Tree shrew body wall
The back should already be skinned and the thoracodorsal fascia removed. If not begin with the shoulder lab for preliminary instructions. If you have not already done so cut and reflect laterally the trapezius, latissimus dorsi, and rhomboid muscles.
Note: All abdominal musculature is visible in Figure 1.
External oblique - Arises from the lower 7-8 ribs, and from the lumbodorsal fascia runs posteromedially to insert on the ilium, inguinal ligament, and linea alba. Acting unilaterally rotates the trunk, and acting bilaterally flexes the trunk. Also supports the abdominal viscera.
Internal oblique - Deep to the external oblique, this muscle arises
from the inguinal ligament, ilium, and thoracolumbar fascia, and inserts into
the linea alba. The ventral half of this muscle becomes fascial and transparent
thereby exposing the transversus abdominus. Acting unilaterally rotates the
trunk, and acting bilaterally flexes the trunk. Also supports the abdominal
viscera.
It is difficult to see the distinction between the internal oblique and transversus
abdominus without substantial magnification. Therefore, I have indicated the
border with a yellow line. Notice that transversus abdominus appears to be lighter
in color than internal oblique and runs in a medial to lateral direction.
Transversus abdominus - Arises from the lumbodorsal fascia and the inguinal
ligament deep to the internal oblique and inserts on the linea alba. This muscle
is especially well-developed ventrally, where the internal oblique muscle becomes
fascial. Supports the abdominal viscera.
Rectus abdominus - Arises from the pubic crest, and inserts on the sternum and costal cartilages. The attachments were easily detected cranially, and became increasingly difficult to identify past the fifth rib. Flexes the trunk and supports the abdominal viscera. This muscle was cut in an earlier lab, and can only be seen as a thin remnant in Figure 1.
External intercostals - Arises from the inferior border of one rib and inserts on the superior border of the next rib. Pulls the ribs cranially during the inhaling respiration. Look for these muscle bodies between each rib in Figure 1.
Internal intercostals - Deep to the external intercostals these muscles arise from the inferior border of a higher rib and inserts on the superior border of the next more posterior rib. Pulls the ribs caudally during the exhalation phase of respiration.
Innermost intercostals - Deep to the internal intercostals, these muscles arise from the inferior border of a higher rib and inserts on the superior border of the next lowest rib. Pulls the ribs cranially during respiration.
Reflect the rib cage inferiorly to open the thoracic cavity, and use a sharp probe to dissect away the innermost intercostals dorsally. Look for intercostal nerves, arteries, and veins running along the inferior border of each rib between the internal and innermost intercostals.
Transversus thoracis - On the deep side of the rib cage, transversus thoracis arises from the sternum and inserts on the dorsal, deep, surface of the costal cartilages. Transversus thoracis pulls the ribs caudally during respiration. Also look for the internal thoracic nerve artery and vein on either side of the sternum.
Serratus ventralis cervicis - Arises from the transverse processes of the cervical vertebrae and caudal ribs, and inserts on the vertebral border of the scapula. The serratus muscles are part of the appendicular skeleton, but they attach at least partially to the vertebrae. Helps to rotate the scapula and raise the glenoid fossa by depressing the vertebral border of the scapula. Also, protracts and holds the scapula in place. In addition, may help to pull the ribs cranially during respiration. Look for this muscle deep to latissimus dorsi in Figure 1.
Serratus dorsalis anterior - Arises from the spinous processes of the cervical vertebrae and inserts onto ribs 4 - 10. Pulls the ribs cranially during respiration. It is very difficult to see serratus posterior superior and inferior without substantial magnification. Therefore, in Figure 2 I have indicated the border and fiber direction of each muscle with a green line, inferior, and yellow line, superior. Also look for a difference in texture between the outlined and surrounding areas. Additionally, the superior muscle runs more anteroposteriorly and the inferior muscle runs mediolaterally. These muscles are visible upon close inspection with the unaided eye.
Serratus dorsalis posterior - Arises from the spinous processes of the 7-8 most caudal vertebrae, and inserts onto the last 6-7 ribs. The fibers of serratus posterior inferior merge with those of serratus posterior superior around rib 10. It was difficult to determine what relationship they possess at this point. Pulls the ribs caudally during respiration.
Erector spinae (sacrospinalis) is composed of three muscle masses, all
of which are distinguishable from one another along their entire length.
Multifidus spinae - Connects adjacent spinous and transverse processes.
This most medial muscle of the erector spinae extends the trunk when acting
bilaterally and bends the trunk laterally when acting unilaterally. All three
muscles are divisible along their entire length.
Longissimus - Arises from the iliac crest and sacrum and inserts on the caudalmost ribs and spinous processes of lumbar, thoracic, and possibly caudal cervical vertebrae. This middle muscle of the erector spinae extends the trunk when acting bilaterally and bends the trunk laterally when acting unilaterally. Longissimus is visible in Figure 2.
Iliocostalis - Arises from the iliac crest and inserts on the caudal 6 ribs. This most lateral muscle of the erector spinae extends the trunk when acting bilaterally and bends the trunk laterally when acting unilaterally. Illiocostalis is visible in Figure 2.
Splenius capitis - Arises from the nuchal raphé and inserts on
the occipital protuberance. Acting bilaterally extends the head and neck. Acting
unilaterally rotates and bends the head laterally.
Longissimus capitis - Arises from the transverse processes of the cervical vertebrae and inserts on the mastoid process. Acting bilaterally flexes the head. Acting unilaterally turns the head laterally.
A vertebral column allowing extensive flexion and extension, such as that found
in Tupaia, can add significantly to stride length, thereby increasing
an animal's speed [1,2]. These movements require specializations in the axial
skeleton and correspondingly powerful back musculature, such as the erector
spinae. A comparison of the highly arboreal Ptilocercus lowii with the
mostly terrestrial Tupaia glis, T. tana, T. minor,
Urogale, Dendrogale, and Anathana reveals that the former
has several adaptations in the axial skeleton for stability, while the Tupaiinae
are adapted for flexibility and powerful flexion and extension of the vertebral
column. Surprisingly the one arboreal species of the Tupaiinae, T. minor,
shares the suite of characters associated with increased flexibility. For example,
Ptilocercus has ribs that are much wider than any of the tupaiids, thereby
adding to the stability of the vertebral column [3]. Such stability is assumed
to be useful during bridging movements across gaps. Tupaiids have six lumbar
vertebrae compared to only five for Ptilocercus. Since the lumbar region
adds the most to stride length, this is a possible adaptation for increased
sagittal movement of the vertebral column. Ptilocercus however shows
an increased number of thoracic vertebrae, highly stable structures, over tupaiids.
Tupaiids also have a suite of characters associated with a more flexible neck
relative to Ptilocercus: thin atlas, caudally oriented spinous process
of the axis, large intervertebral space. While this added flexibility is difficult
to explain, it may be related to the rooting foraging method frequently employed
by the terrestrial tupaiids in leaf litter.
Furthermore, tupaiids have long, thin thoracic spinous processes, which presumably add to the flexibility of the vertebral column [4]. In addition, this extended length in the thoracic and also lumbar spinous processes provides a better lever arm for the vertebral extensors, which attach on these processes [3]. The lumbar transverse processes in tupaiids are also long and directed ventrally relative to Ptilocercus. Such structures allow a better lever arm for the flexors of the back, and create a larger volume available to the erector spinae muscles. Tupaiids have several specializations of the axial skeleton for increasing flexion/extension of the vertebral column thereby adding significantly to stride length. While Ptilocercus appears to have many structures suited for increased stability, it is not yet clear what biological role this serves. Most comparisons are made to slow climbers that frequently use bridging. However, it is not clear how much bridging Ptilocercus actually uses, or what the vertebral adaptations of similar animals are like. It is particularly interesting to note that the highly arboreal T. minor does not share any of the adaptations for added stability. It is possible that the ancestor of all Tupaia was terrestrial, and that the benefit gained from added sagittal bending of the vertebral column outweighs the benefits of stability.
1 Jenkins, F.A. 1974. Tree shrew locomotion and the origins of primate arborealism.
In: F.A. Jenkins, ed.: Primate Locomotion. New York:
Academic Press. pp. 85-115.
2 Schilling, N. and M.S. Fischer. 1999. Kinematic analysis of treadmill locomotion
of tree shrews, Tupaia glis (Scandentia: Tupaiidae). Z. Saugetierkunde.
64: 129-153.
3 Sargis, E.J. 2001. A preliminary qualitative analysis of the axial skeleton
of tupaiids (Mammalia, Scandentia): functional morphology and phylogenetic implications.
J. Zool. Lond. 253: 473-483.
4 Gambaryan, P.P. 1974. How mammals run. New York: Wiley.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale. Field Mus. Publ. Chicago Zool. 20:383-405.
Le Gros Clark, W.E.1924. The myology of the tree shrew (Tupaia minor). Proceedings of the Zoological Society of London 1924: 559-567.
Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus lowii). Proceedings of the Zoological Society of London 1926: 559-567.
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