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but our admiration is greatly increased.

Here are two

distinct orders of existences, in their general characters altogether different, yet so intimately connected, and so beautifully fitted to each other, that, without the one, the other could not exist at all, or would exist for no end. What gave rise to this remarkable adjustment, this necessary connexion? The answer is obvious,-A Contriving Mind, that could form comprehensive schemes, that could combine mighty powers naturally distinct, that could foresee and provide for conditions and relations vast, profound, and complicated.

FIFTH WEEK-WEDNESDAY.

THE SENSORIAL ORGANS.

In the volume on Spring, I took notice of the general principles which enter into the animal structure, such as the nature of the tendons, membranes, and ligaments, the functions of digestion, secretion, and muscular power, the circulation of the blood, &c. There are, however, some other important provisions in the animal frame, which have either not been mentioned at all, or have only been mentioned incidentally, and which now seem to invite particular notice, as introductory to a consideration of animal existences, in their state of full developement. I allude chiefly to the different organs of sense, which shall form the subject of this paper.

Living creatures, being intended to perform voluntary actions, required some means of communication with the external world. This it has pleased Providence to afford them, by five different avenues. How many others might

have been bestowed, in order to give them a complete acquaintance with natural things, it is impossible even to conjecture; yet while, on the one hand, we cannot but be aware that there are many existences, and many qualities of bodies, which lie beyond the ken of our senses, and which we can, therefore, no more perceive than a

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man born blind can perceive an object of vision, we ought gratefully to acknowledge that the powers of sensation and perception actually allotted to us, are quite sufficient for all the purposes of sublunary life.

To be made sensible of the presence of bodies with which we are in immediate contact, seems to be the most simple mode of communication with the external world; which is effected by the sense of touch. But what a complicated and curious system does even this require. The seat of perception is in the brain; and, that the knowledge in question may be obtained, it was necessary that there should be messengers and channels of communication to it from every part of the body. This is accomplished by peculiar, and, in some respects, recondite contrivances, in what is called the nervous system. The sense of touch is chiefly confined to the outer surface of living bodies, because it is here that they come in contact with the external world. To understand the material mechanism through which it is perceived, it must be observed, that the skin consists of three parts, of which the corium forms the principal portion, and the cuticle the outmost layer, between which there occurs a thin layer of a substance called the rete mucosum. The corium is constructed of an intermixture of dense and tough fibres, through which a multitude of blood-vessels and nerves are interspersed; but its exterior surface is more vascular than any other part, exhibiting a fine and delicate network of vessels, named by anatomists the vascular plexus. It is here that the sensation of touch is principally situated; and we may infer that it contains the terminations of all the nervous filaments appropriated to this sense, which are here found to be divided to an extreme degree of minuteness. When examined with the microscope, this surface presents a great number of minute projections which have received the name of papillæ, and which seem essential to the perfection of the sensation, as they are particularly conspicuous on those parts of the skin which are most essentially appropriated to this sense, such as the tongue, the lips, and, in man, also the tips of the fingers. To protect this structure of exquisite sensibility, the cuticle

is provided, an external covering entirely insensible, which envelopes the whole.

Such is a condensed account of the appearance and distribution of the nerves of touch; and I have only further to add, that this system may be traced till all its minute ramifications are concentrated and terminated in the brain. Beyond this, all is mystery.

The senses of taste and smell are intended to convey impressions resulting from the chemical qualities of bodies, the one in the liquid or solid, the other in the gaseous state; and there appears to be a considerable analogy between these two senses; indeed, they may be said to be only peculiar modifications of the sensation of touch. The organ of taste is the surface of the tongue, the skin of which is furnished with a large proportion of bloodvessels and nerves, and here the vascular plexus is particularly conspicuous. It is intended, as its primary use, to guide animals in the choice of their food, and to warn them against the introduction of any noxious substance into the stomach, and is therefore placed in the mouth, through which aliment is received.

The

The sense of smell, on the other hand, is chiefly intended to protect the animal from the reception of noxious effluvia into the lungs, and hence it is usually made to occupy the beginning of the passages of respiration. It also materially assists the sense of taste, giving rise to what is called flavor; and, on this account, is placed in the immediate neighborhood of the latter organ. cavity of the nostrils is lined by a soft membrane, constantly kept moist, which is supplied with numerous bloodvessels, and upon which are spread the ultimate ramifications of the olfactory nerves. In quadrupeds, as well as in man, these nerves are not collected into a single trunk, in their course towards the brain, but compose a great number of filaments, which pass separately into the cavity of the skull, through minute perforations in a plate of bone.

As there is an analogy between the three senses already mentioned, so there is this analogy between the other two, that it is by hearing and sight that animals

chiefly acquire the knowledge of the existence and movements of distant objects; sound results from certain tremulous or vibratory motions of the particles of an elastic medium, such as air or water, excited by some sudden impulse or concussion. These sonorous vibrations, being transmitted with great velocity, strike on the external ear, and then, after being concentrated in the internal passages of the organ, they are made to act on the filaments of a particular nerve, called the acoustic or auditory nerve, the structure of which is adapted to receive these particular impressions, and convey them to the brain.

Sight is the most refined and admirable of all the senses. It is intended to convey to animals a knowledge of the existence, situation, and color of distant objects, which it does by means of the light reflected in all directions from these objects. The sensation is produced through the medium of the retina of the eye, as I have already had occasion to remark, on which the rays of light are thrown, in a concentrated form, and from which an immediate communication is established with the brain.

Such is, in few words, the material apparatus, through the medium of which the knowledge of external objects is conveyed to that principle in animals, whatever it may be, which possesses the faculty of sensation and perception. Even this necessarily slight and imperfect sketch is sufficient to impress the mind with a conviction that this diversified mechanism, and its results, could only originate in benevolent intention and profound contrivance. A more minute inquiry not only confirms this conviction, but, in proportion to the skill with which it is conducted, increases our admiration of the Divine Contriver, till we are lost in astonishment; and, as we stretch our mental powers to comprehend this subject, we become sensible of their utter inadequacy.

I have, in a previous paper, adverted to the mechanical structure of the eye as an optical instrument; but there are very many circumstances of adjustment and construction, both in the instrument itself, and in the parts connected with it, no less obviously the result of design and

consummate skill, to which I have not adverted. Take, for example, the crystalline lens. This central part in the eye of a codfish has been accurately analyzed by Sir David Brewster; and, by the help of a powerful microscope, he has discovered that, minute as it is, and perfectly transparent, it is composed of upwards of five millions of fibres, which lock into one another by means of more than sixty-two thousand five hundred millions of teeth! This is only a single instance of those mechanical wonders which are to be found in the examination of every part of the animal frame, and especially of the sensorial organs. How infinitely, then, does the workmanship, in its details, as well as in its united effects, surpass all our powers of conception !

FIFTH WEEK-THURSDAY.

SENSATION AND PERCEPTION.

HAVING adverted to the organic structure by which the existence and qualities of external objects are perceived, some general observations seem to be called for, on the nature of that structure, and of the faculties connected with it. This agreeable task I shall perform, by making a free use of the observations of Dr. Roget,* to whom I have been so often indebted for information on all subjects connected with physiology.

To a person unused to reflection, the phenomena of sensation and perception may appear to require no elaborate investigation. That he may behold external objects, nothing more seems necessary than to direct his eyes towards them. He feels as if the sight of these objects were a necessary consequence of the motion of his eyeballs, and he never suspects that there can be any thing marvellous in the function of the eye, or that any other organ is concerned in the simple act of vision. If he

* Bridgewater Treatise, Chapter on the Sensorial Functions, vol. ii.

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