The voice is produced by the modifications which the teeth, tongue, lips and throat make upon the sound produced by the vocal cords. The vocal cords are stretched across the upper end of the trachea, which is called the larynx. The air from the lungs is forced past them, setting them in vibration, thus producing sound.
The nervous system consists first of all of the brain. This is composed of nervous matter and constitutes the mass contained in the skull. It is divided into three parts: the cerebrum, the cerebellum, and the medulla oblongata. These three parts seem to preside over different functions. Thus, the cerebrum is the seat of the faculties of the mind, while the cerebellum presides over the muscular actions. The brain is connected with distant parts of the body by means of fibers which are distributed to all parts. These fibers are of two kinds, one set carrying the impressions to the brain, the other carrying the stimulus from the brain to the organs. The former are called sensitive, the latter motor nerves. A deep fissure separates the brain into two lateral halves, and these parts are connected with opposite sides of the body. Besides these nerves, which originate in the brain and pass through the spinal cord, there are twelve pairs that pass directly through the skull to the organs which they supply. These are called cranial nerves, and are distributed to the eyes, the nose, the ears, the larynx, the lungs, the face, stomach, etc. Ten of these pairs of nerves originate in the medulla oblongata. Of the functions presided over by these nerves may be mentioned those performed by the tongue, (taste), eyes, ears, and nose.
Many of these nerves are not sensitive, in the ordinary use of the word. Thus, the retina of the eye is the expansion of the optic nerve, and, while it is sensitive to light, it is not to ordinary impressions, such as material contact. Also, the nerve of the ear is only sensitive to the vibrations of fluids. We see by light reflected from objects. This light passes through a set of lenses, and by means of these an image is formed on the retina, which impression is carried to the brain. Just how all this is accomplished is not known. The nerve of the ear floats in a fluid called lymph. This fluid receives the vibrations of bodies through the air, through the membranes and chain of bones, and thus the nerves receive and transmit them to the brain, which act constitutes hearing.
Volumes could not tell all that one single fiber of muscle contains that is instructive, much less the entire functions, constructions, and mysteries of a single organ of special sense. And to perform all the allotted functions every part must be in the best repair. This constitutes health. Health is maintained by cleanliness, by repose, by muscular activity, by moderate eating, by plenty of fresh air, by a contented disposition, and a clear and active mind. Watch over your body with a jealous care, for all your future depends upon its good condition.
[SUNDAY READINGS.]
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SELECTED BY THE REV. J. H. VINCENT, D.D.
[April 1.]
THE LAW OF THE HOUSEHOLD.
By E. A. WASHBURN, D.D.
“Honor thy father and thy mother; that thy days may be long in the land which the Lord thy God giveth thee.”—Exod. xx: 12.