Text and photos: Terence Mitchell
My specimen of Tupaia belangeri is a small (19 cm from the last sacral vertebra to the tip of the snout) mammal from Southeast Asia. It has a dark brown pelage with lighter patches on its abdomen and muzzle. Its hind limbs, measuring 13 cm in length, are significantly longer than its forelimbs, which measure 9 cm in length. The tips of the distal phalanges on the manus (hand) and pes (foot) possess curved claws. The pinnae are large and mobile. The vibrissae are short, less than 1.5 cm. Its eyes do not possess a nictitating membrane. It has a moderately long tail, 17 cm, relative to its body size. The specimen is male and has a penis bordered by a right and left testicle.
Remove the skin - Pull up the skin over the spine and adjacent to the tail with a pair of forceps, and insert the sharp probe. Run the probe up the length of the spine while pulling up on the skin. Cut down the probe making an incision from the tail to the skull. Using the probe in a similar fashion make mediolateral cuts across the shoulder and caudal to the scapula. Use forceps to gently peel back the skin. In most cases this will yield a nice separation, leaving only deep fascia superficial to the muscles. Throughout the dissection only skin sections of the body as needed to prevent premature desiccation. When skinning a new area, try to peel from an existing edge.
Defining borders - Use the blunt probe when possible (and sharp probe when necessary) to define the borders of each muscle. Gently place the probe beneath the muscle to separate it from surrounding layers.
Reflecting muscles - Make sure the borders are well defined and that the muscle is as mobile as possible without tearing any muscle tissue. Insert a probe beneath the attachment you will be reflecting, and use blunt dissection or a scalpel to cut the muscle.
Removing deep fascia - The deep fascia will often adhere to the muscle after the other layers are peeled back. This fascial layer can often obscure the view of smaller muscles and origins/insertions for any muscle. It is often possible to detach much of this tissue with forceps or fingers. If the fascia is tightly bound, create a small hole with the sharp probe. Insert the probe between the fascia and muscle and separate the two as much as possible. Repeat this process to create several large breaks in the deep fascia. Remove the remaining tissue with forceps.
Trapezius - The trapezius is the superficial muscle originating from the external occipital protuberance to the most caudal thoracic vertebrae and inserting into the scapular spine. Insert the probe beneath the trapezius as close to the spine as possible. Using blunt dissection or a scalpel reflect the trapezius without altering the scapular attachment. In Figure 1 the lateral border of trapezius has been torn and is highlighted in yellow. Originally the cut inferior border covered the now visible spino-humeralis, teres major, and spinodeltoideus, and inserted into the scapular spine with the superior fibers. Trapezius elevates, retracts and rotates the scapula.
Latissimus dorsi - The latissimus dorsi is the superficial muscle originating along the spine from the lumbar vertebrae to T6 via the thoracolumbar fascia and inserting into the intertubercular groove of the humerus. Reflect at the medial attachment as with the trapezius. Look for this muscle inferior to trapezius in Figure 1. Latissimus dorsi flexes, adducts, and medially rotates the humerus.
You should now be able to see the following muscles:
Rhomboideus - Deep to the trapezius the rhomboids originate from the external occipital protuberance to T4-6 and inserts on the medial lip of the vertebral border of the scapula. In Figure 2, rhomboideus is medial to omohyoideus. [Web Editor's Note: Figure 2 not included b/c of size constraints] The rhomboids retract and elevate the scapula.
Levator scapulae - Deep to the trapezius the levator
scapulae arises from the atlas and inserts onto the spine of the scapula. This
insertion can be seen in Figure 2. Also, look for the inferior portion of this
muscle superior to the intersection of the spinodeltoideus and acromiodeltoideus.
Levator scapulae moves the scapula cranially.
Omohyoideus - Arises from the hyoid bone and inserts into the cranial border of the scapula. In Figure 2, look for this muscle just medial to the insertion of the rhomboids. Initially it is easier to locate this muscle dorsally. [Web Editor's Note: Figure 2 not included b/c of size constraints]
Subclavius - Arises from the manubrium and attaches to the caudal surface of the clavicle. Subclavius depresses the clavicle.
The following pectoral muscles can be seen in Figure 3.
Pectoralis major - The pectoralis major is a superficial muscle arising along the midline from the manubrium to the xiphisternum and inserting into the deltoid tuberosity of the humerus. Reflect this muscle along the midline. Pectoralis major adducts and medially rotates the humerus.
Pectoralis abdominalis - Inferior to the pectoralis major, the pectoralis abdominalis arises from the linea alba and inserts into the deltoid tuberosity of the humerus. It appears to be almost continuous with pectoralis major, but look for a difference in fiber direction as a guide to the border of this muscle. There may also be a thick layer of connective tissue between these two muscles. Reflect this muscle along the midline with pectoralis major. Pectoralis abdominalis adducts and medially rotates the humerus.
You should now be able to see the following muscles:
Pectoralis minor - Deep to the pectoralis major, the pectoralis minor originates from ribs 2-7 and inserts on the shoulder capsule. Pectoralis minor helps in flexing the arm.
Panniculus carnosus - Arises from the superficial fasica of the abdomen and inserts into the deltoid tuberosity of the humerus. This muscle joins pectoralis abdominalis near their common point of insertion. Do not reflect at this time. Panniculus carnosus helps to adduct and medially rotate the humerus. It is also responsible for moving the skin.
NOTE: You must first reflect the trapezius to clearly view these muscles.
Deltoideus - The three deltoid muscles are a large muscle
group covering most of the shoulder.
Claviculodeltoideus arises from the ventral border of the clavicle, the acromiodeltoideus
arises from the acromion, and the spinodeltoideus arises from the scapular spine.
All three insert onto the deltoid tuberosity of the humerus. In Figure 4, the
claviculodeltoideus and acromiodeltoideus appear to be a continuous muscle separated
only by a thin median raphé. The spinodeltoideus, on the other hand,
is almost perpendicular to the other two when it dives beneath the acromiodeltoideus
adjacent to the superior border of teres major. Claviculodeltoideus flexes the
humerus, acromiodeltoideus abducts the humerus, and spinodeltoideus laterally
roatates and extends the humerus. There has been some confusion in the literature
about the origin of spinodeltoideus. In 1924, this muscles was initially classified
as the teres minor by Le Gros Clark. However, in 1966, Kladetsky and Kobold
showed that this muscle was homologous to the spinodeltoideus of insectivores.
In Figures 2 & 4 the notice the following three muscles. Notice that spino-humeralis, teres major, and spinodeltoideus form three parallel and adjacent bands of muscle bordered cranially by supraspinatus. [Web Editor's Note: Figure 2 not included b/c of size constraints]
Teres Major - The teres major arises from the axillary border of the scapula and inserts into the intertubercular groove of the humerus. Teres major extends and laterally rotates the humerus.
Infraspinatus - Deep and superior to teres major, infraspinatus arises from the infraspinous fossa on the posterolateral side of the scapula and inserts into the greater tubercle of the humerus. Cut the medial attachment of spinodeltoideus to locate this muscle. Infraspinatus laterally rotates the arm.
Spino-humeralis - Inferior to teres major, this muscle arises from T2-6 and inserts into the humerus with teres major. Look for this muscle adjacent to the inferior border of teres major. Spino-humeralis extends and laterally rotates the humerus.
Supraspinatus - Arises from the supraspinous fossa on the anterolateral side of the scapula and inserts into the scapular spine. Look for the supraspinatus in Figure 2 adjacent to the superior border of spinodeltoideus. Supraspinatus helps abduct the arm.
Subscapularis - The subscapularis arises from the entire subscapular fossa on the medial side of the scapula and inserts into the lesser tubercle of the humerus. Reflect teres major and spino-humeralis to get a clear view of this muscle. Subscapularis adducts and medially rotates the humerus.
More than any other single individual Le Gros Clark is responsible for the phylogenetic association between tree shrews and primates. In his analysis of the brain, skull, and myology of Tupaia, especially Tupaia minor, he concludes that tree shrews have more non-primitive traits in common with prosimians than with any insectivore [1,2,3]. In an exhaustive dissection, he also shows that Ptilocercus lowii differs from Tupaia in many characteristics - most of which associate Tupaia with primates [4]. However, as a larger array of insectivores and marsupials were described many of the similarities between tree shrews and primates were identified as symplesiomorphies [5,6]. A brief review of the comparative myology demonstrates the difficulties involved with earlier interpretations. While Le Gros Clark initially listed 20 similarities between tree shrews and prosimians, this number was later pared down to 14 [3,7]. Unfortunately, eight of these similarities are now known to be present in insectivores as well as tree shrews and prosimians. An interesting example is the teres minor muscle, which Le Gros Clark claimed to be present in tree shrews and primates, but absent in insectivores. The first problem is that the teres minor has a wide distribution among mammals, occurring in primates, squirrels, and many marsupials [5]. More importantly, the reported teres minor is in fact the spinodeltoideus division of the deltoid group, which is present in many insectivores [8]. Marsupials, prosimians, and tree shrews share the six remaining myological similarities. All of Le Gros Clark's proposed synapomorphies appear to be primitive conditions.
1 Le Gros Clark, W.E.1924a. On the brain of the tree shrew (Tupaia minor).
Proceedings of the Zoological Society of London 1924: 559-567.
2 Le Gros Clark, W.E.1925. On the skull of Tupaia. Proceedings of the
Zoological Society of London 1925: 559-567.
3 Le Gros Clark, W.E.1924b. The myology of the tree shrew (Tupaia minor).
Proceedings of the Zoological Society of London 1924: 559-567.
4 Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew
(Ptilocercus lowii). Proceedings of the Zoological Society of London 1925: 559-567.
5 George, R.M. 1977. The limb musculature of the Tupaiidae. Primates 18:1-
34.
6 Luckett, W.P., ed. 1980. Comparative biology and evolutionary relationships
of tree shrews. New York: Plenum Press.
7 Le Gros Clark, W.E.1971. The antecedents of man. 3rd ed. Quadrangle
Books. Chicago, Illinois.
8 Reed, C.A. 1951. Locomotion and appendicular anatomy in three Soricoid
insectivores. Amer. Midl. Nat. 45:513-671.
Following is a list of all the resources I found for tree shrew anatomy. I have referenced the most useful resource for each body segment in its individual section. Some of the sources listed below are not mentioned elsewhere, but do contain useful information. Where it is not immediately obvious, I have made a brief mention of the kind of information available in each resource.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew
Dendrogale. Field
Mus. Publ. Chicago Zool. 20:383-405.
Treats the skeleton, urogenital system, and digestive system briefly. Detailed discussion of myology.
George, R.M. 1977. The limb musculature of the Tupaiidae.
Primates 18:1-
34.
Haines, R.W. 1955. The anatomy of the hand of certain insectivores. Proc. Zool.
Soc. Lond. 125:761-777.
Le Gros Clark, W.E.1924. On the brain of the tree shrew (Tupaia minor).
Proceedings of the Zoological Society of London 1924: 559-567.
Le Gros Clark, W.E.1925. On the skull of Tupaia. Proceedings of the
Zoological Society of London 1925: 559-567.
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.
Incredibly detailed dissection of most systems.
Straus, W.L. 1941. The phylogeny of the human forearm
extensors. Hum. Biol.
13:23-50, 202-238.
Straus, W.L. 1942. The homologies of the forearm extensors. Amer. J. Anat.
70: 281-316.
Verma, K. 1965. Notes on the biology and anatomy of the Indian tree shrew,
Anathana wroughtoni. Mammalia. 29: 289-330.
Treats the myology and urogenital system briefly. Thorough discussion of the skeleton.
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tree shrew brachium
tree shrew antebrachium and hand