To show the Plan of the Larynx.—Cut from stiff paper four pieces of different shapes as indicated in Fig. 156. (The piece to the left should have a length of about six inches, the others proportionally[pg 369] large.) The largest represents the thyroid cartilage, the next in size the cricoid, and the two smallest the arytenoid cartilages. By means of pins, or threads, connect these with each other according to the description of the larynx on page 253. With this simple model the movements of the different cartilages and their effect upon the vocal cords may be illustrated.
To show the Relation of the Movements of the Vocal Organs to the Production of Different Sounds.—1. Lightly grasp the larynx with the fingers while talking. Observe the changes, both in the position and shape of the larynx, in the production of sounds of different pitch. 2. Observe the difference in the action of the muscles of respiration in the production of loud and faint sounds. 3. Pronounce slowly the vowels, A, E, I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape of the mouth, the position of the tongue, and the action of the lips in each case.
To demonstrate the Ear.—Examine a dissectible model of the ear, locating and naming the different parts. Trace as far as possible the path of the sound waves and find the termination of the auditory nerve. Note also the relative size of the parts, and calculate the number of times the model is larger than the natural ear. Suggestion: The greatest diameter of the internal ear is about three fourths of an inch.
In an extended course it is a profitable exercise to dissect the ear of a sheep or calf, observing the auditory canal, middle ear, bridge of bones, and the tympanic membrane with attached malleus and tensor tympanic muscle. Pass a probe from the nasal pharynx through the Eustachian tube into the middle ear. With bone forceps or a fine saw, split open the petrous portion of the temporal bone and observe the cochlea and the semicircular canals. By a careful dissection other parts of interest may also be shown.
CHAPTER XXII - THE EYE
Sight is considered the most important of the sensations. It is the chief means of bringing the body into proper relations with its surroundings and, even more than the sensation of hearing, is an avenue for the reception of ideas. The sense organs for the production of sight are the eyes; the external stimulus is
Light.—Light, like sound, consists of certain vibrating movements, or waves. They differ from sound waves, however, in form, velocity, and in method of origin and transmission. Light waves are able to pass through a vacuum, thus showing that they are not dependent upon air for their transmission. They are supposed to be transmitted by what the physicist calls ether—a highly elastic and exceedingly thin substance which fills all space and penetrates all matter. As a rule, light waves originate in bodies that are highly heated, being started by the vibrations of the minute particles of matter.
Light is influenced in its movements by various conditions. In a substance of uniform density it moves with an unchanging velocity and in a straight line. If it enters a less dense, or rarer, substance, its velocity increases; if one more dense, its velocity diminishes; and if it enters either the rarer or denser substance in any direction other than perpendicularly, it is bent out of its course, or refracted. If it strikes against a body lying in its course, it may be thrown off (reflected), or it may enter the body and either be passed on through (transmitted) or absorbed (Fig. 157). Light which is absorbed is transformed into heat.