SHAPE OF THE PHARYNGEAL PORTION.

297

thin and delicate on all its edges, as it occupies the chamber of the pharynx, and is subject to constant muscular movement. The sides are rolled slightly upward, while the posterior end is curved downward. The inferior portion, marked D D, should reach only to the base of the uvula, and bridge directly across the chasm at that point; and no effort to imitate the uvula should be made. The extreme posterior end should not reach the posterior wall of the pharynx by a quarter of an inch when all the muscles are relaxed (although subsequent use must determine whether to increase or diminish this space), thus leaving abundant room for respiration and for the passage of nasal sounds.

In cases where it is desirable to make the instrument as far as possible independent of the teeth for its support, the anterior part which occupies the apex of the fissure in the hard palate may lap over upon the floor of one or both nares. Such a projection is seen in Fig. 194, marked B, and like processes are seen in Figs. 189 and 190, marked EE. Were it not for this process in the first case, the palate would drop from the fissure into the mouth, the single clasp at the extreme anterior edge not being sufficient to keep the whole appliance in place throughout its entire length. Caution must be exercised that this projection entering the nares be not too large, or it will obstruct the passage, and give a disagreeable nasal tone to the, voice.

The end of the artificial palate should not come against the pharynx at a right angle, but rather obliquely. Consequently, if the palate is placed at a higher level, a shorter instrument will reach across; while, if it be placed at a lower level, a much longer instrument will be required. Both extremes are objectionable. The longer palate is liable to interfere with deglutition and to split and misdirect the column of sound in its outward passage, while the shorter one will affect the quality of the voice as well as make articulation more difficult. The best form is that which follows the border of the fissure from its apex down to the junction of

298 REASONS FOR THE PECULIAR POSTERIOR FORM.

the uvula, with the palate, and then diverges across the pharyngeal passage, as seen in Figs. 191 and 195. There is no difficulty in discovering the line of departure from the fissure; the uvula, which is bifurcated or rather appears double, one on each side, is always strongly pronounced in its bulbous form, and its junction with the palate is always distinctly marked.

When the fissure is filled or bridged across down to this point, the instrument here forms the superior boundary of the fauces, which theretofore was without boundary because of the fissure. Theoretically it would seem that the artificial palate should stop at this point, and not continue at a different angle across the pharynx; but experience has shown the necessity for elongating it. The reason for this elongation is, that distinct articulation of the voice at times requires the passage of sound to be entirely cut off from the nasal cavity, while at other times the sound must escape in that direction. This cutting off of the nasal passage for sound is accomplished by the simultaneous action of two sets of muscles: 1. The levator muscles lift and in a sense carry back the artificial palate. 2. The constrictors of the pharynx bring forward or contract the pharyngeal wall. This is the physiological action of the palatal and pharyngeal muscles in the articulation of the voice. With an artificial palate filling a congenital cleft and terminating at the uvula, it will be found that when it is raised to the utmost power of the levatores, and the pharynx is contracted or brought forward to its utmost, there is still a gap behind the instrument for the escape of sound, and for this reason the extension must be made. As an approximate guide for this length it may be stated that if the artificial palate is of the form described and placed in its best position, the palatal and pharyngeal muscles being all relaxed, there should be a space of about a quarter of an inch between its posterior edge and the wall of the pharynx. But the final test of this must be when the completed palate is introduced.

OBJECTIONS TO PLASTER FOR PALATE MOLDS.

299

All the peculiarities described must be provided for in the gutta-percha model, which, after having been carefully formed upon the cast, may be tried in the mouth to ascertain its length or necessary variations. When its ultimate form. has been determined, provision must be made to duplicate it in soft rubber. A familiar illustration of the process here to be adopted is found in the parallel process employed when a set of teeth is made on a vulcanite base. A model form is made of wax or gutta-percha, bearing the teeth, and in all its prominent characteristics has the shape desired in the com

pleted denture, the rubber duplicate being vulcanized in a plaster mold. In like manner the rubber duplicate of the palate, as before described, may be made in a plaster mold.

If plaster is used for the molds, it must be worked so that the surface shall be free from air-bubbles, or the rubber palate will be covered with excrescences that can not readily be removed. But ordinarily plaster molds will be found too troublesome for general use. They may be put to most excellent use, however, by using one to make a duplicate of the gutta-percha in hard rubber. This is not necessary with

300

SECTIONAL PALATE MOLD.

those who have had much experience, but with beginners it will be difficult to work up the gutta-percha as nicely as may be desired. A duplicate in vulcanite will enable the operator to make a more artistic model of the palate, and one which can be handled with greater freedom.

As in the course of a lifetime a considerable number of elastic vela will be required, the mold which produces them should be made of some durable material. The type-metal of commerce is admirably adapted to this use. A very complete mold is one made of four parts, such as illustrated by Fig. 204, which will produce a palate in one continuous piece.

The blocks C C are accurately adapted to the body of the mold marked A, and are prevented from coming into inaccurate contact with each other by the flanges D D, which overlap and rest upon the sides of the main piece. B shows the top of the mold, and the groove E provides for the surplus rubber in packing. Such a mold makes as perfect an appliance as can be produced. The palate is one homogeneous and inseparable piece. The cut sufficiently indicates the form of the several parts. Each of these pieces is first made in plaster, having exactly the form desired in the typemetal. They are then molded in sand, and the type-metal cast, as in making an ordinary die for swaging. When in

PALATE MADE IN SECTIONS.

301

use a clamp similar to Fig. 205 is placed around the mold to keep the several parts firm in their position.

Such a mold requires nice mechanical skill in fitting all the parts accurately, and, unless the operator has had experience in such a direction, it is better to simplify the matter. By making the palate in two pieces, to be joined after vulcanizing, as shown in Fig. 206, the mold may be made in two pieces and with much less trouble. Fig. 207 shows such a mold, made also of type-metal, but inclosed in a flask for greater convenience. The flasks are made of brass expressly for this purpose; but they are not so unlike the flasks in common use in dentists' laboratories that the latter will not

FIG. 206.

answer. The common flask is simply unnecessarily thick or deep.

This mold is produced in the following manner: The palate pattern or model having been made in hard vulcanite and separated into two parts, as seen in Fig. 206, the pieces are placed side by side in a bed of soft plaster in one half of the flask. When the plaster is set, remove the patterns and trim and varnish the surface. This form may be copied in type-metal by molding in sand and casting. To make the sand-mold, take a ring of sheet-iron of the same diameter as the flask and three or four inches high; slip it over the flask and pack full of sand. Separate them, remove the plaster from the flask, return the empty flask to the sand-mold, and fill with melted metal through a hole made in the side or