_I._ The most important are _contractibility_ and _sensibility_.

_E._ Let me advertise you that I am particularly hostile to technical words--all because I do not understand them, I allow, but please humor this ignorance by avoiding them.

_I._ And thus perpetuate your ignorance, my dear Ellinora? No; this will not do; for my chief object in these conversations is that you may be prepared to profit by lectures, essays and conversation hereafter. You will often be thrown into the company of those who express themselves in the easiest and most proper manner, that is, by the use of technical words and phrases. These will embarra.s.s you, and prevent that improvement which would be derived, if these terms were understood.

Interrupt me as often as you please with questions; and if we spend the remainder of the evening in compiling a physiological glossary, we may all reap advantage from the exercise. To return to the vital principles--vital is from _vita_, life--_contractibility_ and _sensibility_. The former is the property of the muscles. The muscles, you know, are what we call flesh. They are composed of fibres, which terminate in tendons.

_Alice._ Please give form to my ideas of the tendons.

_I._ With the muscles, they const.i.tute the agents of all motion in us.

Place your hand on the inside of your arm, and then bend your elbow. You perceive that cord, do you not? That is a tendon. You have observed them in animals, doubtless.

_Ann._ I have. They are round, white, and l.u.s.trous; and these are the muscular terminations.

_I._ Yes; this tendon which you perceive, is the termination of the muscles of the fore-arm, and it is inserted into the lower arm to a.s.sist in its elevation.

_E._ Now we are coming to it. Please tell me how I move a finger--how I raise my hand in this manner.

_I._ It is to the contractile power of the muscles that you are indebted for this power. I will read what Dr. Paley says of muscular contraction; it will make it clearer than any explanation of mine. He says, "A muscle acts only by contraction. Its force is exerted in no other way. When the exertion ceases, it relaxes itself, that is, it returns by relaxation to its former state, but without energy."

_E._ Just as this India-rubber springs back after extension, for ill.u.s.tration.

_I._ Very well, Ellinora. He adds, "This is the nature of the muscular fibre; and being so, it is evident that the reciprocal _energetic_ motion of the limbs, by which we mean _with force_ in opposite directions, can only be produced by the instrumentality of opposite or antagonist muscles--of flexors and extensors answering to each other.

For instance, the biceps and brachiaeus _internus_ muscles, placed in the front part of the upper arm, by their contraction, bend the elbow, and with such a degree of force as the case requires, or the strength admits. The relaxation of these muscles, after the effort, would merely let the fore-arm drop down. For the _back stroke_ therefore, and that the arm may not only bend at the elbow, but also extend and straighten itself with force, other muscles, the longus, and brevis brachiaeus _externus_, and the aconaeus, placed on the hinder part of the arms, by their contractile twitch, fetch back the fore-arm into a straight line with the cubit, with no less force than that with which it was bent out.

The same thing obtains in all the limbs, and in every moveable part of the body. A finger is not bent and straightened without the _contraction_ of two muscles taking place. It is evident, therefore, that the animal functions require that particular disposition of the muscles which we describe by the name of antagonist muscles."

_A._ Thank you, Isabel. This does indeed make the subject very plain.

These muscles contract at will.

_E._ But how can the will operate in this manner? I have always wished to understand.

_I._ And I regret that I cannot satisfy you on this point. If we trace the cause of muscular action by the nerves to the brain, we are no nearer a solution of the mystery; for we cannot know what power sets the organs of the brain at work--whether it be foreign to or of itself.

We will come now, if you please to _sensibility_, which belongs to the nerves.

_A._ I have a very indefinite idea of the nerves.

_E._ My _ideal_ is sufficiently definite in its shape, but so droll! I do not think of them as "being flesh of my flesh," but as a _species_ of the _genus_ fairy. They are to us, what the Nereides are to the green wave, the Dryades to the oak, and the Hamadryades to the little flower.

They are quite omnipotent in their operations. They make us cry or they make us laugh; thrill us with rapture or woe as they please. And, my dear Isabel, I shall not allow you to cheat me out of this pleasing fancy. You may tell us just what they are, but I shall be as incredulous as possible.

_I._ They are very slender white cords, extending from the brain and spinal marrow--twelve pairs from the former, and thirty from the latter.

These send out branches so numerous that we cannot touch the point of a pin to a spot that has not its nerve. The mucous membrane is--

_F._ Oh, these technicals! What is the mucous membrane?

_I._ It is a texture, or web of fibres, which lines all cavities exposed to the atmosphere--for instance, the mouth, windpipe and stomach. It is the seat of the senses of taste and smell.

_E._ And the nerves are the little witches that inform the brain how one thing is sweet, another bitter; one fragrant, another nauseous.

Alimentiveness ever after frowns or smiles accordingly. So it seems that the actions of the brain, and of the external senses, are reciprocated by the nerves, or something of this sort. How is it, Isabel? Oh, I see!

You say sensibility belongs to the nerves. So sights by means of--of what?

_I._ Of the optical nerves.

_E._ Yes; and sounds by means of the--

_I._ Auditory nerves.

_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_ b.u.mps. 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 pa.s.sage 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 pa.s.s 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 circ.u.mference; 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.

_Ann._ I have observed that when the mucus is dried by fever, food is nearly tasteless. I now understand the reason.

_E._ _Apropos_ to the senses, let me ask if feeling and touch are the same. Alfred says they are; I contend they are not, precisely.

_I._ Hayward thinks a distinction between them unnecessary. He says they are both seated in the same organs, and have the same nerves. But the sense of feeling is more general, extending over the whole surface of the skin and mucus membrane, while that of touch is limited to particular parts, being in man most perfect in the hand; and the sense of feeling is pa.s.sive, while that of touch is active. This sense is in the skin, and is most perfect where the epidermis, or external coat, is the thinnest. We will look through this little magnifying gla.s.s at the skin on my hand. You will see very minute prominences all over the surface. These points are called papillae. They are supposed to be the termination of the nerves, and the _locale_ of sensation.

_E._ Will you _shape_ my ideas of sensation?

_I._ According to Lord Brougham, one of the English editors of this edition of Paley, it is "the effect produced upon the mind by the operation of the senses; and involves nothing like an exertion of the mind itself."

Of the sense of hearing, I can tell you but little. Physiologists have doubts relative to many parts of the ear; and I do not understand the subject well enough to give you much information. I will merely name some of the parts and their relative situations. We have first the external ear, which projecting as it does from the head, is perfectly adapted to the office of gathering sounds, and transmitting them to the membrane of the tympanum, commonly called the drum of the ear, from its resembling somewhat, in its use and structure, the head of a drum. The tympanum is a cavity, of a cylindrical or tunnel form, and its office is supposed to be the transmission to the internal ear of the vibrations made upon the membrane. These vibrations are first communicated to the malleus or hammer. This is the first of four bones, united in a kind of chain, extending and conveying vibrations from the tympanum to the labyrinth of the ear beyond. The other bones are the incus, or anvil, the round bone, and the stapes, or stirrup--the latter so called from its resemblance to a stirrup-iron. It is placed over an oval aperture, which leads to the labyrinth, and which is closed by means of a membranous curtain. These bones are provided with very small muscles, and move with the vibrations of the tympanum. The equilibrium of the air in the tympanum and atmosphere is maintained by the means of the Eustachian tube, which extends from the back part of the fauces, or throat, to the cavity of the tympanum. The parts last mentioned const.i.tute the middle ear. Of the internal ear little is known. It has its semicircular ca.n.a.ls, vestibules, and cochlea; but their agencies are not ascertained.

The organ of smell is more simple. This sense lies, or is supposed to lie, in the mucous membrane which lines the nostrils and the openings in connection. Particles are constantly escaping from odorous bodies; and, by being inhaled in respiration, they are thrown in contact with the mucous membrane.

_A._ Before leaving the head, will you tell us something of the organs of voice?

_I._ By placing your finger on the top of your windpipe, you will perceive a slight prominence. In males this is very large. This is the thorax. It is formed of four cartilages, two of which are connected with a third, by means of four chords, called vocal chords, from their performing an important part in producing the voice. Experiments have been made, which prove that a greater part of the larynx, except these chords, may be removed without destroying the voice. Magendie thus accounts for the production of the voice. He says, "The air, in pa.s.sing from the lungs in expiration, is forced out of small cavities, as the air-cells and the minute branches of the windpipe, into a large ca.n.a.l; it is thence sent through a narrow pa.s.sage, on each side of which is a vibratory chord, and it is by the action of the air on these chords, that the sonorous undulations are produced which are called voice."

_E._ Do not the lips and tongue contribute essentially to speech?

_I._ They do not. Hayward says he can bear witness to the fact that the articulation remains unimpaired after the tongue has been removed. The l.a.b.i.als, _f_ and _v_, cannot be perfectly articulated without the action of the lips.--What subject shall we take next?