Fox body wall
Photo and Text: Nick Zolkowski and Elizabeth Farrell
The majority of muscles in the body are hypaxial muscles, which are divided into prevertebral muscles, which flex the vertebral column, muscles of the limbs, and muscles of the body wall. The muscles of the body wall are further divided into four layers. One, the rectus series, runs longitudinally around the midsection, forming the rectus abdominis, rectus thoracis, and rectus cervicis muscles. The remaining three concentric layers lie on each side of the rectus series. They form, from deepest to most superficial, the innermost intercostal, internal intercostal, and external intercostal muscles of the thorax, and the transversus abdominis, internal oblique, and external oblique muscles of the stomach. These layers also extend into the neck to form the scalene muscles [1].
A number of epaxial muscles are also present in the body wall, and serve to add length and force to each stride by extending the flexed vertebral column. For the most part, the epaxial musclulature can be divided into superficial, intermediate, and deep layers. The superficial layer includes the erector spinae muscles, such as the longissimus and spinalis muscles, and the splenius muscles, comprised of the splenius cervicis and splenius capitis muscles. These muscles run laterally and cranially from the midline. The intermediate layer includes the multifidus muscles and the semispinalis muscles, which run medially and cranially. The deep layer, not visible in this lab, includes the rotator muscles, levator costarum muscles, interspinales msucles, and intertransversarii muscles [1].
The vertebral column in canines is more rigid than that found in some other mammals, and therefore imposes limits on the type of locomotion possible. Conversely, the flexible spine of a feline allows a greater range of motion. The thoracic vertebrae support the rib cage, which is large, allowing for a large heart and expansive lungs. The sternebrae of the sternum are not fused, but remain separate with cartilage between them, further allowing the chest to expand as the animal breathes. The lumbar region is specialized for tail function [2].
We started the dissection on the ventral surface of the abdominal wall. We cut the latissimus dorsi muscle along its ventral attachment and reflected it to expose the external oblique muscle. We then cut through the ventral portion of the external oblique muscles along the midline, and detached the connective tissue between the external oblique muscles and the muscular layers underneath. After reflecting the external obliques we were able to see the internal oblique muscles deep to them. The internal oblique muscles lie perpendicular to the external obliques, running medially and cranially from the lateral aspect of the pelvis to insert on the rectus abdominis muscle, forming the rectus sheath. The external oblique muscles run medially and caudally from the lower ribs and insert on the front of the rectus sheath. The external obliques are much thicker than the internal obliques. Deep to the internal obliques are the transversus abdominis muscles, which run from the transverse processes of the lumbar vertebrae and insert on the rectus sheath. The abdominal muscles, taken together, support the contents of the abdominal cavity and aid in urination, respiration, and locomotion
The thoracic muscles corresponding to the abdominal obliques, the intercostals, were also seen. The innermost intercostal muscles, corresponding to the transversus abdominis muscle, were the smallest of the three. The internal intercostal muscles, serially homologous with the internal obliques, run medially and caudally between the ribs. Ventrally, near the sternum, the muscles become ligamentous. Conversely, the external intercostal muscles, homologous with the external obliques, run laterally and caudally between the ribs, and become ligamentous dorsally, near the vertebral column. These muscles also aid in respiration. We were not able to see the intercostal veins, arteries and nerves, but would expect them to run in close association in the intercostal space between the two deepest layers of hypaxial muscle.
The back and spinal cord area was dissected last. We first reflected the latissimus dorsi and rhomboid muscles and cut away a lot of fatty tissue that was deep to these muscles. We were then able to trace out three long muscles running from the pelvis to the lower neck that are used to extend the spinal column. From medial to lateral, they are the spinalis muscle, the longissimus thoracis muscle, and the iliocostalis muscle. Closer to the cervical region are the spinalis capitis medially and the semispinalis capitis muscle more laterally. These raise and flex the neck, respectively, which allows the fox to have strong snapping motions when trying to subdue prey [3]. The scalenus anterior, scalenus medius, and scalenus posterior muscles, which run from the cervical transverse processes of the vertebral column to the first two ribs and act to lift the ribs upon inhalation, were not dissected out. We also were not able to see the brachial plexus, which runs between the scalenus medius and scalenus anterior muscles, or the internal jugular vein, which runs between the scalenus anterior muscle and the sternocleidomastoid muscle.
1. Cartmill, M. et al. 1987. Human Structure. Harvard University Press, Cambridge.
2. Alderton, David. 1994. Foxes, Wolves, and Wild Dogs of the World. Facts
on File, New York.
3. Evans, Howard. 1993. Miller's Anatomy of the Dog. 3rd. Ed. W. B. Saunders
Co., Philadelphia.