E. Yes; convey impressions of externals to the brain. And "Upon this hint" the brain acts in its consequent reflections, and in the nervous impulses which induce muscular contractibility. And this muscular contractibility is a contraction of the fibres of the muscles. This contraction, of course, shortens them, and this latter must result in the bending of the arm. I think I understand it. What are the brain and spine, Isabel? How are they connected?
I. You will get correct ideas of the texture of the brain by observing that of animals. It occupies the whole cavity of the skull, is rounded and irregular in its form, full of prominences, alias bumps. These appear to fit themselves to the skull; but doubtless the bone is moulded by the brain. The brain is divided into two parts; the upper and frontal part is called the cerebrum, the other the cerebellum. The former is the larger division, and is the seat of the moral sentiments and intellectual faculties. The latter is the seat of the propensities, domestic and selfish.
A. I thank you, Isabel. Now, what is this spine, of which there is so much "complaint" now-a-days?
I. I will answer you from Paley: "The spine, or backbone, is a chain of joints of very wonderful construction. It was to be firm, yet flexible; firm, to support the erect position of the body; flexible, to allow of the bending of the the trunk in all degrees of curvature. It was further, also, to become a pipe or conduit for the safe conveyance from the brain of the most important fluid of the animal frame, that, namely, upon which all voluntary motion depends, the spinal marrow; a substance not only of the first necessity to action, if not to life, but of a nature so delicate and tender, so susceptible and impatient of injury, that any unusual pressure upon it, or any considerable obstruction of its course, is followed by paralysis or death. Now, the spine was not only to furnish the main trunk for the passage of the medullary substance from the brain, but to give out, in the course of its progress, small pipes therefrom, which, being afterwards indefinitely subdivided, might, under the name of nerves, distribute this exquisite supply to every part of the body."
Alice. I understand now why disease of the spine causes such involuntary contortions and gestures, in some instances. Its connection with the brain and nerves is so immediate, that it cannot suffer disease without affecting the whole nervous system.
I. It cannot. The spinal cord or marrow is a continuation of the brain. But we must not devote any more time to this subject.
Bertha. I want to ask you something about the different parts of the eye, Isabel. When —— —— lectured on optics, I lost nearly all the benefit of his lecture, except a newly awakened desire for knowledge on this subject. He talked of the retina, cornea, iris, &c.; please tell me precisely what they are.
I. The retina is a nervous membrane; in other words a thin net-work, formed of very minute sensitive filaments. It is supposed by some to be an expansion of the optic nerve; and on this the images of objects we see are formed. It is situated at the back part of the eye. Rays pass through the round opening in the iris, which we call the pupil.
B. What did the lecturer say is the cause of the color of the pupil?
I. He said that its want of color is to be imputed to the fact that rays of light which enter there are not returned; they fall on the retina, forming there images of objects. And you recollect he said that "absence of rays is blackness." The iris is a kind of curtain, covering the aqueous humor—aqueous is from the Latin aqua, water. It is confined only at its outer edge, or circumference; and is supplied with muscular fibres which confer the power of adjustment to every degree of light. It contracts or dilates involuntarily, as the light is more or less intense, as you must have observed. The rays of light falling on that part of the iris which immediately surrounds the pupil, cause it to be either black, blue, or hazel. We will not linger on this ground, for it belongs more properly to Natural Philosophy. We will discuss the other four senses as briefly as possible. "The sense of taste," says Hayward, "resides in the mucus membrane of the tongue, the lips, the cheeks, and the fauces." Branches of nerves extend to every part of the mouth where the sense of taste resides. The fluid with which the mouth is constantly moistened is called mucus, and chiefly subserves to the sense of taste.