1 And linked to reason's guiding reins1o 5. See how yon beam of seeming white Wakes the hushed spirit through thine ear 6. Then mark the cloven sphere1 that holds To make these mystic temples thine'! [A full explanation of the foregoing exquisitely beautiful verses would lead to a gen eral review of the entire subject of Physiology. Every pupil should give as much expla uation, at least, as is contained in the following notes.] 1 The human frame. 2 The air-cells of the lungs. 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" Woven net"-the net-work of veins and capillaries. and 60. See Fourth Reader, p. 51 7 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. 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 p. 89. 17 That receives impressions through the "nerves of feeling," or sensory nerves. p. 91. 20 18 That sends forth its commands through the motor nerves. 19 The nerve-cells, forming the gray substance of the brain. Glassy threads"-the nerve threads or nerve fibres. See p. 90. See 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. As we 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 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 6 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 mode of action being by an instrument, if that instrument becomes imperfect the action becomes imperfect too."-DRAPER. *The subject of Mental Philosophy will be taken up in the Sixth Reader. back-bone. In the engraving at the foot of page 91 is represented an internal side view of the right half of the brain. 5. The brain is composed of a soft jelly-like substance, very much like the marrow which is found in bones. The interior portion, which is of a white color, is composed of exceedingly minute tubes, which are the beginnings of the nerves. These little nerve tubes are exceedingly minute; but where they start from the brain a bundle of them is generally inclosed in a sheath; then this bundle is divided and subdivided, branching out in smaller and smaller divisions, until each little nerve tube is connected with some one muscular fibre or some one sensitive point in the body. Each of these little tubes, although sometimes too small to be distinctly seen by the microscope, is supposed to be entirely separate from its fellow nerves, and unconnected with them from its beginning to its termination. 6. We will now explain the uses of these little nerve tubes, of which there are millions in the body, and we will do it by supposing a particular case in which they are used. If the mind wishes to move the right hand it sends a message downward from the brain along the course of the spinal cord in the neck, and thence down along the arm through a bundle of the nerves that run to the hand. If the mind wishes the Fig. 3 represents the base of the brain as seen from below, together with some of the double sets of nerves branching from it. Here the figures 1, 1, represent the anterior or front lobes, and 3, 3, the lobes of the cerebellum, which lies at the base of the back part of the skull. From 4 to 18 is the line of the median fissure. Figure 13 points to the medulla oblongata, or severed portion of the upper part of the spinal marrow; 22, to one of the olfactory nerves, or nerves of smell; 24, to the beginning of the pair of optic nerves, or nerves of sight; while 25 and 28 point to one, each, of pairs of oculomotor nerves, or nerves that direct the 12motions of the eyes; 27 and 29 to nerves that move the muscles of the face, and 30 to the auditory nerves, or -14 nerves of hearing. The doubleness of the nerves is here well illustrated; and this is the characteristic of the whole nervous system, the cranial and spinal nerves coming forth by pairs to their distribution on the right and left sides of the body. The object of this arrangement is evidently to increase the precision of nervous action, and to compensate readily for any incidental defects. whole hand to move, it sends the message through all the nerves that run there, and one tells the little finger, another the fore-finger, and another the thumb, etc., what to do. If the mind wishes the fingers to strike the keys of the piano, it tells each finger, through its own nerves, what key to strike. These nerves, through which the mind sends out its commands to the fingers, and to every muscle in every part of the body, telling each when and how to act, are called motor nerves, or nerves of motion. If the bundle of motor nerves running to the hand should be cut in two, the hand would not move! And why'? Because it could receive no command from the brain; and it will not move without orders from head-quarters. 7. But there is another set of nerves running to the hand besides the nerves of motion. What if the fore-finger had been directed, in the dark, to touch a certain key of the piano, and had chanced to press upon the sharp point of a needle or the blade of a knife! How could the mind be informed of the danger? Not by the nerves of motion, for their only office is to convey commands outward from the brain. Another set of nerves is needed, and such has been provided in the nerves of feeling. As soon as the finger presses upon the needle's point, some of these numerous and minute nerves of feeling are pierced, and instantaneously they convey the intelligence to the brain. As quick as thought, a command is then sent down, through the nerves of motion, to withdraw the finger, if possible, before any serious injury is received. If the bundle of nerves of feeling running to the hand should be severed, the finger might be pierced through, or cut off, Fig. 4 represents an external side view of the right half of the brain. At 1 is seen the medulla oblongata, or beginning of the spinal marrow; at 3, a side view of the cerebellum, or little brain. The chief office of the cerebellum is believed to be that of combining, regulating, and directing all the muscular movements. From experiments on animals, it is found that when it is cut it gives rise to neither motion nor sensation, but when it is removed in slices the animal gradually loses all power of regulating its motions. The weight of the cerebellum is about one eighth of that of the cerebrum. The average weight of the entire brain in man is about fifty ounces; in females about forty-five. |
