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INTRODUCTION
The sternocleidomastoid muscle is a muscle that di-
vides the side of the neck into anterior and posterior triangles.
It is an important surgical landmark as it is related to many
neurovascular structures in the neck. It originates from two
heads. The sternal head is rounded and tendinous. It
originates from the upper part of the anterior surface of the
manubrium sterni. The clavicular head is flattened and takes
origin from the medial one third of the superior surface of
the clavicle. The two heads, near their origin are separated
from lesser supraclavicular fossa on the surface of the neck.
The muscle gets inserted to the lateral surface of the mastoid
process and lateral part of the superior nuchal line. The fibres
of the muscle cross in such a manner that the clavicular fibres
get inserted on to the mastoid process and the sternal fibres
get inserted to the superior nuchal line.
The sternocleidomastoid gets its motor supply from
the spinal accessory nerve and the proprioceptive innervation
from the cervical spinal nerves. The muscle derives its arterial
supply from the occipital, posterior auricular, superior thyroid
and suprascapular arteries.
The muscle while acting alone flexes the neck
laterally and turns the face to the opposite side. When the
muscles of the two sides contract simultaneously, they flex
the head and neck.
CASE REPORT
During routine dissections for first year medical
students, an additional head (third head) was found in relation
to the sternocleidomastoid muscle in a male cadaver aged
approximately 65 years. The variation was found on the left
side of the neck and was unilateral (Figs. 1 and 2). The
additional third head took its origin from the superior surface
of the middle one third of the clavicle as a fleshy belly. The
size of this belly was half the size of normal clavicular head
of the muscle. Its fibres passed deep to the normal clavicular
head of the muscle and then blended with the other fibres of
sternocleidomastoid. The additional head covered the major
neurovascular structures in the subclavian triangle. It was
supplied by a branch of spinal accessory nerve.
DISCUSSION
Sternocleidomastoid muscle presents several
variations commonly at its origin. Variations in the insertion
are very rare. The sternocleidomastoid shares the same
developmental source as the trapezius and hence it can be
fused with the trapezius muscle. The fusion of these two
muscles is considered to be a normal feature by Bergman et
al. (1988). The tendinous intersections have been noted in
sternocleidomastoid by Bergman et al. These intersections
Int. J. Morphol.,
26(1):99-101, 2008.
A Rare Case of Unilateral Third Head
of Sternocleidomastoid Muscle
Un Caso Raro de una Tercera Cabeza Unilateral del Músculo Esternocleidomastoideo
Saju Binu Cherian & Satheesha Nayak
CHERIAN, S. J. & NAYAK, S. A rare case of unilateral third head of sternocleidomastoid muscle. Int. J. Morphol., 26(1):99-101, 2008.
SUMMARY: A rare case of unilateral third head of sternocleidomastoid muscle was noted during routine dissections for
undergraduate medical students. The additional third head originated from the middle third of the clavicle and joined the normal two
heads (sternal and clavicular) of the muscle in the middle of the neck. The insertion and nerve supply of the muscle was normal. This case
may be important for head and neck surgeons and for plastic surgeons doing muscle graft surgeries.
KEY WORDS: Sternocleidomastoid muscle; Anatomical variation; Neck muscle.
Case Report
Department of Anatomy, Melaka Manipal Medical College (Manipal Campus), International Centre for Health Sciences, India.
