Page images
PDF
EPUB

bolus (Fig. 51). The apex of the tongue is now raised and pressed against the hard palate, and by the successive elevations of the different parts of the dorsum of the tongue the bolus is gradually pushed backward toward the isthmus of the fauces.

[merged small][graphic]

Section through a portion of Dentine next the pulp cavity of a growing tooth.
(a) An isolated odontoblast; (b) Growing part; (c) Odontoblasts: (d) Fila-
ments of protoplasm projecting from the tubules of hard dentine. (Beale.)

FIG. 50.

The Pterygoid Muscles seen from without after removal of the super-
ficial parts, the temporal muscle, the zygomatic arch, and a portion
of the lower jaw and masseter. (1) External, (2) Internal pterygoid
muscle.

The root of the tongue with the hyoid bone is at the same time drawn upward and forward, so that the bolus easily slips down

along the retreating

slope leading from

the mouth cavity,

and gets within the reach of the constrictors of the fauces. Immediately before the morsel of food is grasped by the muscles of the fauces, the levator palati draws the soft palate upward and backward to completely close the posterior openings of the nasal cavity, as is shown by

the fact that during

[blocks in formation]

the act of swallowing the pressure in the nasal cavity is raised. At the same moment the intrinsic muscles of the larynx, which surround the rima glottidis like 1, a constrictor, firmly close that opening

f

-12

Muscles of Tongue and Pharynx,

2, 3, Muscles from styloid process (6) to the tongue, hyoid
bone (d) and pharynx respectively; 4, 5, 6, 7, 8, muscles
of tongue; 9, 10, 11, constrictors of pharynx; 12, œsoph-
agus; 13, is placed on larynx (e). (Allen Thomson.)

by approximating the cords and arytenoid cartilages. The entire
larynx is at the same time drawn up behind the hyoid bone by the
thyro-hyoid muscle. The rima glottidis is thus tucked in under
the cushion of epiglottis, while the leaf of the epiglottis is pulled
down over the larynx by the oblique aryteno-epiglottidean and
thyro-epiglottidean muscles.

While the closure of the nasal and pulmonary air passages is

1

going on, the bolus has passed out of the cavity of the mouth and has been caught by the palato-glossal and palato-pharyngeal muscles, which force it into the pharynx and at the same time close the isthmus faucium behind the descending morsel. The stylo-pharyngeis and the pharyngeal constrictors now grasp the bolus spasmodically, and the latter contract in rapid succession, moving the bolus onward, and drawing themselves over it, pass it on to the œsophagus, where, by a progressing ring-like con

8

FIG. 52.

Deep Muscles of Cheek, Pharynx, etc. (1) Orbicularis oris; (2) buccinator; (3) superior, (4) middle and (5) inferior constrictors of the pharynx; (6) œsophagus: (7) styloid muscles cut across; (8,9,

10) muscles attached to the hyoid bone (d) and thy

roid cartilage (e). (Allen Thomson.)

traction of the circular muscles and a simultaneous shortening of the longitudinal layer of fibres, the mass is slowly squeezed down to the cardiac orifice of the stomach. The movements of the œsophagus are essentially peristaltic in character, the peculiarities of which form of motion will be discussed when speaking of the intestinal

movements.

The process of swallowing is performed by a continuous series of coördinated muscular movements, quite independent of gravitation, as may be seen in animals drinking with their heads downward. Although these complex sets of movements any check or interval,

follow each other regularly and without the act of deglutition is commonly divided into three stages,

between which, as there is no pause, it is not easy to draw a hard and fast line.

The first stage is simply the initiatory step of placing the morsel of food or some liquid in such a position as to excite the second or spasmodic act of deglutition. This first step is a voluntary act, and it is the only part of the movements of swallowing over which we can exert complete control. The progress of the morsel between the

tongue and palate toward
the fauces may be as slow
and gradual as we wish,

but the moment a certain
point is reached volition
is at an end, and we are d
unable to check the com-
pletion of the act.

By the second stage is
meant the period occupied
by the passage of the food
bolus through the pharynx
and past the top of the
larynx. Although we are
not able to influence it in
any way by our will, we
are conscious of the food
passing in this region. It
is a rapid,
rapid, involuntary
spasm in which a great
number of muscles take

FIG. 53.

[graphic]

Transverse section of Esophagus.
(Horsley.)

a. Outer fibrous covering.

part, all of which are made b. Bundles of longitudinal muscle cut across.

up of striated muscle tissue.

c. Transverse muscular coat cut obliquely.
d. Sub-mucous coat with glands in section.
e. Muscular layer of the mucous membrane.

The third stage includes all the rest of the time &

Mucous membrane with cut vessels.
Stratified epithelium.

during which the bolus is passing from the grasp of the lower pharyngeal constrictor and along the oesophagus. Not only has our will no influence over this stage of deglutition, but we are hardly conscious of its taking place, since no sensations accompany

the greater part of it.

Thus the more essential movements of the act of swallowing are purely reflex and involuntary, though we can call forth this series of reflexions by voluntary stimulation of a certain part of the fauces by means of a morsel of food or a drop of liquid, and without such a stimulus as food or liquid we cannot by our will excite swallowing. We think we can perform the muscular movements of swallowing when we please, without any food or fluid, but in this we are mistaken, as careful observation of our own performance of the act will show.

The pharyngeal spasm is always preceded by the deposition in the region of the isthmus faucium of some drop of saliva collected from the mouth or fauces themselves. In fact, without a slight preliminary movement of the posterior part of the tongue -which might be called the last act of mastication-the more essential stages of deglutition cannot be excited.

Nervous Mechanism of Mastication and Deglutition. -The voluntary influences which regulate the motions of the muscles of mastication pass along the efferent branches of the fifth nerves (trigemini) which accompany its inferior division. The muscles which depress the jaw to open the teeth and the intrinsic muscles of the tongue are supplied by the ninth pair of nerves (except the posterior belly of the digastric, which has a branch from the facial, and the mylohyoid and anterior belly of the digastric, which are supplied from the third division of the fifth). The coördination of the movements of mastication and suction seem to reside in the medulla oblongata, but are obviously under the control of the will.

The afferent impulses which excite the nerve centres in the medulla, and give rise to reflex acts which cause the swallowing movements, pass from the mucous membrane of the fauces along (1) the descending palatine branches of the spheno-palatine ganglion and the second division of the trigeminus, also along (2) the pharyngeal branches of the superior laryngeal branch of the vagus to the medulla, where the coördination of pharyngeal spasm and œsophageal peristalsis is accomplished. Thence the efferent impulses pass by (1) the hypoglossal to the hyoid and

« PreviousContinue »