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increased action in both these sets of vessels; as on exposure to cold and damp temperatures ; in cases of spare and coarse diet; or of old age. And the result of this double decrease of energy is dryness, as in the former instance, but coinbined with leanness and corrugation of the organs that are thus affected. It is hence the bones of old people are more easily broken, and the skin is harsher and more wrinkled than in the middle of life; hence the shri. velled and squalid appearance of gipsies and beggars; and hence, in a consi. derable degree, the low and stinted stature of the Esquimaux, Laplanders, and Tongooses.

For all the usual purposes of health and organic nutrition, the common action and common degree of action evinced by these respondent systems of vessels are perfectly sufficient, though not more than sufficient. It may happen, however, that in consequence of severe violence from external injury or internal disease, a considerable portion of an organ, as a part of some of the muscles that belong to an arm or a leg, may be totally destroyed or killed, and, consequently, rendered incapable of performing its proper function. How is nature, or, which is the same thing, the remedial principle of life, to act in such circumstances ? If the dead part remain, it is manifest that it must impede the living parts that surround it in the execution of their appropriate ofñce: independently of which they want the space which the dead part occupies, and the aid which it formerly contributed. It is obvious that two processes are here necessary: the dead part must be carried off, and its post must be filled up by a substitute of new matter possessing the precise properties of the old. And here we meet with a clear and striking instance of that wonderful instinctive power which pervades every portion of the vital systems, both of the animal and vegetable world, and which is perpetually prompting them to a repair of whatever evils they may encounter, by the most skilful and definite methods.

In order to comply with this double demand of carrying off the dead matter, and of providing a substitute of new, each of the systems before us commences, in the living substance that immediately surrounds that which requires removal, a new mode and a new degree of action.

A boundary line is first instinctively drawn between the dead and useless, and the living and active parts; and the latter retract and separate themselves from the former, as though the two had been skilfully divided by a knife. This process being completed, the mouths of the surrounding absorbent vessels set to work with new and increased power, and drink up and carry off whatever the material may be of which the dead part consists, whether fat, muscle, ligament, cartilage, or bone; the whole is equally imbibed and taken away, and a hollow is produced, where the dead part existed. At the same time the mouths of the corresponding secernent vessels commence a similar increase and newness of action, and instead of the usual lymph, pour forth into the hollow a soft, bland, creamy, and inodorous fluid which is denomi. nated pus; that progressively fills up the cavity, presses gradually against the superincumbent skin, in the gentlest manner possible distends and attenuates it, and at length bursts it open, and exposes the whole of the interior to the action of the gases of the atmosphere.

It was at one time conceived, and by writers of considerable eminence and judgment, and of as late a date as the time of Mr. Hewson, that the injured and dead parts were themselves dissolved and converted into pus; but this opinion has been disproved in the most satisfactory manner by the minute and accurate experiments of Mr. John Hunter, Sir Everard Home, and Mr. Cruickshank ; and the process has been completely established as I have now related it.

In what immediate way the gases of the atmosphere operate so as to assist the secernent mouths of what is now the clean and exposed surface of a wound, in producing incarnation, or the formation of new matter of the very same kind and power as that which has been carried off, and enable them to fill up the cavity with such new matter, and perfect the cure, we do not exactly know. Various theories have been offered upon this very curious subject; but at present they are theories, and nothing more ; and I shall not, therefore, detain you with a relation of them. Thus much, however, we do know, that the co-operation of the atmosphere with the action of the mouths of the secernent system engaged in the work of restoration is, in some way or other, pecuiiarly beneficial; and that, generally speaking, the wider the opening, and the freer the access of atmospheric air of a due temperature to the surface of the wound, or, which is the same thing, the freer it comes in contact with the mouths of the secernent vessels, the more rapidly and auspiciously the work of impletion and assimilation proceeds. Neither do we know, precisely, why pus, rather than any other kind of fluid, should in the first instance be poured forth, for the purpose of filling up the hollow, and producing a rupture of the skin ; but we know to a certainty that some such general process is in most cases absolutely necessary; we know that such a rupture must take place in the natural mode of cure; that the atmosphere must come into close contact with the mouths of the restorative secernents; that a milder or softer fluid could not possibly be secreted for such a purpose; and that the entire process exhibits proofs of most admirable skill and sagacity. It is at times possible for us to assist the process by the lancet, which accelerates the opening. Yet, even in this case, we do no more than assist it, and are only, as we ought ever to be in all similar cases, humble coadjutors and imitators of nature, and admirers of that all-perfect and ever-present wisdom which we are so often called upon to witness, but are never capable of rivalling.

A process closely similar to this is perpetually unfolding in vegetable life. And it was merely by taking advantage of this process that Mr. Forsythe was able to make old, but well-rooted, stumps of fruit-trees throw forth, far more rapidly than he could saplings, a thrifty family of vigorous and well-bearing shoots: for the compost for which he was so celebrated does nothing more than merely increase the secernent and absorbent action of the vegetable frame by its stimulating property, and defend the wounded part to which it is applied from being injured by the inclemency of the weather.

From what has thus far been observed, it appears obvious that all the different parts of the living body are assimilating organs, or, in other words, are capable of converting the common nutriment of the blood into their own respective natures, and for their own respective uses. And it has also appeared, that under particular circumstances every part is capable, moreover, of secreting a material different from that of its own nature; as, for example, the material of pus, whenever such a substance is necessary.

This view of the subject will lead us to understand with facility how it is possible for various organs of the system to maintain two distinct secretions at the same time: one of a matter similar to its own substance, and exclusively for its own use; and another of a matter distinct from its own substance, and in many instances subservient to the system in general.

or this last kind are the stomach, the liver, the respiratory organ, and the brain: each of which secretes, independently of the matter for its own nourishment, a matter absolutely necessary to the health and perfection of the general machine: as the gastric juice, the curious and wonderful properties of which I described on a former occasion; the oxygenous principle of the inspired air, and, as some suppose, those of light or caloric; the bile; and the nervous fluid, or material of sensation.

There are various other organs of a smaller kind, and simpler texture, which also perform the same double office, and secrete materials of a much more local use, or which are intended to be altogether thrown away from the system, as waste or noxious bodies. And to the one or the other of these classes belong the kidneys, the intestinal tube, the minute and very simple perspiratory follicles of the skin, the delicate organs that separate the saliva and mucus that serve to lubricate the mouth and nostrils, and those that elaborate the tears, the wax of the inner ear, and the fat.

The organs, of whatever size or texture, that perform this double function, are called secretory glands; and they are distinguished into different sets,

either from their peculiar office or peculiar structure: as salivary, lachryinal, mucous, which are denominated from the former character, and apply to the smallest and simplest of them ; conglobate, which are of a larger sorm, and of an intricate convolution, and belong exclusively to the absorbent system,as the mesenteric and lumbar; and glomerate and conglomerate, which are composed of a congeries of sanguineous vessels

, without any cavity, but with one or more mouths, or excretory ducts as they are called, which, in the latter, open into one common trunk,—as the mammary and pancreatic; both which kinds are denominated from the character of their structure.

It is by this peculiar organization in aninials and plants that all those nice and infinitely varying exhalations or other Auids are thrown forth from different parts of them, by which such parts, or the whole individual, or the entire species of individuals, are respectively characterized. Our own senses are too dull to trace a discharge of any kind of essence or vapour from the surface of the human skin in its ordinary action; but the discoloration which soon takes place upon the purest linen, when worn in the purest atmosphere, sufficiently proves ihe existence of such an efflux; and there are various animals whose olfactory organs are much acuter than our own, as our domestic dogs, for example, that are able to discern a difference in the odour of the vapour which issues from the skin of every individual, and that in fact identify their respective masters, and distinguish them from other individuals, by this character alone.

It is to this sense chiefly that quadrupeds, birds, fishes, and most insect tribes trust themselves in their search after food; and hence the superior acuteness of this power in animals of such kinds is a strong proof of that unerring Wisdom which regulates the world, and is equally conspicuous in every part of it. Under peculiar circumstances, however, the sense of smell appears to be far more lively among mankind than when such circumstances do not exist. M. Virey, who has written a very learned treatise upon this subject, asserts, that it occurs among savages in a sar higher degree of activity than among civilized nations, whose olfactory nerves are blunted by an habitual exposure to strong odours, or intricate combination of odours, and by the use of high-flavoured foods. And among persons in a keen morbid state of irritability it has been often found, even in civilized life, much sharper than among savages. The Journal des Sçavans, an 1667, gives a curious history of a monk who was said to be able to ascertain, by the difference of odour alone, the sex and age of a person, whether he were married or single, and the manner of life to which he was accustomed.*

When the exhalation from the human skin is increased by muscular exer. cise, or any other exertion, it is rendered visible; and in this state it is generally found to combine with it a certain portion of dissolved animal oil or fat. Even without much increased action of the system, it is possible at times to obtain a knowledge of its existence under particular circumstances, or by particular applications. Thus, in cold subterraneous caverns, where the air is dense and heavy, the natural evaporation often escapes from the surface of the body in the form of thick clouds; and a bright mirror, when held near a warm and naked skin, in the temperature of the atmosphere, soon becomes obscured by a moist vapour.

The quantity of this fluid discharged, either in a state of quiescence or of increased action, has not been determined with any great degrees of exactness. According to M. de Sauvages,f a man of middle stature and age, weighing 146lbs., takes daily of food and drink about 56 ounces (circiter quinquaginta sex uncias), his dinner being about twice as much as his supper. In the same period he perspires about 28 ounces; viz. about twelve during the third part of his time in which he sleeps, and sixteen during the two-thirds in which he is awake. It appears certain, from the experiments of Gorter, that

** In a paper on the Petiveria, in the Swedish Academy Transactions, there are a variety of curious observations on the peculiar properties given to the smell, flesh, &c. of different animals in consequence of their feeding on different foods. li is entitled Petiveria, en Americansk vaxt. Anal. Trans. tom. i p 346.

† Nosol. Method. u. 369

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 fuids, 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 fluid 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 fill with rapidity during the most rigid abstinence from drinks 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 they

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

Series 1. Lecture ix.
Phil. Mag. xix. 197. Young's Nat. Phil. ii. 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 asafetida, or asafetida plant, and the stapelia hirsuta, or carrion-flower, are sufficient examples.

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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 tor. pedo; 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 Auid 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

* Hooker's Monography of British Jungerm.

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