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property doubtless depends on its nervous connections, as we shall learn hereafter.

There being two hearts is designed to effect two circulations, and for this purpose each heart is furnished with two sets of vessels, arteries, and veins. The right heart only sends the blood to the lungs, by its arteries, and returns it by its veins to the left heart. This left heart sends the blood throughout the rest of the entire body, and returns it to the right heart, whence it is conveyed to the lungs again, for change and purification, before it is adapted to its offices in the circulation. Of course there is no direct opening from the right to the left heart, each being entirely separate from the moment of breathing: the former is said to be the organ of the lesser circulation, the latter that of the greater; the one through the lungs, the other through the body.

Having made this allusion to the lungs, I am obliged also to refer to the stomach. By the circulation, the blood, in performing its offices on the route over the body, is found to deteriorate both in quantity and quality, and hence the supply of its waste in either has to be supplied by the constant activity of the stomach, this being the great laboratory in which the material is prepared of which the blood is made: and from this organ a set of vessels are ever employed, with open mouths, in taking up this material, and, by a most intricate and surprising mechanism, conveying it to a returning vein near the heart, where it is mixed drop by drop with the blood, for the purpose of restoring to this fluid an equivalent for the loss it has sustained in every successive performance of its rounds in the circulation.

One of the most striking wonders of structure in the heart is found in the valves, of which mention has already been made. They are found in each heart, between its auricle and ventricle, opening like a gate, to let the blood pass one way, but closing effectually to prevent its passing the other. So also they are placed at the opening of vessels leading from the heart, and perform a similar office, preventing the regurgitation of the blood. But we are not at liberty to dwell bere, else the detail of the multiplied wonders of organization in the heart alone might furnish us with an ample topic for a lecture. But we are obliged to pass to the ARTERIES, next in order.

Strictly speaking, there is but one artery in the body, called the aorta, and all the others are its branches. Still, however, each of the branches is named and treated of, in the books, separately. The aorta is the large vessel which arises out of the left ventricle of the heart, and through which the blood is propelled into the thousand tortuous tubes which are found throughout the body. At first this large vessel ascends above the top of the heart, and then, by a graceful curve, it descends along the spine until it passes to the last bone of the spinal column, when it divides into two trunks, or large branches, to supply the inferior extremities. At the arch in the top of the chest, branches are sent off to the head and superior extremities. Every artery, and branch of an artery, which has any size, is named by the portion of the body, or the office it is to perform, or the organ it is destined to supply, and each artery consists of three coats or membranes, and is itself nourished by very minute arteries, called the vasa vasorum. These arteries, by their muscular power, contribute to the propulsion of the blood, and are auxiliary to the heart's action. Every artery, as it recedes from the heart, continues to subdivide, until the branches become too small to be visible, and terminating in the skin, where they meet with the corresponding termination of the veins, they are called capillaries.

These innumerable arteries are not mere passive tubes, like the hollow logs of an aqueduct, but are living organized muscular tissues, subject to disease and decay, and peculiarly liable, in old age, to be converted into bone. Their pulsation throughout their whole course of itself proves their contractile power, as well as their structure, which demonstrates it.

As already intimated, it is from the extremities of these arteries under the skin that the blood passes into another distinct series of vessels, which are called VEINS. These vessels accompany the arteries, throughout their whule course, and return the blood to the heart by means of a large vein called the vena cava, which empties into the right auricle. The veins differ in their structure from the arteries; they have no pulsation. With few exceptions, they have valves, on an average, every inch of their length, the office of which is to keep the blood from regurgitating, and in the arms, when a ligature is applied above the elbow, and the superficial veins are thus distended, these valves may be recognized by the knot formed in the course of the veins.

The manner in which the blood is propelled through the veins is not yet understood, though it is generally ascribed to some inscrutable agency exerted by the capillaries.

Having already explained the greater and lesser circulations carried on by the right and left side of the heart, it is only necessary to add that a corresponding series of arteries and veins are connected with the right ventricle of the heart, by means of which the circulation of the blood through the lungs is carried on. By this means, the blood returned by the veins from performing its office work in the greater circulation, is exposed to atmospheric influence in the air-cells of the lungs, and here undergoes the change already spoken of, which fits it for a renewed performance of its functions as before.

If the physiological view of this subject were not necessarily ex. cluded from consideration in our present notice of anatomical struc. ture, there are many points of intense interest in the circulation of the blood which are calculated to inspire surprise and admiration..

I dismiss this topic, however, and pass to the next, the BRAIN and

NERVOUS SYSTEM.

The brain is the term applied to the mass of nervous matter occupying the skull, and which very nearly fills the entire cavity of the head, and is composed of the greater and lesser brains, the cerebrum and cerebellum. The brain is also farther divided into two hemispheres, and into the anterior, posterior, and middle lobes, while its prolonga. tion down the spinal column is called the spinal marrow, from which, as well as the brain, an innumerable multitude of individual nerves, and cords of nerves, are given off, with admirable uniformity, to su pply the entire body with these instruments of sensation, and render every portion of the system capable of performing the offices assigned it in the economy of life.

The human brain is, without exaggeration, the most splendid and magnificent structure of mechanism in the material universe ; and although the protracted lives of many of the most enlightened men have been wholly expended in its study, yet the intricacies and wonders of its complicated organization compose a labyrinth which has never been fully explored. It is made up of an infinite congeries of delicate cords, and equally attenuated blood vessels, contained in a pulp of a character perfectly sui generis. It is so soft that, without its enveloping membranes, it would fall into fluidity by its own weight, and hence, after death, it very soon melts down into an oily fluid by early decay. This soft brain is covered, not merely in the head, but in its prolongation in the spinal marrow, as well as its still farther attenuation into individual nerves, until they terminate in so fine threads as to be invisible in the skin, by three several coats, called dura mater, pia mater, and tunica arachnoides, or hard, transparent, and soft coats. These constitute three several membranes between the skull and brain, and they also constitute three several tubes surround. ing the spinal cord, and every individual nerve, for all these are but elongations of the same pulpy matter of which the brain is composed. With what show of reason phrenologists prate about this soft, pulpy brain, by its developments molding the form of the head; and by its pressure, or mechanical action, accommodating them with bumps upon its surface indicative of individual character, can scarcely be esti. mated except by those who have consented to part, first, with their own brains. You will pardon this digression, as the thought involuntarily occurs to all who contemplate the structure of this delicate organ, in these degenerate days, when “philosophy has run mad.”

There is very much more in the structure of the brain and nerves, as well as their mysterious functions, upon which, under other cir. cumstances, we might enlarge; but we must forego it at present. Some idea may be formed of the extent and intricacies of this topic, when we learn that directly from the brain itself there are nine pairs of nerves, chiefly the nerves of sense, and expended on the eye, ear, nose, and tongue. The spinal cord, being itself

composed of three large filaments, sends off thirty pairs of nerves. All the branches thus pro. ceeding from the brain and spinal marrow are divided and subdivided into an inconceivable number of threads, of greater or less thickness, and distributed to the remotest portions of the body, until they are lost in the external or internal surfaces. These threads or cords min. gle together, at some points forming a plexus, and enlarge into little bulbs at other points, as in the ganglions ; or they cross each other, blending their fibres with others, and each one, however slender and hair-like, is found to possess some special function, and often more than one, wbile often within the same sheath is found a number of nerves, performing different, and even opposite offices. Nor can any distinction be discovered in the physical or chemical composition of the nerves, though one goes to the eye, and gives the capacity for vision; another to the tongue, and imparts that of taste ; a third to the nose, by means of which we have the sense of smell ; and a fourth to the ear, by which that organ is made sensible to sound. So also the nerves of sense, and those of motion, are found within the same sheath, and going to the same organ, or muscle, and yet they are often inseparable,

and of uniform structure. And yet, notwithstanding sensation is said to be a property of the nervous fibre, and it is demonstrated that the nerves are the organs of sense and of sensation, yet our difficulties on this mysterious subject are increased by the fact that the brain itself feels no pain, when wounded, and after fractures of the skull, very large portions of the brain have escaped, and been removed by the surgeon without inflicting pain, notwithstanding the brain is the very centre of sensation, as it is also the “ organ of the mind.” But these matters belong rather to physiology, and we forbear.

In pursuance of the subject of human organization, it may now be appropriate to notice, briefly, the contents of the TRUNK, having con. sidered those of the head, including the brain and its appendages. The trunk, in man, is divided into two parts, the chest and the abdomen, the upper and lower half of the body, by a horizontal muscle, convex on its upper surface, and concave on the lower, which is called the diaphragm. This muscle performs an important part in respiration, rising and falling with the inflation and collapse of the lungs. The upper part of the body, or thorax, is a cavity lined by a membrane or coat called the pleura, within which is contained the lungs, the heart, great arteries and veins of the heart, and various tubes, glands, vessels, and nerves, of vast importance to the living body. Some of these struc. tures have already been described, as the heart, lungs, large vessels, and nerves, as well as the walls of the chest, when speaking of the bones and muscles; and a more minute description is uncalled for here. But in the cavity of the lower part of the body are found the organs chiefly instrumental in digestion, as the liver, stomach, intestines, spleen, kidneys, and their numerous appendages. The Liver is the largest and heaviest organ of the body, and furnishes the bile, a fluid necessary to the functions of the stomach. The apparatus by which the blood is here converted into this fluid, and the tubes contrived with so admirable adaptation to the office of conveying it to its destined organ, are all topics of much interest, but cannot be here detailed. The liver occupies the right side, the spleen is found upon the left, in the middle the stomach is placed, while the other organs of this cavity fill up its inferior portion. On these, however, it is alike unnecessary and inadmissible to enlarge.

Having said thus much upon the contents of the three great cavities, including the internal organs of the head, chest, and abdomen, it remains briefly to contemplate the organs of the external senses, and the apparatus for the voice. And as we shall be limited to the structure of these several examples of mechanism, they need not consume much time.

The external senses are thus called to distinguish them from the internal senses of memory, imagination, conscience, and the passions. The external are seeing, hearing, smelling, tasting, and feeling.

The sense of vision has its seat in the expansion of the optic nerve, within the globe of the eye, on its posterior and internal surface. The construction of the visual organs is one of the most astonishing organ. izations in the human frame, and by reason of its numerous and complicated parts is very difficult to describe. The science of optics, with all that is known in relation to the laws of light, have been the result of imitations and modifications of the human eye, as are all the telescopes, microscopes, mirrors, &c., which have ever been invented. The difference, however, is apparent, since they are all receiving instruments, while the eye is a perceiving one, because inanimate in the one case, and animate in the other.

First we find the socket or orbit, for the eye, which is composed of portions of several bones, and this is lined with a deposit of fat or cellular substance, that the globe may always move with freedom. The prominence of the eyeball is thus occasioned, and its absorption, during long sickness, accounts for the sinking of the ball.

The globe is not a perfect circle, but is composed of two thirds of a sphere posteriorly, and a portion of a lesser circle laid upon it anteriorly, by which structure the sphere of vision is greatly extended. To move the ball of the eye six muscles are found attached to it within the orbit. The globe itself is composed of three principal coats, which fit within each other like a nest of boxes, or the brass tubes of a spy. glass. They are severally called sclerotic, choroid, and retina. Anteriorly we have the cornea, or window of the eye, which fits like a watch glass, and is transparent. On looking through this into the interior of the eye, a vertical partition is visible, which is black, blue, or hazel, in different persons, having a round opening in its centre.

This partition is called the iris, and the central orifice is called the pupil, which contracts or expands in proportion to the degree of light. Behind the iris is a second curtain, having also a pupil through it corresponding to the other, and this is called the ciliary processes. The folds of these resemble those of a ruffle, and it is found, upon a minute dissection, that there are no less than seventy folds in the hu. man eye, nicely laid over each other, and of equal width.

The humors of the eye are three, which distend the ball, and pre. serve its rotundity. Between the cornea and the iris we have the anterior chamber, and the transparent humor occupying it is called the aqueous humor, and this fluid also occupies the posterior chamber, which is immediately behind the iris, which seems to float in this hu. mor. Directly behind the pupil is found the crystalline lens, a transparent body resembling a button of transparent glass, in the axis of vision; it is convex on both sides, and this is covered by a capsule. Behind this, and filling the posterior part of the globe, is the vitreous bumor, much thicker than the aqueous, but yet fluid, resembling the white of an egg.

The optic nerve conveys to the mind the perception or sensation of the existence of external things. It proceeds from the brain, enters the globe of the eye, and is there expanded on its interior and posterior surface, constituting the retina.

It only remains to mention the transparent skin which covers the anterior part of the globe of the eye, and is reflected back upon the interior of the eyelids, and which constitutes what is called the white of the eye. This is called the conjunctiva. At its union with the external skin with the eyelids are found the eyelashes, a row of pro. jecting hairs, which serve to protect the eye from the approach of motes, or foreign bodies, when the eyelids close by a peculiar instinct, and in which the will has seldom any share. And within the orbit is found a small gland, from which the tears are ever flowing for moist. ening the surface of the globe; while the open mouths of ducts are

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