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Gross and histological structure of the pharyngeal constrictors in the rabbit.

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Gross and Histological Structure of the Pharyngeal
Constrictors in the Rabbit
Department of Anatomy, Queen's University,
Kingston, Ontario, Canada
In a continuing study of the functional
anatomy and electromyography of the
pharynx, rabbits were chosen as subjects
because there is a basic similarity of
pharyngeal anatomy in various mammalian forms and because of the great convenience of experimenting with this
species. However, it soon became apparent that no detailed information is available on the special anatomy of the pharynx
in the rabbit and even the description given
by Bensley and Craigie ('48) is not sufficient and complete. This resulted in the
present study of the gross and histological
structure of the pharyngeal constrictors of
the rabbit without which further functional studies would have been hampered.
Because our study revealed important facts
which may be of general use, the essential
findings are being reported in this paper.
Ten adult rabbits each weighing about
3 kg were killed by intravenous chloroform and embalmed via the abdominal
aorta. The preserving fluid contained antiseptics (carbolic acid and dettol), hardening fluids (alcohol and formalin), hygroscopic agents (glycerine and potassium
acetate) and salts. Twenty-four hours after
embalming, red latex was injected under
pressure into the aorta to facilitate the detection of the arteries during dissection.
Steps in dissection
With the subject supine, neck extended
and fore-limbs widely abducted and drawn
downwards, a median vertical incision was
made in the front of the neck and another
horizontal incision was made along the
lower border of the mandible up to the
mastoid process on each side. The resulting pair of triangular skin flaps were
turned backward and laterally along with
platysma and the two depressor conchae
(anterior and posterior) and the external
jugular vein on each side (fig. 1). A very
thin layer of deep fascia was removed to
expose the deeper structures in the anterior triangle of the neck and the sternocleido-mastoid was retracted on each side.
The submaxillary salivary gland was removed on the left side and retracted on the
right side. The digastricus, made up of a
single belly in the rabbit, was found on
the medial aspect of medial pterygoid. A
portion of digastricus was removed on one
side to expose the superior and middle
constrictors, the styloglossus, and the stylohyoideus minor. The mylohyoid muscles
were seen between the two digastrici forming the floor of the mouth. The infrahyoid
muscles, such as sternohyoid and sternothyroid, were removed from one side to
expose the origin of the inferior constrictor, cricothyroid and thyrohyoid muscles, thyroid and cricoid cartilages, thyroid gland and trachea. One lobe of the
thyroid gland was removed from one side
to expose the contents of the carotid
sheath, the ansa hypoglossi and the upper
deep cervical lymph nodes; then the nodes
were removed completely to expose the
sympathetic trunk. The hypoglossal nerve
was seen passing across the hyoglossus
muscle. Both nerve and muscle were removed to expose the lingual artery on the
surface of the middle constrictor. The
stylohyoideus major muscle was cut away
from the tip of the greater horn of the
hyoid bone and retracted towards its origin (jugular process of the occipital bone)
to expose the stylopharyngeus muscle with
the glossopharyngeal nerve (figs. 1, 2).
To expose the direction and insertion of the
three constrictor muscles on the back of
Fig. 1
Ventral view of dissection. Inset shows enlargement of contents of carotid sheath.
the pharynx, a coronal section was made
just behind the pharynx (fig. 2). Another
median sagittal section of the pharynx was
made to expose both the interior of the
pharynx and the origin and direction of
the inner longitudinal muscle coat of the
pharynx (fig. 3).
Histological studies
In three rabbits, the total number of
muscle fibers in each constrictor muscle
was counted by teasing the fibers apart
under a dissecting microscope ( X 40) and
then counting them under a binocular
phase contrast microscope ( X 100). The
breadth of the muscle fibers were measured with the help of the stage and ocular
To determine the number of muscle fibers innervated by a single motor neuron,
fresh specimens of constrictor muscles
were taken from three rabbits and fixed in
5% formosaline for 24 hours. Then they
were teased apart under the dissecting
microscope. The teased specimens were
immersed in 0.1% methylene blue for two
minutes thus staining the nerve fibers
and their endings.
The muscular coat of the pharynx is
covered outside by loose areolar fascia and
lined inside by a thick pharyngeal aponeurosis attached to the base of the skull
and pterygoid process. The muscular coat
comprises 6 paired voluntary striated muscles (though their function is essentially
involuntary). The outer circular coat comprises three paired constrictors, namely,
superior, middle and inferior. The inner
longitudinal coat also comprises three
paired muscles, namely, salpingo-, palato-,
and stylo-pharyngeus (figs. 2, 3). The
three constrictors are incomplete in front
where the nose, mouth and larynx open
into the pharynx.
Superior constrictor
As in the human being, superior constrictor is a fan shaped muscle with a narrow origin and a broad insertion (figs. 1,
2, 3).
hetic trunk
n. IX
ntetnal carotid A.
Corn. carotid A.
Corn. carotid
Sterno-thyroid m.
Fig. 2 Dorsal view of dissection of pharyngeal constrictors and related muscles.
IE\\ PalaTophar ynqeus
S u p e r i o r constrictor
Middle constr ickor
- Inferior constrictor
’ :- - - 1
M ylohy 01deus
Hyoid bone
Palatine t o n s t l
Fig. 3
Median sagittal section and dissection from interior of pharynx.
Origin. It arises: ( a ) from the base
of the external acoustic meatus by a thin
band, ( b ) from the pterygoid hamulus and
adjacent part of the posterior border of the
medial pterygoid plate (pterygo-pharyngeus), (c) from the soft palate along with
palato-pharyngeus, and ( d ) from the side
of the tongue.
Course and insertion. The upper fibers
run dorsally, upward, and medially to be
inserted into the pharyngeal tubercle of
the occipital bone. The middle fibers run
horizontally to be inserted into the pharyngeal raphe. The lower fibers run downward, backward and medially also to be
inserted into the pharyngeal raphe. These
fibers are overlapped by the middle constrictor.
Middle constrictor (hyo-pharyngeus)
It is also a fan shaped muscle with a
narrow origin but broad insertion.
Origin. It arises deep to hyoglossus
from the greater and lesser horns of the
hyoid bone and the angle between them.
Course and insertion. All the fibers are
inserted into the pharyngeal raphe. The
upper fibers run obliquely upward, backward and medially; the middle fibers run
horizontally; and the lower fibers run
obliquely downward, backward and medidly to be inserted into pharyngeal raphe.
The lower fibers are overlapped by the
upper fibers of inferior constrictor.
Inferior constrictor
Unlike the human inferior constrictor,
it is a quadrangular shaped muscle with
a vertical height of 6-7 mm, a breadth of
7-8 mm, and a thickness (at its central
part) of 1-2 mm.
Origin. It arises : ( a ) from the oblique
line of the thyroid cartilage behind the attachments of sternothyroid and thyrohyoid
muscles (this part of the muscle being
called thyro-pharyngeus) and (b) from the
lateral aspect of the cricoid cartilage and
fibrous arch covering the cricothyroideus
(this part being known as crico-pharyngeus).
Course and insertion. All the fibers run
parallel with each other obliquely upward,
backward, and medially to be inserted into
the pharyngeal raphe. The lowest fibers
blend with the longitudinal fibers of the
Relationships of constrictors
Superior constrictor. The relations are :
( a ) dorsally-prevertebral fascia and muscles, and the pharyngeal venous plexus :
( b ) laterally-stylo-pharyngeus,
pterygoid and middle constrictor muscles,
glossopharyngeal and lingual nerves, symphatic trunk and carotid sheath; ( c ) internally-palato- and salpingo-pharyngeus,
pharyngeal aponeurosis and tonsil; ( d )
superiorly-the sinus of Morgagni which
is traversed by levator palati muscle and
auditory (Eustachian) tube; (e) inferiorly
(i.e., between superior and middle constrictors)-stylo-pharyngeus
muscle and
glossopharyngeal nerve.
Middle constrictor. Its relations are :
( a ) dorsally-prevertebral fascia and muscles; (b) laterally-carotid sheath, pharyngeal plexus of nerves, covered near its
origin by hyoglossus (separated by lingual
artery) and covered at its lower part by
inferior constrictor; (c) internally-superior constrictor, stylo- and palato-pharyngeus muscles and pharyngeal aponeurosis.
Between the middle and inferior constrictor muscles no structure pierces the
thyrohyoid membrane.
Inferior constrictor. Its relations are :
( a ) dorsally- same as middle constrictor; (b) laterally-thyroid gland, carotid
sheath, sternothyroid muscle and superior
laryngeal vessels and nerve which pierce
the muscle and thyroid cartilage on their
lateral surface to enter the larynx; (c)
internally-middle constrictor, stylo-pharyngeus muscle and fibrous coat; (d) inferiorly-the recurrent laryngeal nerve and
the inferior laryngeal branch of the superior thyroid artery entering the larynx deep
to it.
Nerzie supply of constrictors
The pharyngeal constrictors are supplied
chiefly by the pharyngeal branch of the
vagus through the pharyngeal plexus. The
latter is formed on the surface of the middle constrictor by the pharyngeal branches
of the vagus and glossopharyngeal nerves
and the sympathetic trunk. From this
plexus three main trunks arise and each
trunk runs diagonally across the middle
of individual constrictor muscles accompanied by a vessel (fig. 4).
Fig. 4 Drawing of a nerve bundle ending on
muscle fibers and accompanied by a vessel, as
seen under low power phase contrast.
Histological characteristics of constrictor muscles in three rabbits
Sup. constrictor
Middle constrictor
Inf. constrictor
Length of muscle
5-6 mm
6-7 mm
7-8 mm
Breadth of muscle
15-20 /A
15-20 /A
20-30 /A
Arrangement of
Arranged in a complicated interlacing manner
Arranged in a
Arranged in a
single sheet at
single sheet at
its upper and
its edges but 3 4
lower edge but
layers at its cenat its center
tralpart. They
they are arare all parallel
with each other.
ranged in 2-3
layers. The fibers There is considare almost parerable amount of
allel with each
fat between the
muscle fibers
Color of muscle
Number of muscle fibers (by actual count) i n constrictor muscles of three rabbits
Histological findings
The characteristics of the muscle fibers
in three rabbits are summarized in table
1. It should also be noted that all the
muscle fibers show good cross-striations.
The total number of muscle fibers in
these three rabbits are shown in table 2.
T h e size of motor units. In the stained
and teased tissue seen under the phasecontrast microscope, nerve trunks are accompanied by a vessel and cross the middles of the muscle fibers. As each crosses
its muscle, it gives off branches to a series
of fibers alternately on one side and then
the other (fig. 4). Each branch ends on an
end-plate that is situated near the middle
of each muscle fiber. Under higher magnification ( X 500) with phase-contrast, individual nerve fibers are seen to innervate
4-6 muscle fibers in the inferior constrictor (fig. 5) and 2-4 muscle fibers in
the middle and superior constrictors.
Total number of motor units. This was
calculated by dividing the number of muscle fibers by the average size of the motor
Fig. 5 Drawing of photograph showing the
ending of nerve fibers on muscle fibers ( X ca.
unit. In this short series, the figure for
the superior constrictor is about 1071, for
middle constrictor, 1295, and for inferior
constrictor, 1021. In other words, there
are over 1000 motor units in each pharyngeal constrictor.
The above findings confirm that there
is a marked basic similarity in the gross
structure of the constrictor muscles of the
pharynx in man and rabbit. This basic
similarity is not surprising and is related
to the following factors: ( a ) swallowing
and respiration are the primitive functions
of the pharynx in both man and rabbit and
there is no essential differences in these
functions in the two species; and (b) in
both species the pharynx develops from
the cephalic end of the foregut in about
the same way.
Possibly related to man's higher evolution or to his erect posture, there are a
few noteworthy differences. In man,
superior constrictor muscle extends its origin down to the mandible. This seems to
be due to its important function of closing
the nasopharynx during speech. The lower
fibers of the inferior constrictor in man run
horizontally to form the crico-esophageal
sphincter. This muscle, according to Negus
('49) remains tonically contracted during
ordinary respiration to prevent the entrance of air into the esophagus as a result of the inspiratory descent of the diaphragm and expansion of the thorax. This
sphincter opens with closure of the larynx
during swallowing by virtue of the attachment of the anterior wall of the esophagus
to the back of the cricoid cartilage.
In this study a direct count of the muscle fibers was made after teasing them
apart under a dissecting microscope.
There was a considerable range of results
as shown in table 2. Nonetheless, averages
of about 3200, 3900 and 5100 fibers for
superior, middle and inferior constrictors,
respectively, may be quite useful, especially since they were obtained by direct
counting. Other observers, however, have
made their counts (or rather, estimates)
from cross sections of stained specimens.
For example, van Harreveld ('47) estimated that there are 6282 fibers in the
sartorius of the rabbit. Berlendis and De
Caro ('55) gave 3600 fibers for the rabbit's
stapedius and 4100 for tensor tympani.
The motor end-plates are found to be
located near the middles of the muscle fibers. This was also shown by Coers and
Woolf ('59) in human skeletal muscle, by
Gurkow and Bast ('58) in trapezius and
sterno-mastoid of the hamster, and by
Jarcho et al. ('52) in the gracilis of the
The size of the motor units in our study
was determined by tracing the individual
nerve fibers along their final distribution
to the muscle fibers. Other observers have
calculated the total number of muscle fibers in a muscle and the total number of
nerve fibers in its motor nerve. Then, by
dividing the former by the latter figure,
they have calculated the size of the motor
units. The latter method is rather questionable because we know that the motor
nerve of a muscle contains many sensory
and sympathetic fibers as well as motor
fibers (fig. 6). Nonetheless, it is a method
that does produce reasonable approximations. Tergast (1873) estimated that the
motor units of the sheep extra-ocular muscles have 3-10 muscle fibers; Bors ('26)
estimated 5-6 for human extra-ocular muscles. More particularly, Feinstein et al.
('55) reported 9 muscle fibers per motor
unit in the human lateral rectus, 25 in
platysma, 108 in the first lumbrical of the
hand and 2000 in the medial head of
gastrocnemius. Van Harreveld ('47) reported 100-125 muscle fibers per motor
unit in the sartorius of the rabbit; Berlendis and De Car0 ('55), 27 in the stapedius and 30 in the tensor tympani of the
rabbit; Wersdl ('581, 10 in the human
tensor tympani; and Ruedi ('59), 2-3 muscle fibers per motor unit in the human
laryngeal muscles.
Generally it has been agreed that muscles controlling fine movements and adjustments (such as those attached to the
ossicles of the ear and to the eyeball and
the larynx) have the smallest number of
muscle fibers per motor unit. This statement would now appear to include the
pharyngeal constrictor muscles. On the
other hand, large coarse-acting muscles,
e.g., those in the limbs, have larger motor
It will be noted that we have also calculated by a direct count of muscle fibers
and a direct tracing of the terminal
branches of motor nerve fibers the number
of motor units in the pharyngeal constrictors. We find each contains just over 1000
motor units. By the less precise but still
useful technique of estimation generally
Neruous 2
Sqs ?em
- iVasodilator
Sensory Nerve
I <@ Corpuscle
Sensory Nerve
Fig. 6
Naked Nerve
(muscZe substanm
Scheme showing multiple innervation of skeletal muscIe (after Solandt, '42).
employed, Feinstein et al. ('55) reported
3000 units in the human lateral rectus,
and Berlendis and De Car0 ('55) reported
100-120 motor units in the stapedius and
125-190 motor units in the tensor tympani of the rabbit. It would appear to us
that the total number of motor units has
no intrinsic value in that this is simply a
function of the size of the muscle and the
size of the motor units in it. Strength of
contraction is eventually dependent on the
number of muscle fibers in an individual
muscle and not on the number of motor
units. Fine control, however, is dependent
on the smallness of the motor units within
the muscle.
A study of the pharyngeal anatomy
(gross and histological) was carried out
in 10 adult rabbits. The gross structure
of the constrictor muscles varies from
that in the human being, especially in the
superior and inferior constrictor.
In three rabbits the muscle fibers were
counted after teasing them apart and the
nerve fibers were traced. The motor units
in the inferior constrictor contain 4 to 6
muscle fibers each; those in the middle
and superior constrictors contain two to 4
each. The smallness of the motor units in
the pharyngeal constrictors places these
muscles in the group of muscles that can
produce very fine or delicate movements.
In this short series, the average total
number of muscle fibers were: for inferior
constrictor, about 5100; for middle constrictor, about 3900; for superior constrictor, about 3200.
Berlendis, P. A,, and L. G. De Car0 1955
L’unitB motoria del muscolo stapedio. Boll.
SOC.Med.-chir. Pavia, 69: 33-36.
Bors, E. 1926 Ueber das Zahlenverhaltnis
zwischen Nerven- und Muskelfasern. Anat.
Anz., 60: 415416.
Coers, C., and A. L. Woolf 1959 The Innervation of Muscle, a Biopsy Study. Charles C
Thomas, Springfield, Ill.
Craigie, E. H. 1948 B.ensIey’s Practical Anatomy of the Rabbit, ed. 8. University of Toronto
Press, Toronto.
Feinstein, B., B. Lindeghd, E. Nyman and G.
Wohlfart 1955 Morphological studies of motor units in normal human muscles. Acta
Anat., 23: 127-142.
Gurkow, H. J., and T. H. Bast 1958 Innervation of striated skeletal muscle. Am. J. Phys.
Med., 37: 269-277.
Jarcho, L. W., C. Eyzaguirre, B. Bennan and J.
L. Lilienthal 1952 Spread of excitation i n
skeletal muscle: some factors contributing to
the form of the electromyogram. Am. J. Physiol., 168: 44-57.
Negus, V. E. 1949 The Comparative Anatomy
and Physiology of the Larynx. William Heinemann, Ltd., London.
Riiedi, L. 1959 Some observations on the histology and function of the larynx. Semon lecture,
1958, J. Laryngol. Otol., 73: 1-20.
Solandt, D. Y. 1942 Atrophy in skeletal muscle. J. Am. Med. Assoc., 120: 511-513.
Tergast, P. 1873 Ueber das Verhaltnis von
Nerve und Muskel. Arch. Mikr. Anat., 9: 36-46.
van Harreveld, A. 1947 On the force and size
of the motor units in the rabbit’s sartorius
muscle, Am. J. Physiol., 151: 96-106.
Wersall, R. 1958 The tympanic muscles and
their reflexes, Acta Oto-laryngologica, Supplementum 139.
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structure, gross, rabbits, pharyngeal, constrictors, histological
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