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opening in its centre, called the pupil. Immediately back of the pupil is the crystalline3 lens, composed of numerous layers or coatings, which increase in density toward the centre; an arrangement which prevents that spherical aberration, or too great dispersion of the rays of light, which it has been found so difficult to overcome in artificial lenses. Back of the crystalline lens, and filling a large part of the cavity of the eye,
, is the vitreous, or glassy humor, and spread over this is the thin and delicate membrane of the retina,+ which is the expansion of the optic nerve.
4. It is on the retina, where it concentrates at the back part of the ball to form the optic nerve, that the images of objects at which the eye looks, whether near or distant, are beautifully pictured or daguerreotyped. We can not look without wonder
the smallness yet correctness of these pictures. Thus a landscape of several miles in extent is brought into the
space of a sixpence, yet the objects which it contains are all distinctly portrayed in their relative magnitudes, positions, figures, and colors, with a fineness and delicacy of touch to which art can make no approach.
5. Yet the mechanical part of this apparatus—its beautiful structure, its perfect adaptation to the laws of light, and its ready adjustment to meet the ever-varying degrees of light, and shade, and distance—are far less wonderful than the mental or spiritual part, the manner in which the pictures on the retina are made known to the mind or soul within, through the medium of the optic nerve. The former is a mechanical wonder, of which we comprehend sufficient to excite our unbounded admiration; the latter is a spiritual mystery, of which we know nothing but the bare fact itself.
6. Mr. Addison, in a number of the Spectator, has drawn a much-admired picture of the sense of sight, in the introduction to the first of his celebrated Essays on the Pleasures of the Imagination. We select the opening passages, which Mr: Blair so highly commends for their rhetorical grace and beauty.
7. “Our sight is the most perfect and most delightful of all our senses. It fills the mind with the largest variety of ideas, converses with its objects at the greatest distance, and continues the longest in action without being tired or satiated with its proper enjoyments.
8. “The sense of feeling can indeed give us the idea of extension, figure, and all the other properties of matter which are perceived by the eye except colors; but, at the same time, it is very much straitened and confined in its operations with regard to the number, bulk, and distance of its objects.
9. “Our sight seems designed to supply all these defects, and may be considered as a more delicate and diffusive kind of touch, that spreads it
self over an infinite multitude of bodies, comprehends the largest figures, and brings into our reach some of the most remote parts of the universe.
10. “ It is this sense which furnishes the imagination with its ideas : and by the pleasures of the imagination or fancy (terms which I shall use promiscuously) I here mean such as arise from visible objects, either when we have them actually in our view, or when we call up their ideas in our minds by paintings, statues, descriptions, or other similar means.
11. “We can not, indeed, have a single image in the fancy that did not make its first entrance through the sight; but we have the power of retaining, altering, and compounding those images which we have once received, and of forming them into all the varieties of picture and vision that are most agreeable to the imagination; for, by this faculty, a man in a dungeon is capable of entertaining himself with scenes and landscapes more beautiful than any that can be found in the whole compass of nature.” 1 LŪ'-BRI-CĀ-TED, made smooth or slippery 3 Crys'-TAL-LINE, clear; resembling crystal. by moisture.
4 RĚT'-I-NA, plural rět'-i-no. HU-MOR, (Jū-mor, or hū'-mor).
LESSON II.-THE LIVING TEMPLE.]
OLIVER WENDELL HOLMES. 1. Not in the world of light alone,
Where God has built his blazing throne',
Eternal wisdom still the same'!
Flows murmuring through its hidden caves, a
From the warm fountains of the heart.
Forever quivering o'er his task,
Creeps back to find the throbbing heart.
Behold the outward moving frame';
And linked to reason's guiding reins10
Which claims it as the Master's own. 5. See how yon beam of seeming white
Is braided out of seven-hned light;13
With music it is heaven to hear. 6. Then mark the cloven sphere16 that holds
All thought in its mysterious folds;
Along its hollow glassy threads !20 7. O Father'! grant thy love divine
To make these mystic temples thine'!
And mould it into heavenly forms ! [A full explanation of the foregoing exquisitely beautiful verses would lead to a general review of the entire subject of Physiology. Every pupil should give as much explanation, at least, as is contained in the following notes.]
1 The human frame.
3 The blood, by being purified in the lungs, is changed from a dark purple to a light crimson hue. See Fourth Reader, p. 48.
4 “ Ebbing current"—the expired air. A great portion of the decayed and worn-out particles of the body are thrown out from the lungs in the form of carbonic acid and vapor. See Fourth Reader, p. 50.
5 The heart. See Fourth Reader, p. 51.
6 6 Woven net"—the net-work of veins and capillaries. See Fourth Reader, p. 51 and 60.
? The blood supplies new material to all parts of the body, and bears back to the lungs the decaying and worn-out particles.
8 The warm blood is often spoken of as the flame of life.
9. Tendons, cords, and sinews knit the "marbles," or bony frame-work strongly together, as with thongs.
10 The frame-work of the body is linked to the "guiding reins," or the mind, by those “ trembling chains," the nerve tubes, or nerve fibres. See page 90.
11 All the tissues of the body are formed primarily of cells. Cells, opening, form rings; and these rings unite, in certain cases, to form nerve fibres. Hence these nerve fibres may well be described as “myriad rings in trembling chains."
12 The threaded zone," or hollow of each nerve fibre, contains a fluid substance like that found in the brain itself.
13 The seeming white" light is made up of the seven primary, colors, red, orange, yellow, green, blue, indigo, and violet. 14 The balls of the eye, through whose humors the rays of light pass to the retina.
15 The air, whose vibrations in the chambers of the ear give the sensation which we call sound.
16 “ Cloven sphere"-the two hemispheres of the brain; the seat of "all thought." See 17 That receives impressions through the nerves of feeling," or sensory nerves. See 18 That sends forth its commands through the motor nerves. See p. 90. 19 The nerve-cells, forming the gray substance of the brain. See p. 92. 20 " Glassy threads"—the nerve threads or nerve fibres.
LESSON III.-THE BRAIN: THE NERVES OF VOLUNTARY
MOTION AND THE NERVES OF FEELING.
1. In that part of the Fourth Reader which was devoted to “Human Physiology and Health,” we treated of the bones', and the injuries to which they are liable'; of the muscles', and the laws of their healthy action'; of the organs that prepare nourishment for the body', their proper treatment', and the variety of ways in which we too often abuse' them; of the organs of circulation and of respiration, and their mysterious workings'; of the skin, its uses and abuses, and its complicated mechanism'; of the phenomena of growth and decay, of life and death'; and generally' of the laws of health which depend upon the harmonious action of all the bodily organs. The functions of which we treated were those chiefly of organic life, which, to a certain extent, are common to both plants and animals; for both live and grow, decay and die, through organic processes that are essentially alike. have before stated, the microscope has shown, in a most striking manner, that vegetables and animals are alike constructed of cells.
2. But the parts and functions which we have described are, in all animals, subordinate to the NERVOUS SYSTEM, the higher department of animal physiology, to the study of which we shall devote several of the lessons in the present Part of this volume. It is through this system that all governing power is exerted in the body'; that the muscles are made to move', and the blood to flow; that respiration and digestion are carried on'; that growth is regulated', and every action directed in the thousand mysterious processes of life'; and it is through the same channel also that the mind derives sensations and perceptions from, and holds communion with, the external world. It is also found that, throughout all animal life, from the lowest grades up to the highest, the degree of intelligence bears a close relation to the degree of development of the nervous system.
3. What, then, is this nervous system in man, that ranks so
high above every other in the body as to be the direct agent on which all the functions of life depend, and which, in our mortal state, is the immediate minister and messenger of the mind, and of the principle of life itself'? It is a brief and easy answer to say that this nervous system consists of all the nerves in the body, of which the chief bundles or masses are the brain, and the spinal marrow, and several other small nervous bundles called ganglia. But to explain the functions of these is a more complicated matter; and their study will be found to have an intimate connection with the study of mind itself, or mental philosophy.*
4. The brain is that large organized mass which, with its enveloping membranes, completely fills the cavity of the skull. It is divided vertically nearly into two halves by a deep fissure or cleft, as is seen in the illustration, Fig. 2, given below; and its surface is singularly roughened by elevations and depressions, which have the appearance of folds closely crowded upon each other. The chief mass of the brain is called the cerebrum,' or great brain; below, and somewhat back of this, is the cerebellum,2 or little brain; and connected with and proceeding from both is the spinal cord, or spinal marrow, which extends downward through the spinal column or
Fig. 2. This engraving represents the
appearance of the upper surface of the UPPER SURFACE OF THE BRAIN.
brain, after its covering, the skull, has been removed. The figures 1, 1, show the anterior or front lobes, and 2, 2, the posterior lobes; while from 3 to 3 extends what is called the great median fissure, which divides the brain into two hemispheres, or halves. The figures 5, 5, point to what are called the anterior parietal convolutions; 6, 6, to the posterior; 7,7, to the rudimentary; 8, 8, to the frontal convolutions; and 9, 9, to the occipital. Not only is the brain a double
organ, sending forth its nerves by pairs, 4 but the same symmetrical doubleness is
continued throughout the whole nervous system. It is believed that each half of the brain can act separately, but that both can best act simultaneously.
“While it is true that any unusual and healthy development of brain is attended with correspondingly increased mental powers, yet in this we must not overlook the merely instrumental nature of the organ, Though imperfections in it may produce a manifest inferiority, that inferiority is by no means to be referred
to the intellectual principle itself. The if that instrument becomes imperfect the action becomes imperfect too." —DRAPER.
mode of action being by an instrument, * The subject of Mental Philosophy will be taken up in the Sixth Reader.