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 passive, 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 glass at the skin on my hand. You will see very minute prominences all over the surface. These points are called papillæ. 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 constitute the middle ear. Of the internal ear little is known. It has its semicircular canals, 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 passing 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 canal; it is thence sent through a narrow passage, 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?