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Facial and Masticatory Muscles

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Digestive System

Urinary and Reproductive Systems

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Facial and Masticatory Muscles of the Opossum (Monodelphis domestica)

Text and Photos by Sarah Ogburn and Linda Evonne Brogdon

Overview :

Monodelphis domestica is omnivorous, with a diet that includes carrion, insects, berries, fruit, and seeds.  It has large, mobile pinnae (ears), long vibrissae (whiskers), small eyes, and a long snout. 

The jaw and facial musculature of M. domestica is extremely simple and undifferentiated compared to that of many other mammals.  The movement of its jaw during mastication is a simple abduction and adduction, requiring few specialized or differentiated muscles.  The jaw musculature of opossums is considered to be very similar to the primitive mammalian condition, and is often used in comparative work on mastication. 

The facial musculature is considered “primitive” in that it is closer to the condition in non-mammals--few independent muscles have differentiated and the superficial musculature appears as a single sheet of muscle overlying the face [1]. Elaborate facial muscles are found only in mammals.  Facial muscles are critically important during lactation—they allow the suckling infant to form a seal around the nipple.  The facial muscles are also integral in sensory functions—namely, moving the vibrissae during exploration.  Communication via facial expression is also achieved by the facial muscles. 

Dissection:

We began the dissection by cutting mid-dorsally with small scissors from the neck towards the rostrum.  We then began separating the skin from the thick fascia and delicate facial muscles using the probe and scissors.  Removing the skin proved to be extremely difficult, as many of the small facial muscles insert directly onto the skin.  Some of these were inadvertently removed during skinning.  Skinning was particularly difficult around the eyes, vibrissae, and lips.  To remove the skin from the lower jaw, we made a cut along the midline with the small scissors.  We left the skin surrounding the pinnae intact, as this area was extremely delicate. 

The superficial muscles responsible for moving the pinnae were easily visible as thin reddish sheets of muscle with prominent fibers.  These muscles include: the cervicoauricularis superficialis, cervicoauricularis medius, and interscutularis muscles.  The interscutularis muscles are the most extensive and the most rostral. They are extremely thin and sheet-like.  They span transversely between the pinnae with no bony origin and insert on the medio-rostral aspect of the pinnae. The cervicoauricularis superficialis muscles are strap-like--slightly thicker, longer, and more caudal than the interscutularis muscles.  They extend from the dorsal midline of the skull in the nuchal region to the medial surface of the pinnae, distal to the base.  The cervicoauricularis medius muscles are the most caudal and smallest of the superficial pinnae muscles.  They extend from the dorsal midline of the skull in the region of the sagittal crest to the base of the pinnae.  There are eleven other small muscles involved in moving the pinnae [1].

Part of the orbicularis oculi muscle was visible as a very small, thin ring around the eye.  We believe part of it was removed during skinning, but our sources also suggest that this muscle is very small and weak in opossums [1].

Part of the orbicularis oris muscle was also visible as a ring of muscle around the mouth.  A slip of the buccinator muscle was visible deep to the orbicularis oris muscle. 

We cut and reflected the buccinator and platysma muscles to reveal the superficial masseter muscle.  The superficial masseter muscle is large and fan-shaped and runs from the maxilla to the angle of the mandible.  Just superior and deep to it was the external adductor muscle, which passes under the zygomatic arch.  The internal adductor muscle is caudal and deep to the external adductor muscle, and somewhat smaller.  The posterior adductor muscle is continuous with the internal adductor muscle. 

We did a superficial examination of the suprahyoid muscles.  The anterior bellies of the digastric muscle are large and located on the lateral sides of the lower jaw.  The mylohyoid muscle is rostral and medial to it.  A small portion of the geniohyoid muscle is visible rostral to the mylohyoid muscle. 

In most mammals, the major muscles of mastication are divided into the temporalis muscle, the masseter muscle, the zygomatico-mandibular muscle and the superficial masseter muscle.  In the opossum the muscles are undifferentiated and the muscles cannot be divided, except for the superficial masseter.  The temporalis and masseter muscles a single unit inserting on the coronoid process of the mandible.  This unit is considered to have three parts: the external adductor, the internal adductor, and the posterior adductor.  These parts are not technically separate muscles, but are somewhat distinguishable as different regions and bulges fibers.  The superficial masseter is recognizable as a separate muscle, which runs from the maxilla to the inferior surface of the mandibular angle [2]. 

M. domestica has large pinnae in proportion to its body size and the corresponding musculature for moving the pinnae is large as well.  A total of fourteen muscles are responsible for moving the pinnae [1].  Musculature for moving the vibrissae is also prominent. 

M. domestica relies more on its sense of smell, hearing, and touch than its sight.  The eyes and ocular musculature is small compared to the other sense organs, the orbicular portion of the facial musculature consists of only one muscle—orbicularis oculi [1]. 

The buccolabial musculature of M. domestica consists of two muscles: the superficial orbicularis oris muscle and the deeper buccinator muscle.  In many other mammals, there are additional buccolabial muscles that are not present in the opossum (i.e., the levator anguli oris and depressor anguli oris muscles).  In other mammals, the buccinator muscle may be differentiated further into the mentalis and maxillonasolabialis muscles, which are absent in M. domestica [1]. 

Muscles’ Origin, Insertion, and Function (adapted from sources [1], [2], and [3] and our own observations):

References:

1. Minkoff, E.; Mikkelsen, P.; Cunningham, W.; Taylor, K. 1979. The facial musculature of the opossum (Didelphis virginiana).  Journal of Mammalogy. 60: 46-57.

2. Hiiemae, K.; Jenkins, F. 1969. The anatomy and internal architecture of the muscles of mastication in Didelphis marsupialis. Postilla 140: 1-49.

3. Ellsworth, A. 1976. The North American Opossum: An Anatomical Atlas. Robert E. Krieger Publishing Company, New York.