« AnteriorContinuar »
lachrymal, mucous, which are denominated from the former character, and apply to the smallest and simplest of them; conglobate, which are of a larger form, and of an intricate convolution, and belong exclusively to the absorbent system, as the mysenteric 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 animals 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 the 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 far 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 exercise, 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.
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. It is entitled Petiveria, en Americansk vaxt. Amal. Trans. tom. i. p. 346.
The quantity of this fluid discharged, either in a state of quiescence or of increased action, has not been determined with any great degree of exactness. According to M. de Sauvages,* a man of middle stature and age weighing 146 lbs., 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 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 carbone or charcoal in every four-andtwenty 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,‡ 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.§ Smith, for England alone, gives 731 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 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 manifest 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
*Nosol. Method. ii. 369.
† Ser. I. Lect. XIII.
Ser. L. Lect. IX. Young's Nat. Phil. ii, 369.
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 possest 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 peas 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 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 hedge-hogs, 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 Linnéus; 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 eventide :* and from the florets of the elegant anthoxanthum odoratum, 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 assafœtida, or assafetida 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, a 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 secretion of particular materials, the absorbent system is in like manner only capable of imbibing and introducing into the general frame particular materials 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 medicinal to a second, and even highly nutritive to a third. Thus, swine are poisoned by pepper-seeds, which to a 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 dram 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
* Hooker's Monography of British Jungerm.
origin of such mischievous 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, cancers, 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.
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 impressions; and the particular facts, or things impressed, and of which the impressions retain, as it were, the print or picture, ideas.
The sensory has a 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 elegantly 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