duck and merganser, that in like manner summer in the woods and lakes of Lapland, are able to track the pathless void of the atmosphere with the utmost nicety, and arrive on our own coasts uniformly in the beginning of October ? or that the cod, the whiting, and the herring should visit us in innumerable shoals from quarters equally remote, and with an equal exactness of calculation? the cod pursuing the whiting, which flies before it, from the banks of New. foundland to the southern coasts of Spain; and the cachalot, or spermaceti whale, driving vast armies of herrings from the arctic regions, and devouring thousands of those that are in the rear every hour.

We know nothing of this sense, or the means by which all this is produced : and, knowing nothing of it, and feeling nothing of it, we have no terms by which to reason concerning it.

Yet it is a sense not limited to migratory animals. A carrier-pigeon has been brought in a bag from Norwich to this metropolis, constituting a distance of 120 miles ; and having been let off with a letter tied round its neck, from

* Ser Hunter's Anim. Economy, p. 261, 262.

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