Dissection of beaver body wall, BAA 289L 2004

In the two forms of locomotion that beavers employ, swimming and plantigrade quadrupedality, beavers do not engage in a great deal of flexion and extension of the vertebral column. When a beaver swims, it uses its tail as a rudder to make course corrections, and typically the spine does not move. This is a necessity of maintaining a streamlined shape when swimming. The deep back muscles are important in keeping the vertebral column straight and thus maintaining the “torpedo-like” shape. Even when a beaver is moving terrestrially, the motion is slow, and thus the vertebral column does not need a great deal of flexibility. Compared to the more caudal hip and tail muscles, which are related to the hindleg’s function in aquatic locomotion, the back muscles are relatively short and small. More flexion and extension are required at the neck, and this is represented in the multiple subdivisions of the deep back muscles into cervical components that are more divisible cranially.

The abdominal muscles are relatively thin, and serve no major purpose other than to support the organs of the abdominal cavity. The four layers are, from external to internal: external abdominal oblique, internal abdominal oblique, transversus abdominis, and rectus abdominis. One exception to this layering pattern is that cranially, the transversus abdominis passes deep to the rectus abdominis. These are the main muscles of support for the abdominal viscera. Cranially, the internal obliques are homologous to the internal intercostals and the external obliques are homologous to the external intercostals. There are large fat deposits throughout the layers of body wall musculature. One major layer is an extraperitoneal layer of fat that covers the entire ventral portion of the abdominal cavity. There are also large dorsolateral pockets of fat on either side.

We began the lab by focusing on the epaxial muscles. Therefore, we removed most of the hypaxial muscles on the dorsal side of the beaver including trapezius. After removing major portions of the hypaxial musculature of the back, we encountered three strips of muscles running along the vertebral column. We postulated that these muscles are spinalis, longissimus, and iliocostalis. All three muscles are well-defined in the beaver. We identified faint portions of the external intercostals lateral to these muscles situated between the ribs. In order to locate the insertions of the epaxial muscles, we made several incisions caudally to remove the skin and fat close to the tail. By removing the skin and fat caudal to the beaver’s thorax, we were able to uncover certain portions of transversus abdominis, which wraps around the abdomen. While we were able to identify one abdominal hypaxial muscle, we had difficulty locating the insertions of the epaxial muscles. With most of the superficial epaxial muscles uncovered, we next located the deeper epaxial muscles. After removing certain portions of spinalis and longissimus, we were able to locate the multifidus spinae, which is located in between spinalis and longissimus.

We shifted our focus to the ventral side of the beaver, where the abdominal hypaxial muscles are located. After turning the beaver onto its back, we made several incisions caudally to expose the hypaxial muscles. While removing the skin on the abdomen, we encountered a huge layer of fat, evenly overlying the external oblique muscle. Medial to the external oblique muscle is the rectus abdominis. In order to view the remaining abdominal hypaxial muscles, we made an incision caudally at the linea alba. Afterwards, we lifted the separated muscle to better view the internal oblique and transverse abdominis muscles.

Muscle	Origin(s)	Insertion(s)	Action(s)
Abdominal Hypaxial
External oblique	lateral surface of the last 11 ribs	linea alba, the cranial brim of the pubis and the crest of the ilium	compresses the abdomen and holds the internal organs in place
Internal oblique	internal margin of the iliac crest	pubic symphysis and into the linea alba	compresses the abdomen and holds the internal organs in place
Transversus abdominis	internal surface of the costal cartilages of the sternal ribs, and from the transverse processes of the lumbar vertebrae	xiphoid process and the linea alba	compresses the abdomen and holds the internal organs in place
Rectus abdominis	sternal end of the first rib, the ventral surface of the sternum , and the extreme medial ends of the cartilages of the sternal ribs	small area on the anterior border of the pubis at the symphysis	provides support for the abdominal viscera
Thoracic Hypaxial
External intercostals	caudal border of a rib	cranial border of the next caudal rib	elevates ribs during inspiration
Internal intercostals	inner margin of the caudal border of a rib	cranial border of the next caudal rib	pulls the ribs caudally during exhalation
Serratus dorsalis
a) Anterior part	lateral surfaces of the fifth to the eleventh ribs	caudal half of the ligamentum nuchae, supraspinous ligament, and spines of the first seven thoracic vertebrae	during the process of respiration, it pulls the ribs cranially
b) Posterior part	lateral surfaces of the seventh to fourteenth ribs	spinous processes of the sixth to thirteenth thoracic vertebrae	pulls the ribs caudally during exhalation
Additional Hypaxial
Psoas minor	from the ventral surfaces of the last four thoracic vertebrae and first three lumbar vertebrae	ilio-pectineal eminence of the ilium	flex the lumbar region of the vertebral column
Quadratus lumborum	transverse processes of the lumbar vertebrae	tubercular crest of the ilium	laterally flexes the vertebral column
Epaxial musculature
Multifidus spinae	transverse processes of the thoracic vertebrae and the articular processes of the cervical vertebrae	lateral surfacse of the vertebral spines	extends and rotates the vertebral column
Sacrospinalis
a) Semispinalis dorsi	caudal articular processes of the ninth, tenth, and eleventh thoracic vertebrae	caudal margins of the shafts of the first seven thoracic vertebrae	extends the thoracic region of the vertebral column
b) Semispinalis cervicis	transverse processes of the last five cervical vertebrae and the first three thoracic vertebrae	posterior margin of the spine	extends the neck
c) Longissimus dorsi	last three thoracic and lumbar spines, and from the lumbar transverse processes	transverse processes of all the thoracic vertebrae and into the caudal borders of the vertebral extremities of the first eight ribs	extends the vertebral column
d) Iliocostalis
1) Iliocostalis dorsi	cranial borders of the ninth to twelfth ribs	caudal borders of the third to sixth ribs	extends the vertebral column and tightens the rib cage
2) Iliocostalis cervicis	cranial borders of the angles of the fourth, fifth, sixth, and seventh ribs	caudal borders of the first two ribs and into the transverse processes of the last three cervical vertebrae	extends the upper thoracic region of the vertebral column and the neck
Splenius capitis	the entire ligamentum nuchae and supraspinous ligaments of the first three or four thoracic vertebrae	occipital crest and laterally into the mastoid process of the temporal bone underneath the trapezius muscle	extends and rotates cervical spine
Longissimus capitis	transverse processes of the last two cervical vertebrae	mastoid process of the temporal bone	flexes and rotates the head laterally

The muscles of the beaver body wall follow the generalized mammalian pattern. Few differences were noted between the muscles of the beaver body wall and those of the other mammals. One interesting difference to note is the striation pattern of the external obliques of the rat compared to that of the beaver. The beaver external obliques were oriented in the normal mammalian pattern, with muscles fibers running craniolaterally to caudomedially. In contrast, the rat external obliques ere oriented perpendicular to the this normal pattern.

1. Young, F.W. (1937). Studies of Osteology and Myology of the Beaver (Castor canadensis). Michigan State College, East Lansing.