For comparison, see body wall dissections of a bat, a fox, and a tree shrew.

Adapted from Klingener 1979.

The muscles of the neck and trunk of the ferret are classified as either hypaxial or epaxial, depending on their relationship to the transverse processes of the vertebral column. The muscles dorsal to the transverse processes are considered epaxial muscles and these extend the spine [1]. Those ventral to the transverse processes are called hypaxial muscles and these flex the spine [1]. The spine bends when either set of muscles acts independently [1].

We began the dissection by skinning the entire back and abdomen. We first dissected the hypaxial musculature of the abdomen. We made a shallow cut along the linea alba, the ventral midline of the abdomen, and carefully peeled away the outermost layer of the abdominal muscles.

The abdominal muscles of the ferret are not separated by a fascial sheath, but are in direct contact with one another. Each muscle can be distinguished by the direction of its fibers. This thin outermost layer is the external oblique, which has fibers that start dorsally and run posterioventrad. The abdominal muscle layers are very thin and therefore difficult to separate from one another. When we reflected the external oblique, a small amount of the underlying layer adhered to the muscle. We identified this underlying layer as the internal oblique based on the direction of its fibers, which start dorsally and run anterioventrad. The internal oblique is even thinner than the external oblique. The third layer of the abdominal wall is the transverse abdominus. The fibers of this muscle run directly dorsoventrally. These three muscles can be differentiated based on the various directions of their fibers. The arcuate line can be found at the posterior end of the rectus sheath. The arcuate line marks where the fibers of the abdominal muscles end and their aponeuroses continue posterior.

As the oblique muscles approach the linea alba, they gradually become fascial sheets that meet and form a sheath for the rectus abdominus muscle [2]. We first identified the rectus abdominus muscle as a thickening along the linea alba. This muscle has fibers that run from caudal to cranial. The rectus abdominus is much thicker than the external oblique, internal oblique, and transverse abdominus muscles. It is very important in maintaining the arch of the vertebral column [2]. The vertebral ligaments and muscles tend to straighten the vertebral column, and the rectus abdominus muscle resists this action [2]. All four of these abdominal muscles compress the abdominal cavity and support the abdominal viscera.

After we identified the abdominal hypaxial musculature, we began to dissect the thoracic hypaxial musculature. The serratus dorsalis is the outermost muscle of the thorax. It is quite thin and its fibers run directly ventrodorsally from the ribs to the vertebral column. We reflected this muscle to observe the intercostals muscles, which run between the ribs. The intercostal muscles are extremely thin and run in directions analogous to the directions of the abdominal muscles. When we reflected the serratus ventralis, we found fibers of the external intercostals attached to the reflected muscle. Between the ribs, we saw fibers that started dorsally and ran anterioventrad. This fiber direction indicated that these were the internal intercostals. The innermost intercostals are directly deep to the internal intercostals. All of the intercostal muscles are involved in rib cage movement during respiration.

To view the deep muscles of the neck, the superficial clavotrapezius must be reflected. The splenius capitis, an epaxial muscle and the broadest muscle in the neck, is deep to the clavotrapezius. The atlantoscapularis is also deep to the clavotrapezius and superficial to the ventral edge of the splenius capitis. It originates at the transverse process of the atlas, inserts into the scapular spine, and is involved in scapular rotation. This long, thin muscle has to be reflected to observe the splenius capitis in its entirety. Reflecting the splenius capitis reveals the semispinalis capitis and the longissimus capitis. The semispinalis capitis is one division of the sacrospinalis. This muscle covers a large area of the lateral side of the neck, and is deep to the longissimus capitis. The longissimus capitis is a cervical division of the sacrospinalis. It originates from the longissimus dorsi and inserts into the nuchal crest of skull. The longissimus capitis is remarkably thinner than the semispinalis capitis and is superficial to the ventral region of this muscle. These two muscles, along with the splenius capitis, extend the head and rotate the neck.

The scalene muscles are ventral and slightly caudal to the other neck muscles. The three scalene muscles, which aid in respiration and vary in size, are just dorsal to the cleidomastoid. We had difficulty revealing these muscles and had to remove all of the muscles superficial to the scalenes in order to view them. The most ventral scalene is the longest of the three, though all of them are approximately the same thickness.

The epaxial musculature of the ferret back is poorly differentiated, especially in the posterior region. This bundle of muscles is collectively known as the sacrospinalis and functions in bending the vertebral column from side to side. Clinically, these muscles are important as a major site of intramuscular injections. They are also very important in the locomotion of carnivores because they are involved in dorsoventral flexion and extension, which adds significantly to stride length [2]. The entire muscle is covered in a thick fascial layer, which is thicker in the lumbodorsal region. We removed this fascial layer to observe the differentiation of the sacrospinalis muscle. This differentiation occurs in only the thoracic and cervical regions. In the thoracic region, this muscle divides into the iliocostalis, longissimus dorsi, and the transversospinalis. The iliocostalis is the lateral most section of the sacrospinalis and is also the thickest of the three divisions. The longissimus dorsi is in between the other two divisions and is the narrowest of the three. The origin of the longissimus dorsi is cranial to and much narrower than the origin of the iliocostalis. The transversospinalis, the most medial of the three divisions is directly adjacent to the dorsal midline. The cervical divisions of this muscle are the semispinalis capitis, which originates from the transversospinalis, and the longissimus capitis, which originates from the longissimus dorsi.

References

1. Fox, J.G. 1998. Biology and Diseases of the Ferret (2nd Ed.). Williams & Wilkins, Baltimore.

2. Klingener, D. 1979. Laboratory Anatomy of the Mink (2nd Ed.). William C. Brown, Dubuque, Iowa.

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