Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Body Wall Muscles of the Opossum (Monodelphis domestica)

Text and photos by Sarah Ogburn and Linda Brogdon

Abdominal hypaxial musculature:

Dissection:

In order to examine the different layers of body wall muscles, we made an incision and cut cranio-caudally along the linea alba.  We then made two horizontal medio-lateral incisions at the caudal and cranial ends of the first cut, exposing a square window.  We then used the probe to try and separate the different layers of muscle.  The linea alba was still attached to the reflected square of muscle and prevented us from fully separating the muscles, so we made a cranio-caudal cut though the deepest muscle—the transversus abdominis muscle layer—about 2mm lateral to the linea alba.  We reflected the transversus abdominis layer dorsally towards the body cavity, thus exposing the dorsal side of the internal and external oblique muscles and the rectus abdominis muscle.  The internal and external oblique muscle layers could not be separated.  We shone a light through these layers of muscle in order to view the direction of the fibers and identify the layers. 

The entire abdominal hypaxial musculature is extremely thin, measuring only millimeters in thickness.  The transversus abdominis muscle is the deepest, with a medio-lateral fiber orientation. The transversus abdominis muscle forms a sheath around the rectus abdominis muscles, which has a cranio-caudal fiber orientation.  The rectus abdominis muscle is restricted to the more medial aspect of the body wall in the rat, but is fairly wide in the opossum.  The opossum has no tendinous inscriptions of the rectus abdominis muscle, unlike the rat and the rabbit.  Superficial to this layer is the internal oblique muscle, with fibers oriented medio-caudally.  Superficial to the internal oblique muscle is the external oblique muscle, with a latero-medial fiber orientation.  The internal and external oblique muscles are impossible to separate from one another. 

The pyramidalis muscle extends from the palpable epipubic bone to the linea alba.

Function:

The abdominal hypaxial musculature functions mainly to support the internal organs.  It does play some role in locomotion, however.  The internal and external oblique muscles help with lateral bending of the body and the rectus abdominis muscle stiffens the trunk longitudinally to prevent sagittal bending [1]. 

The pyramidalis muscle is very large in animals with epipubic bones, such as the opossum.  Several hypotheses have been proposed to explain the presence of epipubic bones.  One it that the bone and its associated muscles help support the young in the pouch.  However, studies of opossums during locomotion show that the epipubic bone acts as a lever that stiffens the body across the limb pair that is on the ground. Some scientists believe that this particular series of muscle attachments and subsequent stiffening function was important during the transition from reptile to mammalian locomotion.  Erect postures became more efficient because of the ability to resist the forces of gravitation and ground reaction [1].

Thoracic hypaxial musculature:

Dissection:

We cut the latissimus dorsi muscle away from the spine at the midline to expose the thoracic hypaxial and epaxial musculature.  The serratus dorsalis muscles were damaged when we removed the latissimus dorsi muscles.  They originate from the nuchal area of the skull and the spines of the upper thoracic vertebrae and insert onto the superior edge of ribs 2-4.  The intercostal muscles were extremely difficult to differentiate because of the very small intercostal spaces made it difficult to dissect.  The external intercostal muscles ran in a latero-caudal direction between the ribs and are visible from the outside of the rib cage. The internal intercostal muscles run at 90 degrees to the external intercostal muscles and are visible from the inside of the rib cage.  We could not locate the innermost intercostal muscles, as they are an inconstant layer. 

Epaxial musculature:

Dissection:

The sacrospinalis muscle group consists of the semispinalis muscles, the longissimus dorsi muscles, the longissimus capitis muscles, and the iliocostalis muscles. These muscles are oriented cranio-caudally along the longitudinal axis of the back and are long and cord-like, with many tendinous areas.  The spinalis, longissimus, and iliocostalis muscles all originate as one muscle unit from the lower thoracic and lumbar vertebral spines.  The three muscles split apart and insert at different locations.  The iliocostalis muscle inserts onto the ribs; the longissimus dorsi, longissimus capitis and the spinalis muscles insert onto the upper vertebrae and nuchal area of the head.

We cut the sacrospinalis muscle group along the midline and reflected it to reveal the deeper epaxial musculature.  The semispinalis muscles originate on the spines of the vertebrae and insert onto the transverse processes of the vertebrae caudal to it, spanning several vertebrae at a time.  The semispinalis muscles are present from mid-thorax to nuchal area of the skull.  The splenius capitis muscle had been largely destroyed in our dissection of the brain.  It is a flat, strap-like muscle originating on the cervical vertebrae and inserting on the nuchal region of the skull.

Function:

Scientists used to believe that sagittal bending of the trunk during locomotion replaced the lateral bending characteristic of early tetrapod gaits in the evolution of early mammals.  However, it is more likely that sagittal bending of the spinal column is a specialization evolved in association with galloping and bounding.

Part of this change in theory comes from research on Monodelphis.  Studies show that the vertebral column bends laterally during slow walking, with the most bending occurring in the lumbar region.  Other placental mammals, such as the domestic cat, exhibit similar lateral bending of the trunk during slow locomotion.  At faster speeds, the amount of lateral bending is reduced, but marsupials show more lateral bending than placentals at all speeds.  The retention of some lateral bending at fast speeds in marsupials is correlated with their posture.  Lateral bending contributes to stride length, but is only used at low speeds—scientists have not been able to pose an adequate explanation for this.  The epaxial musculature is probably primarily responsible for this lateral bending. However, some scientists argue that trunk movement is merely a consequence of limb and pelvis movement [2].

Muscles’ Origin, Insertion, and Function (from our own observations and source [3])

References:

1. Reilly, S. and White, T. 2003. Hypaxial motor patterns and the function of epipubic bones in primitive mammals.  Science: 299: 400-402.

2. Pridmore, P. 1992. Trunk movements during locomotion in the marsupial Monodelphis domestica (Didelphidae). Journal of Morphology: 211: 137-146.

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