The
digastric muscle (also
digastricus) (named
digastric as it has two bellies) is a small
muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
located under the
jawThe jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
.
It lies below the body of the
mandibleThe mandible or inferior maxillary bone forms the lower jaw and holds the lower teeth in place. It also refers to both the upper and lower sections of the beaks of birds; in this case the "lower mandible" corresponds to the mandible of humans while the "upper mandible" is functionally equivalent...
, and extends, in a curved form, from the
mastoid processThe mastoid process is a conical prominence projecting from the undersurface of the mastoid portion of the temporal bone. It is located just behind the external acoustic meatus, and lateral to the styloid process...
to the
symphysis mentiThe external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction of the two pieces of which the bone is composed at an early period of life....
. It belongs to the
suprahyoid musclesThe term suprahyoid refers to the region above the hyoid bone in the neck. The suprahyoid muscles include digastric, stylohyoid, geniohyoid, and mylohyoid.-Function:...
group.
A broad aponeurotic layer is given off from the tendon of the digastricus on either side, to be attached to the body and
greater cornuThe greater cornua of the hyoid bone project backward from the lateral borders of the body; they are flattened from above downward and diminish in size from before backward; each ends in a tubercle to which is fixed the lateral hyothyroid ligament.The upper surface is rough close to its lateral...
of the
hyoid boneThe hyoid bone is a horseshoe shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra behind.It is the only bone in the human skeleton not...
; this is termed the suprahyoid aponeurosis.
Structure
The digastricus (digastric muscle) consists of two fleshy bellies united by an intermediate rounded tendon.
The two bellies of the digastric muscle have different
embryologicalEmbryology is the study of the development of an embryo. An embryo is defined as any organism in an early stage well before birth or hatching, or in plants, before germination occurs....
origins, and are supplied by different cranial nerves.
Posterior belly
The posterior belly, shorter than the anterior belly, arises on the inferior surface of the skull, from the mastoid notch on the medial surface of the
mastoid processThe mastoid process is a conical prominence projecting from the undersurface of the mastoid portion of the temporal bone. It is located just behind the external acoustic meatus, and lateral to the styloid process...
of the
temporal boneThe temporal bones are situated at the sides and base of the skull.The temporal bone supports that part of the face known as the temple.-Parts:Each consists of four parts:* Squama temporalis* Mastoid portion* Petrous portion* Tympanic part...
and a deep groove between the mastoid process and the
styloid processThe styloid process is a pointed piece of bone that extends down from the human skull, just below the ear.-Structure:The styloid process is a slender pointed piece of bone just below the ear...
called the digastric groove.
The posterior belly is supplied by the
digastric branch of facial nerveThe digastric branch of facial nerve arises close to the stylomastoid foramen, and divides into several filaments, which supply the posterior belly of the Digastricus; one of these filaments joins the glossopharyngeal nerve.-External links:...
.
The digastric muscle stretches between the mastoid process of the cranium to the mandible at the chin, and part-way between, it becomes a tendon which passes through a tendinous pulley attached to the hyoid bone.
Anterior belly
The anterior belly arises from a depression on the inner side of the lower border of the mandible, close to the
symphysisA symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.Unlike synchondroses, symphyses are permanent.-Examples:The more prominent symphyses are:* the pubic symphysis...
, and passes downward and backward.
The anterior body is supplied by the
trigeminalThe trigeminal nerve is responsible for sensation in the face. Sensory information from the face and body is processed by parallel pathways in the central nervous system....
via the
mylohyoid nerveThe mylohyoid nerve is a nerve that innervates the mylohyoid muscle and the anterior belly of the digastric muscle.-Structure:...
, a branch of the
inferior alveolar nerveThe inferior alveolar nerve is a branch of the mandibular nerve, which is itself the third branch of the trigeminal nerve .-Path:...
, itself a branch of the
mandibular division of the trigeminal nerveThe mandibular nerve is the largest of the three branches of the trigeminal nerve.-Roots:It is made up of two roots:* a large sensory root proceeding from the inferior angle of the trigeminal ganglion....
.
Intermediate tendon
The two bellies end in an intermediate
tendonA tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fascia as they are all made of collagen except that ligaments join one bone to another bone, and fascia connect muscles to other...
which perforates the Stylohyoideus muscle, and is held in connection with the side of the body and the
greater cornuThe greater cornua of the hyoid bone project backward from the lateral borders of the body; they are flattened from above downward and diminish in size from before backward; each ends in a tubercle to which is fixed the lateral hyothyroid ligament.The upper surface is rough close to its lateral...
of the
hyoid boneThe hyoid bone is a horseshoe shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra behind.It is the only bone in the human skeleton not...
by a fibrous loop, which is sometimes lined by a mucous sheath.
Action
When the digastric muscle contracts, it acts to elevate the
hyoid boneThe hyoid bone is a horseshoe shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra behind.It is the only bone in the human skeleton not...
.
If the hyoid is being held in place (by the infrahyoid muscles), it will tend to depress the
mandibleThe mandible or inferior maxillary bone forms the lower jaw and holds the lower teeth in place. It also refers to both the upper and lower sections of the beaks of birds; in this case the "lower mandible" corresponds to the mandible of humans while the "upper mandible" is functionally equivalent...
(open the mouth).
Variations
Variations are numerous.
The posterior belly may arise partly or entirely from the styloid process, or be connected by a slip to the middle or inferior constrictor; the anterior belly may be double or extra slips from this belly may pass to the jaw or Mylohyoideus or decussate with a similar slip on opposite side; anterior belly may be absent and posterior belly inserted into the middle of the jaw or hyoid bone.
The tendon may pass in front, more rarely behind the Stylohoideus. The Mentohyoideus muscle passes from the body of hyoid bone to chin.
Triangles
The Digastricus divides the
anterior triangle of the neck-Boundaries:-Divisions:This space is subdivided into four smaller triangles by the Digastricus above, and the superior belly of the Omohyoideus below.These smaller triangles are named:* the muscular triangle* the carotid triangle* the submandibular triangle...
into three smaller triangles.
- (1) the submaxillary triangle
The submandibular triangle corresponds to the region of the neck immediately beneath the body of the mandible.-Boundaries and coverings:It is bounded:...
(also called Digastric Triangl), bounded above by the lower border of the body of the mandible, and a line drawn from its angle to the Sternocleidomastoideus, below by the posterior belly of the Digastricus and the Stylohyoideus, in front by the anterior belly of the Diagastricus;
- (2) the carotid triangle, bounded above by the posterior belly of the Digastricus and Stylohyoideus, behind by the Sternocleidomastoideus, below by the Omohyoideus;
- (3) the suprahyoid or submental triangle, bounded laterally by the anterior belly of the Digastricus, medially by the middle line of the neck from the hyoid bone to the symphysis menti, and inferiorly by the body of the hyoid bone.
External links