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12 Tlie Eye.
the latter—enlarges the sphere of intellect, and adds so largely to our stock of pleasurable sensations, is in its structure less com-' plex than might at first be supposed. It consists of two distinct parts—the ball of the eye, or the proper organ of vision, and the accessary apparatus, destined to shield.it from injury, to preserve it in a fit state for the performance of its functions, and to produce the various movements of which the ball of the eye is susceptible. It is the former only, into the description of which we propose at present to enter, reserving the latter for a future occasion.
The ball or socket of the eye, as all are aware, is situated in a deep bony socket, and thus secured from a thousand accidents, to which it would otherwise be every moment liable. The socket is well lined with fat, which serves both as a soft bed for the eye ball, and, also, to facilitate its motions. When this fat diminishes, as in sickness, the eye sinks back in the head. The ball of the eye is considerably smaller than the socket, to allow room for its motions: it is not perfectly globular, but is in some degree elongated, and is composed of different coats, somewhat like the coats of an onion, which enclose several humours. Externally is a hard, opaque, pearly or bluish white coat, called sclerotica; this forms what is termed the white of the eye. This coat is covered with a fine transparent membrane, continuous with that which lines the eye lids. Within the sclerotic coat, and in close contact with it, is the middle coat of the eye, which is thin, rather soft, and so full of minute blood-vessels, that its inner surface has a fleecy or velvet-like appearance, from their numerous terminations. This is called by anatomists the choroid coat. The innermost coat of the eye is a white and extremely tender film or net work, formed by an expansion of the optic nerve, or nerve of vision. It is called the retina. This description applies, however, only to the posterior and lateral parts of the eye ball, for in front, instead of the opaque exterior coat, we observe a projecting transparent circular part, which from its being firm and transparent, like horn, fs called the cornea. This portion is somewhat more convex than the sclerotic coat, representing the segment of a smaller sphere added to the segment of a greater ; or, in more familiar language, it may be compared to a convex watch glass, fixed in the less convex surface of the watch case. . *
The connexion between the external and middle coats of the eye is very slight, depending upon a fine cellular membrane, and very small blood-vessels and nerves; except around the margin of the cornea, where these coats are firmly connected together by means of a white ring, called the ciliary ring or ligament. The middle coat proceeds anteriorly to this ring, but no longer lines the internal surface of the shell of the eye.—It is now reflected inwards, in a transverse direction, as if to form a parThe Eye. 13
tition, and is plaited or laid in beautiful folds like the ruffle of a shirt. '"?" **
Behind the transparent cornea, is one of the most beautiful parts of the eye, which is fancifully called iris or rainbow, from its varied colour. It is this which, in some eyes, is blue, in others dark brown, or nearly black, and in others hazel or gray. It is a delicate and very sensible^ membrane, attached at its circumference to the ciliary ring and dividing the eye ball into two chambers the one anterior to it, the other posterior. The iris has a round opening in its middle called the pupil, or sometimes, vaguely, the sight of the eye. It is this which produces the dark circle seen in the centre of the eye, varying in size according to the degree of light to which the latter is exposed. It is invariably deep black, whatever may be the colour of the iris itself, so long as the eye is free from disease. Through the pupil the rays of light are admitted into the internal parts of the eye. The dark colour of the pupil is caused by a black paste or pigment which is spread oyer the internal surface of the middle coat of the eye. This paint differs in colour in various animals. In the ox it is green, in the cat and owl white and silvery, in the lion golden yellow, in the dog grayish. -In the peculiar race of men called albinos, as well as in ferrets, white mice, rabbits, and pigeons, the paint is altogether wanting, and the pupil appears red, from the blood contained in the numerous vessels of the choroid coat. ■ The use of the paint'is evidently to modify the intensity of the light admitted into the eye. White and pale colours reflect light, while black and deep colours absorb it; hence animals which prey in the dark, have this paint of a paler colour than in man,, in whom distinct vision, in a full light, is a more useful faculty, than the power of distinguishing objects when the light of day is excluded. The eye of the cat, especially, concentrates all the light which falls upon it, the white paint reflecting it back on the objects near them: hence their eyes are observed to gleam in the dark, becoming in seme measure a torch to light them to their prey. Upon the posterior surface of the iris, is, also, spread a paint upon which its colour depends: it is usually brown even in light-coloured eyes. The different shades of colour in different individuals, result from the degree of transparency possessed by the iris.
The iris is capable of expanding and contracting in such a manner as to lessen the size of the pupil on the approach of a strong light, and to enlarge it in proportion as the light is less vivid. By this beautiful yet simple contrivance, the eye adapts itself instantaneously to the different degrees of light to which it may be exposed. Were the pupil to remain always as much contracted as it is when exposed to the light of noon day, a weaker light, as that of the moon, would not be admitted with sufli14- The Eye.
dent freedom to allow of distinct vision. On the contrary, if the pupil were permanently dilated, we might take advantage of the scattered rays of light, but should be distressed and blinded by the glorious effulgence of the sun.
The ball of the eye is filled with three substances, which differ from each other in consistence, 'but are all called humours of the eye; they are the aqueous, vitreous, and crystalline.
The aqueous or watery humour, which is a perfectly limpid fluid, fdls the space between the iris and cornea, or the anterior chamber of the eye. Its use appears to be to distend the cornea, and preserve its convexity: it likewise affords a medium for the iris- to float in, so that it may perform its motions with perfect freedom. The watery humour accordingly fills the openingof the pupil, and a small quantity of it also lies behind the iris. The portion of the eye behind the iris is much the larger one, and is chiefly filled with a transparent fluid, which, from its appearance, is called the ritfeous or glassy humour; resembling very nearly melted glass, or the white of an egg. There is, besides, a small round transparent body, set in the front of this humour, like a diamond in a ring, immediately behind the pupil. This is the crystalline humour or lens. The glassy humour does not float freely like the watery, but is contained in a very transparent membrane, which is so arranged as to form innumerable little bags, each one containing a drop of the fluid, and so perfectly transparent as not to break the course of a single ray of light. The uses of this humour would appear to be, to keep the ball of the eye distended to the size necessary for the purposes of vision, and to retain the crystalline lens at the proper focal distance. This latter body, as has been already mentioned, is placed immediately behind the pupil, in a cup-like depression of the glassy humour. In form, it is like a very small, thick spectacle glass, or the doubly convex lens of a spy glass. It is composed of very transparent scales, laid one over the other.- In the centre of the lens, these scales lie closer together than they do nearer the surface, forming a kind of firm button, which will leap out if the outer scales be divided. The whole lens is surrounded by a strong, thick, transparent and elastic skin or capsule. The use of the crystalline lens is to concentrate the rays of light, proceeding from objects in the field of vision, so as to form a distinct image of them at the bottom of the eye upon the retina, or nervous coat. From this last, through the medium of the optic nerve, the image is conveyed to the brain.
The lens becomes occasionally opaque, constituting the disease termed cataract. Sight is of course prevented, and on looking into the pupil, it is perceived to be cloudy, gray or white, and not deep black, as in the healthy eye.—The foregoing description
Proximate Principles of Vegetables.
will be better understood by the following drawing, which represents a longitudinal section of the eye ball.
PROXIMATE PRINCIPLES OF VEGETABLES.
A Rrief summary of the nutritive elements of vegetable bodies will, we have reason to believe, be acceptable to our readers, and prepare them for a better understanding of the subjects of aliment, and nutrition in general.
Every distinct compound which exists ready formed in plants, is called a proximate or immediate principle of vegetables. Thus sugar, starch, and gum are proximate principles; "The proximate principles of vegetables are sometimes distributed over the whole plant, while at others they are confined to a particular part. The methods by which they are procured are very variable. Thus gum exudes spontaneously, and the saccharine juice of the maple tree is obtained by incisions made in the bark. In some cases, a particular principle is mixed with such a variety of others, that a distinct process is required for its separation. Of such processes, consists the proximate analysis of vegetables. Sometimes a substance is separated by mechanical means, as in the preparation of starch."*
The chief nutritive principles in vegetables, are gluten, starch or fecula, sugar, gum, and oil, on the quantity and different preparations of which depend their alimentary properties. The first of these principles may be exhibited in the familiar process of making starch from wheat flour. Thus, if we take a paste of moistened flour, enclosed in a piece of linen, and pour water on it, this fluid carries off a part of the flour, and leaves, by repeated washings and squeezing between the fingers, nothing behind but a tenacious mass, called gluten. The water has carried off the fecula or Btarch, which gradually falls to the bottom of the vessel, and a small proportion of sugar and gummy matter which are dissolved in it. The difference in the nutritive properties of grains and esculent roots is explicable, as we have just said, by the various proportions of these proximate principles. In many of the nut tribe, oil is the most abundant principle. In the following table, the figures are expressive of so many parts in the hundred.
70 to 80
Turner's Chemistry—edited by Franklin Bache, M. 6. Philadelphia
History of the Indian Cholera.
Davy makes the proportion of gluten in wheat, to be from 19 to 24 per cent, whilst Vauquelin and Henry, of Paris, find the flour of the Paris bakers to be 10.20 to 10.25 per cent. The difference, therefore, is very great in the varieties of wheat.
The reader who is aware of the nutritive properties of the potato, will be astonished at the small proportion of fecula which it contains, and also at this being the only proximate principle of an alimentary nature. There is another principle, the, woody fibre, or lignin, in the potato, which is in a great measure insoluble in the stomach: it is found in the proportion of from ten to fifteen per cent. In these experiments, the potato is supposed to have 'been peeled.
In the proximate analysis of barley, chemists discover a peculiar principle, called hordein, in the proportion of fifty-five per cent.; but it so nearly resembles fecula that we have classed it under the same head with this latter, which, pure, is represented as constituting thirty-two per cent, of this grain. For the same reason, we have represented sago and tapioca as consisting nearly all of fecula or starch, whereas, in rigid chemical classification, it should be amidine, Or starch modified by dry heat or by boiling water.
The detailed properties of these several proximate and nutritive principles will form the subject of future notices.
HISTORY OP THE INDIAN CHOLERA.
The epidemic cholera, which is now prevailing throughout the north of Europe, adding the horrors of pestilence to those of war, has become a subject of deep interest to every portion of the civilized world. However remotely situated from the present theatre of its ravages, there is no country the inhabitants of which can flatter themselves that they are perfectly secure from its visitation. Travelling, as it does, with an unexampled rapidity; unarrested in its progress by mountains or seas; prevailing alike in the driest weather and during the deluge of periodical rains; in storms and in calms; under the scorching sun of Arabia and amid the frosts of Russia, to determine the point or period at which its further extension will be stayed, defies all human calculation. Commencing at a point almost the farthest removed from ourselves, it has already traversed nearly half of the mighty interval, and, however improbable it may appear to some, that the disease will ever extend across the Atlantic; yet, judging from its progress heretofore, and the space over which it has already passed, this is by no means impossible. The only certain means of preventing the cholera from extending itself to this country, are a timely resort to correct precautionary measures founded upon the nature of the disease and the general principles of hygiene. With this fact in view, we have conceived it to be our duty to lay before our readers a brief sketch of the rise and progress of the pestilence, and, so far as we are able, from the documents in our possession, some account of its character and causes, together with the proper means for its prevention.
The terrific epidemic which is now ravaging the immense districts of the Russian empire, commenced its destructive career in various parts of the delta formed by the Ganges, during the summer of 1817. In 1818, it had reached as far north as Saharunpore, high up on the Jumna, and extended