=I. THE MUSCLES OF THE SKIN.= (Fig. 62.)
=M. cutaneus maximus= (Fig. 62, _b_).--This is a very large, thin muscle
which covers almost the whole side of the body. It arises from the outer
surface of the latissimus dorsi (Fig. 68, _m_) near its ventral end and
from the bicipital arch (Fig. 65, _t′_) in the axilla; from the linea
alba for a considerable distance (two or three inches) caudad of the
base of the xiphoid process, and from the thorax over a line joining the
axilla and the base of the xiphoid. Sometimes a few fibres take origin
from the fascia which covers the pectoantibrachialis on the ventral side
of the arm.
From their origin the fibres diverge. The cranial ones curve about the
base of the forelimb and are inserted into the skin at or near the
middle line caudad of the first thoracic vertebra. The most cranial
fibres of all are inserted about one to one and a half centimeters from
the middorsal line; thence the line of insertion approaches the
middorsal line to reach it at about the eighth or ninth thoracic
vertebra. The middle fibres run parallel to the middorsal line in the
lumbar and sacral regions as far as the root of the tail, a small bundle
passing onto the dorsal side of the tail, another onto the ventral side.
The fibres of the caudal portion pass onto the thigh, the ventral ones
running in the fold of skin which stretches from thigh to abdomen, and
are finally lost in the fascia along a line connecting the knee and the
root of the tail. A strong fascia connects the adjacent borders of the
cutaneus and platysma and lies over the scapular region.
[Illustration: FIG. 62.--MUSCLES OF THE SKIN.
_a_, _a_′, _a_″, M. platysma; _b_, M. cutaneus maximus; _c_, M.
orbicularis oris. Between the platysma and the cutaneus maximus are
seen the following deeper muscles: _d_, M. acromio-trapezius; _e_, M.
levator scapulæ ventralis; _f_, M. spino-deltoideus; _g_, M.
acromio-deltoideus; _h_, M. clavobrachialis; _i_, caput laterale of M.
triceps brachii; _j_, caput longum of M. triceps brachii; _k_, M.
xiphi-humeralis; _l_, M. pectoralis minor. 1, external jugular vein.]
This muscle lies immediately beneath the integument. It covers the first
layer of body muscles.
_Action._--Moves the skin.
=M. platysma= (Figs. 62 and 64, _a_, _a′_, _a″_).--This muscle forms a
thin layer of fibres covering the sides of the neck and face, in close
relation with the integument. Several more or less distinct portions may
be distinguished.
(1) Most of the fibres (_a_) arise from the middorsal line, from the
occiput to the first thoracic vertebra, in a narrow fascia common to the
muscles of the two sides. The most cranial fibres of this region arise
as a small bundle from the external occipital crest, beneath the
levator auris longus (Fig. 63, _g_, _g′_).
From this origin in the middle line the fibres pass craniolaterad. The
most cranial fibres curve about the ventral side of the ear and pass
toward the caudal angle of the eye, where they unite with fibres of the
zygomaticus (Fig. 64, _d_) or corrugator supercilii lateralis (Fig. 64,
_k_), or pass to the lower eyelid. Caudad of these the fibres cover the
side of the face and become lost among the facial muscles, some passing
to the lower eyelid, some to the fibrous pad which supports the
vibrissæ, some to the angle of the mouth, some to the lower lip. The
most ventral fibres meet the fibres of the opposite muscle just ventrad
of the symphysis of the mandible.
The ventral free border of the platysma is separated on the ventral side
of the neck from the border of the opposite muscle by a wedge-shaped
area having its point at the symphysis menti.
The fibres of this portion of the muscle are interrupted by an
attachment to the skin, along a line passing from the base of the ear to
about the middle of the coracoid border of the scapula, The dorsal (_a_)
and ventral (_a′_) portions of the muscle, separated by this line of
attachment, are sometimes described as separate muscles (the
=supercervicocutaneus= and =cervicofacial=, respectively, of
Strauss-Durckheim).
(2) A band of fibres one or two centimeters across (_a″_) arises in the
fascia of the side of the neck just craniad of the middle of the
coracoid border of the scapula, and passes caudoventrad toward the
manubrium, its fibres crossing the fibres of the first part of the
platysma at right angles. These fibres become lost in the fascia ventrad
of the manubrium, or pass across the middle line to intermingle with the
corresponding fibres of the opposite side. This portion of the platysma
is sometimes absent.
The platysma is everywhere subcutaneous, except at its dorsocranial
angle, where a small bundle of fibres is covered by the levator longus
auris. It covers the deeper muscles of the neck and head. Closely
attached to its inner surface are the submentalis and depressor conchæ,
whose fibres bridge over the ventral interval between the borders of the
platysmas of opposite sides.
_Action._--Moves the skin of the face and neck.
=II. THE MUSCLES OF THE HEAD.=
A. SUPERFICIAL MUSCLES.--The most superficial layer of muscles on the
face and head is formed by differentiation of the fibres of the
platysma. The muscles thus formed are not clearly distinct from each
other; in this region sets of fibres differing in direction and in
origin or insertion receive separate names even though the different
sets of fibres are closely interwoven.
In the quadrangle on the dorsal surface of the head enclosed between the
two eyes and the two ears, a thin superficial sheet of fibres is found,
in which a number of different sets may be distinguished (Fig. 63).
These have received the following names.
=M. intermedius scutulorum= (Fig. 63, _a_).--This consists of a broad
thin sheet of transverse fibres between the two external ears. The
fibres are attached at either end to the scutiform cartilage (1) of
the two ears, and pass without interruption across the middle line. At
its cranial edge this muscle is continuous with the corrugator
supercilii medialis (_b_); at its lateral edge with the
frontoscutularis; at its caudal edge with the levator auris longus
(_g_).
_Relations._--Outer surface with the integument. Inner surface with the
galea aponeurotica (to which the muscle is closely united), the
epicranius muscle (_h_) and the temporal muscle (_n_).
_Action._--Draws the two ears dorsad, toward the middle line.
[Illustration: FIG. 63.--MUSCLES ON THE DORSAL SURFACE OF THE HEAD.
On the right side are shown the superficial muscles; on the left side
the superficial muscles have mostly been removed, exposing the deeper
muscles and the bone. _a_, M. intermedius scutulorum; _b_, M.
corrugator supercilii medialis; _c_, M. orbicularis oculi; _d_, M.
corrugator supercilii lateralis; _e_, cranial end fibres of M.
platysma; _f_, M. adductor auris superior; _g_, _g′_, M. levator auris
longus (_g_, cranial portion; _g′_, caudal portion); _h_, _h′_, M.
epicranius (_h_, M. occipitalis; _h′_ M. frontalis); _i_, M.
transversus auriculæ; _k_, M. auricularis superior (cut on the left
side); _l_, M. abductor auris brevis; _m_, M. abductor auris longus;
_n_, M. temporalis; _o_, cut origin of M. frontoscutularis; _p_, _p′_,
M. levator labii superioris alæque nasi (_p′_, the origin from the
maxillary bone); _q_, angular head of M. quadratus labii superioris.
1, scutiform cartilage; 2, external ear; 3, bones of the skull.]
=M. corrugator supercilii medialis= (_b_).--This consists of a thin
sheet of scattered fibres lying craniad of the last and intermingling
with it. The fibres take origin near the middle line, pass laterad, then
curve craniad, converging, to be inserted into the whole extent of the
upper eyelid, especially near the caudal angle. Here the fibres unite
with those of the orbicularis oculi (_c_).
This muscle is continuous caudad with the intermedius scutulorum (_a_),
craniad with the orbicularis oculi (_c_); laterad with the corrugator
supercilii lateralis (_d_). Toward the medial side the fibres lose
themselves in a tendinous sheet that joins the galea aponeurotica.
_Relations._--Outer surface with the integument. Inner surface with the
frontoscutularis and the skull.
_Action._--Raises the upper eyelid.
=M. orbicularis oculi= (Fig. 63, _c_; Fig. 64, _s_).--This consists of
two thin bands of muscle-fibres which lie one in either eyelid parallel
to its border and unite at the angle of the eye.
_Origin_ by short tendon-fibres from a tubercle on the surface of the
frontal process of the maxillary bone just dorsad of the orbital end of
the lachrymal canal and between the two parts of the quadratus labii
superioris (Fig. 63, _p_ and _q_).
The muscle splits into two parts which pass into the two eyelids. At the
outer angle of the eye the two bands unite by the intervention of
tendon-fibres between the muscle-fibre bundles of the two.
_Relations._--Outer surface with the integument. Inner surface with the
inner membrane of the eyelid.
_Action._--Closes the eye.
=M. corrugator supercilii lateralis= (Fig. 63, _d_; Fig. 64, _k_).--This
consists of a number of scattered fibres which arise from among the
fibres of the frontoscutularis, and from the tendon lying just craniad
of the external opening of the ear, to which are united also parts of
the zygomaticus (Fig. 64, _d_) and submentalis (Fig. 64, _c_). The
fibres pass craniad, converging so as to form a narrow band which is
inserted at the caudolateral angle of the eye, where it unites with the
orbicularis oculi (Fig. 64, _s_). This muscle is continuous on the
medial side with the corrugator supercilii medialis (Fig. 64, _j_) and
the frontoauricularis, on the lateral side with the platysma.
_Relations._--Outer surface with the integument. Inner surface with the
frontal bone and the frontoscutularis.
_Action._--Pulls the angle of the eye caudad; at the same time pulls the
external ear craniad.
=M. frontoauricularis.=--A few of the fibres which are attached along
the upper eyelid sometimes pass dorsocaudad, mingled with fibres of the
corrugatores supercilii medialis and lateralis and the frontoscutularis,
to the craniomedial angle of the auricular cartilage, where they unite
with the fibres of the adductor auris superior. These fibres are
sometimes distinguished as the =frontoauricularis= muscle.
=M. levator auris longus= (Fig. 63, _g_ and _g′_). (Part of the
auricularis posterior of man.)--This lies on the caudal half of the
dorsal surface of the head, forming a laterocaudal continuation of the
intermedius scutulorum (_a_).
_Origin_ from the middle line of the neck dorsad of the atlas, and from
the sagittal crest for about one centimeter craniad of the external
occipital tubercle. The fibres form a broad thin sheet which passes
craniolaterad as far as the caudal end of the scutiform cartilage (1).
Here the muscle divides; the major portion is attached to the scutiform
cartilage (1), its fibres intermingling with those of the intermedius
scutulorum (_a_). The caudal portion of the muscle (_g′_) passes onto
the surface of the auricle, extending one or two centimeters distad;
here it is inserted on an oblique line which lies directly craniad of
the insertion of the transversus auriculæ (_i_).
The caudal portion of this muscle (_g′_) having origin above the atlas
and insertion on the auricle, is sometimes considered a separate muscle,
the supercervicoauricular or cervicoauricular. The cranial portion (_g_)
with origin on the sagittal crest and insertion on the scutiform
cartilage might be distinguished as the occipitoscutularis.
The levator auris longus is continuous at its cranial end medially with
M. epicranius (_h_); laterally with M. intermedius scutulorum (_a_).
_Relations._--Outer surface with the integument. Inner surface with the
temporal muscle (_n_), the auricularis superior (_k_), the abductor
longus auris (_m_), a narrow strand of the platysma, and the
clavotrapezius.
_Action._--Pulls the external ear dorsocaudad.
The four following muscles lie partly or entirely beneath those already
described; they are doubtless formed as differentiations of the inner
layers of the platysma.
=M. auricularis superior= (or attollens auris) (Fig. 63, _k_).--This
muscle forms a band about one centimeter broad lying just beneath the
levator longus (_g_).
_Origin_ on the sagittal crest for about one centimeter craniad of the
interparietal bone. The muscle passes laterad onto the surface of the
auricle and is inserted into the auricular cartilage a little caudad of
the middle of its convex surface. At its lateral end the cranial margin
is closely united to the under surface of the levator auris longus
(_g_).
_Relations._--Outer surface at the medial end with the epicranius (_h_),
the abductor auris longus (_m_), and levator auris longus (_g_); at the
lateral end with the skin. Inner surface with the temporal muscle (_n_)
and the auricular cartilage.
_Action._--Pulls the external ear dorsad.
=M. abductor auris longus= (Fig. 63, _m_; Fig. 64, _q_). (Part of the
auricularis posterior of man.)
_Origin_ on the sagittal crest dorsad of the interparietal bone, caudad
of that of the auricularis superior (_k_), which it partly covers.
The muscle passes laterad as a flat band 8 to 10 millimeters wide over
the caudal surface of the concha of the ear, and is inserted (Fig. 64,
_q_) on the lateral surface of the eminentia conchæ, caudad of the
antitragus.
_Relations._--Outer surface with the levator auris longus (_g_) and the
integument. Inner surface with the auricularis superior (_k_), the
abductor auris brevis (_l_), and the concha.
_Action._--Pulls the external ear caudad.
=M. abductor auris brevis= (Fig. 63, _l_).
_Origin_ by a tendon from the lambdoidal crest for one or two
centimeters laterad of the middle.
The muscle passes lateroventrad lying beneath the abductor longus (_m_)
as a flat band 6 to 8 millimeters wide which is inserted into the medial
surface of the most proximal portion of the concha, just distad of its
junction with the cartilaginous auditory meatus.
_Relations._--Outer surface with the abductor longus (_m_), a small
strand of the platysma and the concha. Inner surface with the temporal
muscle (_n_).
_Action._--Pulls the concha caudad.
=M. epicranius= (or occipitofrontalis) (Fig. 63, _h_ and _h′_).
_Origin_ on the sagittal crest or suture, just craniad of the origin of
the levator longus auris, to the inner surface of which this muscle is
closely united. From the origin the fibres pass craniad forming a band
(_h_) about 8 or 10 millimeters in width, the two muscles lying close to
one another and partly united in the middle line. About two centimeters
craniad of their origin the fibres of both muscles end in a tendinous
sheet, the =galea aponeurotica=, which covers the surface of the skull
in the region between the ears and eyes, and is formed by the inner
surface of the intermedius scutulorum (_a_) and other muscles of this
region. The galea aponeurotica passes craniad onto the surface of the
nose, where it gives origin again to a thin sheet of muscle-fibres
(_h′_) which are inserted into the integument near the cranial ends of
the nasal bones.
M. epicranius is thus formed of two muscular portions (_h_ and _h′_),
connected by a long tendinous sheet. The caudal portion is frequently
distinguished as the occipital muscle (=M. occipitalis=, _h_), the
cranial portion, on the nose, as the =frontal= muscle (=M. frontalis=,
_h′_).
_Relations._--Outer surface of the occipitalis (_h_) with the
intermedius scutulorum (_a_), the levator auris longus (_g_), and the
integument; inner surface with the auricularis superior (_h_), the
abductor auris longus, and the bone. Outer surface of the frontalis
(_h′_) with the integument; inner surface with the bones of the skull.
_Action._--Moves the integument of the dorsal surface of the head and of
the nose.
Ventrad of the external ear the following three differentiated portions
of the platysma may be distinguished.
=M. zygomaticus= (major) (Fig. 64, _d_).--A slender band connecting the
angle of the mouth with the scutiform cartilage of the ear.
[Illustration: FIG. 64.--SUPERFICIAL MUSCLES ON THE LATERAL SURFACE OF
THE HEAD AND NECK.
_a_, _a′_, _a″_, M. platysma; _b_, M. depressor conchæ; _c_, M.
submentalis; _d_, M. zygomaticus (_b_, _c_, and _d_ lie beneath the
platysma and are seen through it); _e_, M. zygomaticus minor; _f_′, M.
caninus; _f_, _g_, parts of M. quadratus labii superioris (_f_,
angular head; _g_, M. levator labii superioris alæque nasi); _h_, M.
myrtiformis; _i_, M. orbicularis oris; _j_, M. corrugator supercilii
medialis; _k_, M. corrugator supercilii lateralis; _l_, M. adductor
auris superior; _m_, M. helicis; _n_, M. antitragicus; _o_, M.
adductor auris inferior; _p_, M. conchæus externus; _q_, M. abductor
auris longus; _r_, parts of M. auricularis externus; _s_, M.
orbicularis oculi.]
At the angle of the mouth the fibres arise from among those of the
orbicularis oris (_i_), from both the upper and the lower lip. They pass
thence dorsocaudad across the zygomatic arch as a band 8 or 10
millimeters wide which is attached to a tendinous aponeurosis lying just
craniad of the external opening of the ear. To this aponeurosis are
attached also, wholly or partly, the corrugator supercilii lateralis
(_k_) and the submentalis (_c_). The aponeurosis is united with the
caudoventral angle of the scutiform cartilage and with the ventral edge
of the cranial margin of the auricular cartilage.
This muscle is united at its cranial end with the orbicularis oris
(_i_); throughout its length more or less with the platysma (_a_, _a′_);
at its caudodorsal end with the corrugator supercilii lateralis (_k_) on
its medial side, and with the submentalis (_c_) on its lateral side.
_Relations._--Outer surface with the integument and the platysma (_a′_).
Inner surface with the masseter, temporal, frontoscutularis, and
adductor auris inferior (_o_).
_Action._--Draws the angle of the mouth dorsocaudad; the external ear
ventrocraniad.
=M. submentalis= (Fig. 64, _c_).--A flat band, similar to the last, and
closely connected for some distance to its caudal border.
It arises as scattered fibres near the ventral middle line at about the
level of the larynx, the fibres of the opposite muscles crossing the
middle line and intermingling. The fibres pass dorsad, converging so as
to form a narrower band, which unites dorsad with the zygomaticus (_d_),
to be inserted with it into the tendinous aponeurosis above described.
_Relations._--Outer surface with the integument and platysma (_a′_).
Inner surface with the muscles of the hyoid, the digastricus, the
parotid gland, the masseter, the zygomatic arch and the temporal muscle.
_Action._--Draws the external ear ventrad.
=M. depressor conchæ= (Fig. 64, _b_).--A thin band of fibres, caudad of
the last and parallel with it. The muscle arises as scattered fibres on
the ventral surface of the neck, one or two centimeters craniad of the
manubrium, the fibres of the muscles of opposite sides crossing the
middle line and interdigitating. They pass dorsad, gathering together to
form a narrow band which is inserted into the summit of the antitragus.
_Relations._--Outer surface with the skin at the ventral end, and with
the platysma (_a′_) further dorsad. Inner surface with the deep muscles
of the neck and with the parotid gland.
_Action._--Draws the external ear ventrad.
Lying deeper than the muscles thus far described, but closely connected
with a number of them, are the following:
=M. frontoscutularis.=
_Origin_ (Fig. 63, _o_) on the frontal bone, along the supraorbital
margin, from the craniomedial angle of the eye caudad to the zygomatic
process of the frontal.
The fibres form a large muscle which passes dorsocaudad to the scutiform
cartilage and is mostly attached along its ventrolateral border. Some of
the outer fibres, however, pass distad of the scutiform cartilage toward
the cartilage of the ear, thus joining the adductor auris superior
(_f_).
The outer surface of this muscle is closely connected with fibres of
the intermedius scutulorum (_a_), the corrugatores supercilii medialis
(_b_) and lateralis (_d_), and the frontoauricularis, when this exists.
The outer surface is covered near the origin by the orbicularis oculi
(_c_), farther dorsad by the muscles just mentioned and the integument.
The inner surface is in relation with the frontal bone and the adductor
auris inferior (Fig. 64, _o_).
_Action._--Pulls the ear craniad.
=M. adductor auris inferior.= (Fig. 64, _o_.) (Part of the auricularis
anterior of man.)--A small muscle lying beneath the frontoscutularis.
_Origin_ on the ligament which connects the zygomatic process of the
frontal with the frontal process of the malar. The fibres form a thin
band about 7 millimeters wide which passes craniad, closely united to
the inner surface of the frontoscutularis. At the caudal edge of the
zygomaticus (_d_) the muscle is interrupted by a short tendinous
interval; the fibres then continue to their insertion on the tip of the
antitragus.
_Relations_.--Outer surface with the frontoscutularis, the zygomaticus
(_d_), and the integument. Inner surface with the temporal muscle and M.
antitragicus (_n_).
_Action._--Pulls the ear craniodorsad.
* * * * *
The remainder of the muscles of the ear, which, with the exception of
the tragicus lateralis, merely interconnect the cartilages of the
external ear or parts of these cartilages, are described in connection
with the account of the Auditory Organ.
* * * * *
Owing to the presence of the fibrous pad to which the whiskers are
attached, the facial muscles between the eye and the mouth differ in the
cat in some respects from those of related animals. This fibrous pad
interrupts the muscles, frequently breaking muscles which are elsewhere
single into two.
=M. zygomaticus minor=, or malaris (Fig. 64, _e_).--This muscle is not
always present; when it exists it forms a very thin flat band of fibres
passing from the ventral side of the eye toward the angle of the mouth.
_Origin_ among the fibres of the orbicularis oculi (_s_) in the lower
eyelid. The fibres pass ventrad; the insertion varies. In some specimens
the insertion is among the fibres of the orbicularis oris (_i_), at the
angle of the mouth, beneath the zygomaticus (_d_). In other cases this
muscle is said to be inserted into the pad on which the whiskers rest.
_Relations._--Outer surface with the integument and near its insertion
with the zygomaticus major (_d_). Inner surface with the malar bone and
the masseter muscle.
_Action._--Pulls the angle of the mouth (or the whiskers) dorsad.
=M. orbicularis oris= (Fig. 64, _i_).--This forms a very thin layer of
subcutaneous concentric fibres which surround the mouth, that of the
upper lip being thicker than that of the lower lip. The part of the
muscle in the lower lip has its fibres intermingled with those of the
platysma (_a′_). In the median line the fibres of the upper lip are
interrupted by a raphe, and caudad of this are intermingled with those
of the caninus.
=M. quadratus labii superioris= (Fig. 63, _p_ and _q_; Fig. 64, _f_ and
_g_).--This is a complex of muscle-fibres attached chiefly to the
fibrous pad on which the whiskers rest. In it two parts can be
distinguished more or less completely.
(1) =M. levator labii superioris alæque nasi= (Fig. 63, _p_; Fig. 64,
_g_) on the sides of the nose. This arises (_a_) as a continuation of
the frontal portion of the epicranius, and (_b_) in a small bundle (Fig.
63, _p_) from near the dorsal end of the frontal process of the
maxillary bone, in common with the next. The two slips thus formed
unite craniad and are inserted partly into the integument on the outer
side of the wing of the nose, partly into the skin of the upper lip,
lateroventrad of the nose, and partly into the pad on which the whiskers
are located, in connection with the next.
(2) The =angular= head (=caput angulare=), or =levator labii superioris
proprius= (Fig. 63, _q_; Fig. 64, _f_) is much larger than the last. It
takes origin from a small tubercle at the cranial border of the orbit,
close to the origin of the orbicularis oculi. The fibres diverge from
their origin to their insertion among the whiskers and in the fibrous
pad on which the whiskers rest.
_Action._--Erects the whiskers and raises the upper lip.
=M. caninus= or =levator anguli oris= (Fig. 64, _f′_).
_Origin_ in a depression on the lateral surface of the maxillary bone,
just craniad of the last premolar tooth. The fibres pass craniad,
diverging, toward the whiskers, and have their insertion into the pad of
tissue on which the whiskers rest.
_Action._--Retracts the whiskers and raises the upper lip.
=M. buccinator.=--A thin muscle against the mucous membrane of the upper
lip, partly united with the orbicularis oris and lying beneath M.
caninus.
_Origin_ on the outer surface of the superior maxillary bone in the
depression between the infraorbital foramen and the first premolar
tooth. The fibres pass ventrad, diverging, and mingle with those of the
deep part of the orbicularis oris.
_Relations._--Outer surface with the caninus and with the orbicularis
oris. Inner surface with the mucous membrane of the upper lip.
_Action._--Raises the upper lip.
=M. myrtiformis= (probably corresponds to a part of the human M.
nasalis) (Fig. 64, _h_).--A thin sheet of muscle-fibres lying craniad of
the levator labii superioris alæque nasi and passing from the whiskers
to the nose and upper lip. It is distinguishable from the alæque nasi by
the direction of the fibres.
_Origin_ from the pad upon which the whiskers rest. The fibres diverge
and have their
_Insertion_ upon the wing of the nose and into the skin of the upper lip
near the median line.
_Action._--Dilator of the nares and elevator of the upper lip.
=M. “moustachier”= (Str.-D.) (probably corresponds to one of the Mm.
incisivi).--It lies on the upper lip near the median line.
_Origin_ from the outer surface of the premaxilla near the suture along
the ventral border of the narial opening. The fibres diverge and pass
caudad into the upper lip within the superficial fibres of the
orbicularis oris.
_Insertion._--The skin of the upper lip just craniad of the whiskers.
Some of the fibres pass into the pad on which the whiskers rest.
_Action._--It carries the lip craniad.
=M. quadratus labii inferioris.=--A thin flat band extending almost the
length of the lower lip.
_Origin_ from the alveolar border of the mandible, between the molar
tooth and the canine. The fibres pass dorsad into the lower lip, where
they intermingle with those of the orbicularis oris. The muscle is
perhaps not constant.
_Action._--Depressor of the lower lip.
Sometimes a few fibres arising from the cranial portion of the alveolar
border of the mandibula pass mediad, seeming to join corresponding
fibres from the opposite side. Such fibres would constitute a M.
transversus menti.