Billeder på siden
PDF
ePub

260

STRUCTURE OF THE HUMAN BODY.

LESSON 118.

Structure of the Human Body (continued.)
Secre'tion, the process by which various fuids are separated

from the blood by means of the glands. Vas'cular, full of ves

sels. The mind being formed for society and intercourse with beings of her own kind, she must be endued with powers of expressing and communicating her thoughts by some sensible marks or signs, which shall be both easy to herself, and admit of great variety; and accordingly she is provided with the organs and the faculty of speech, by which she can throw out signs with amazing facility, and vary them without end. Thus we have built up an animal body which would seem to be pretty complete; but as it is the nature of matter to be altered and worked upon by matter, so in a little time such a living creature must be destroyed, if there is no provision for repairing the injuries which she must commit on herself, and those to which she must be exposed from without. Therefore a treasure of blood is actually provided in the heart and vascular system, full of nutritious and healing particles, Auid enough to penetrate into the minutest parts of the animal; impelled by the heart, and conveyed by the arteries, it pervades every part, builds up what was broken down, and sweeps away the old and useless materials. Hence we see the necessity or advantage of the heart and arterial system. The heart consists of four cavities, from one of which, the blood is driven into the arteries through the body, by another, it is received back again by the veins : it then passes into the third, whence it is forced into the lungs. Having there been revivified by coming in contact with the air, it is carried back by a set of veins into the fourth cavity, and thence into that in which it began its course : it is then again forced into the arteries, brought back by the veins, and thus circulates till the end of life. Each cavity of the heart is generally called into action four thousand times every hour. The arteries, into which the blood is forced, branch in every direction through the body, like the roots, branches, and leaves of a tree, running through the substance of the bones, and every part of the animal, till they are lost in such fine tubes as to be wholly invisible. In this man

STRUCTURE OF THE HUMAN BODY.

261

ner, they distribute nourishment, supply perspiration, and renew all the waste of the system ; and by passing through glands in every part of the body, all the various animal secretions are elaborated. In the parts where the arteries are lost to the sight, the veins take their rise, and in their commencement are also imperceptible. The blood is then of a dark colour. In this discoloured state it has lost some of its vital power; but on being driven through the lungs its colour is restored. All this provision, however, would not be sufficient, for the store of blood would soon be consumed, and the fabric would break down, if there was not a provision for fresh supplies. And we actually find that on its passage from the lungs to the heart the blood receives a supply of a new fluid extracted from the food by myriads of fine tubes which carry it to a larger one, that empties itself into a large vein, and being mixed with the blood is conveyed to the heart. We see, therefore, by the very imperfect survey which we have been able to take of this subject, that the animal man must necessarily be complex in his corporeal system, and in its operations. He must have one great and general system, the vascular, branching through the whole circulation ; another, the nervous, with its appendages the organs of sense, for every kind of feeling; and a third for the connexion and union of all these parts. Besides these primary and general systems, he requires others which may be more local or confined. One for strength, support, and protection; another for the requisite motion of the parts among themselves, as well as for moving from place to place, the muscular system ; another to prepare nourishment for the daily recruit of the body, the digestive organs; and others for the various purposes of existence.

QUESTIONS.-1. What are the uses of the blood ? 2 Describe the circulation of the blood. 3. Describe the arteries. 4. What changes does the blood undergo in the course of its circulation? 5. How is provision made for a fresh supply of blood ? [Note. That cavity of the heart from which the blood is driven into the arteries is called the left ventricle ; the next is called the right auricie ; the third the right ventricle ; and the fourth the left auricle.]

[blocks in formation]

The Human Voice. Epiglottis, a small and thin piece of cartilage, placed at the back of the tongue, and having the office of closing the glottis, when

the food is passing. The parts employed in the production of the voice are three in number, the trachea, or wind-pipe, by which the air passes to and from the lungs; the larynx, which is a short cylindrical canal at the head of the trachea ; and the glottis, which is a small oval opening between two semicircular membranes. The glottis being very narrow compared with the size of the trachea, the air can never pass through it without acquiring a considerable degree of velocity; so that the air thus compressed and forced on communicates, as it passes, a vibratory motion to the particles of the two lips of the glottis, which produces the sound. The sound thus produced is reverberated through the different parts of the mouth; and it is the mixture of different reverberations, well proportioned to one another, which produces in the human voice a harmony, which no instrument can equal.

The most wonderful part of the mechanism of the voice is the contraction and dilatation of the gloutis. It is these changes which produce all the variety of tone. The diameter of the glottis never exceeds one tenth of an inch: now suppose a person capable of sounding twelve notes—to which the voice easily reaches,—there must be the difference of the hundred and twentieth part of an inch for each note. But if we consider the subdivision of notes of which the voice is capable, the motion of the sides of the glottis appears still more minute. Suppose that a voice can divide a note into one hundred parts; it will follow that the different openings of the glottis will be twelve hundred in one tenth of an inch, and it is known that each of these will produce sounds perceptibly different to a good ear. But the movement of each side of the glottis being equal, it is necessary to double this number, and the side of the glottis, therefore, actually divides the tenth of an inch into twenty-four bundred equal parts.

Speech is articulated voice, that is, voice modified by the action of the palate, teeth, tongue, and lips. All animals

[blocks in formation]

have a voice, but man alone speaks in the sense now alluded to. Some animals, it is true, have been taught to pronounce a few words; but they express no thoughts by these sounds. It is believed that no sufficient reason can be drawn from mere organization, why man invariably should possess, and animals invariably want the power of speech. If we consider speech simply as a medium of the reciprocal .expressions of present feelings to the little society of citizens and friends of which we are a part, even in this limited view, of what inestimable value does it appear! To communicate to every one around us, in a single moment, the happiness which we feel ourselves,--to express the want, which we have full confidence, will be relieved as soon as it is known, or to have the still greater privilege of being ourselves the ministers of cornfort to wants, which otherwise could not have been relieved by us, because they could not have been discovered, --when the heart which we love is weighed down with imaginary grief, to have it in our power, by a few simple sounds, to convert anguish itself into rapture,--these are surely no slight advantages; and yet compared with the benefit which it affords to man as an intellectual being, even these are inconsiderable. By means of language, spoken or written, the opinions which are perishing in one mind, are rising in another; and often, perhaps, at the last fading ray of the flame of genius, that may have almost dazzled the world by excess of brilliancy, some star may be kindling, which is to shine upon the intellectual universe with equai light and glory.

Questions.-1. What are the parts employed in the production of the voice? 2. How is the sound produced ? 3. What is the most wonderful part of the mechanism of the voice ? 4. What is said of the divisions and subdivisions of the glottis in sounding twelve notes ? 5. What is speech? 6. What is said of the voice of animals ?

LESSON 120.

The Eur. Trun'cated, divided. Sen'tient, perceiving. The ear is adapted in an eminent degree to the purposes it is designed to execute ; and it offers an inviting subject

[blocks in formation]

us to

ideas ;

to such as are disposed to investigate the minute mechanism of an organ, which contributes remarkably to some of our most exquisite and refined enjoyments. Though the rapid glance of the eye, and the immense distance to which it enables us to carry our perceptions have given rise to some of our most pleasurable and magnificent sensations, still the sense which we are now considering has contributed most efficiently to the daily happiness of life. It enab hold communication with our fellow creatures; to improve and exalt our understandings by the mutual interchange of

and thus to increase the circle not only of our physical, but of our moral relations. The charms of eloquence and the pleasure resulting from the concord of sweet sounds are other sources of intellectual enjoyment, which contribute to place this sense among the most delightful as well as the most important we possess.

The organ of hearing, in its simplest form, consists of the expansion of a nerve, gifted with its peculiar sensitive qualities, over the surface of a delicate membrane. In man and the more perfect animals, there is an additional apparatus connected with this, the design of which is to collect and modify those pulses of the air which are finally to be impressed on the nervous membrane. In man this apparatus consists of a piece of cartilage, seated externally to the head, which contracts into a tube leading to the internal parts. The bottom of this tube is truncated obliquely, and its aperture closed by a firm membrane stretched across it, called the drum of the ear, which separates the external part from the succeeding, or middle purtion of the organ. Beyond, or on the opposite side of this membrane, we meet with a small cavity, hollowed out in bone. Of the several openings into it, there is one more particularly demanding attention. It is the internal aperture of a tube, the other extremity of which opens

behind and above the palate. By means of this communication, the external air is admitted into the cavity, and equipoises the weight of the atmosphere on the other side of the membrane. Across the cavity there is extended, though by no means in a straight line, a series of little bones, the exterior one of which is attached to the membrane we have just mentioned, the most internal set being firmly con nected with another membrane, which, in conjunction with it, shuts up the entrance to a still more deepened cavity,

« ForrigeFortsæt »