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 ju- ́ nipers; 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 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 * Hooker's Monography of British Jungerm.

the ferula assafoetida, 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, 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 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 alcohol, 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 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 origin of such mischievous and fatal secretions,

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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 a 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 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