Tree Shrew antebrachium
Text and photos: Terence Mitchell
Muscles of the Dorsal Antebrachium
In this lab, both the muscles of the arm, antebrachium, and hand, manus, will be dissected because many of the muscle bodies for the manus actually originate in the antebrachium. These muscles are known as the extrinsic muscles of the hand, while muscles originating within the manus are known as intrinsic muscles. Tree shrews have five manus digits numbered from I to V beginning at the thumb, pollex.
The following muscles are all visible on the dorsal side of the antebrachium once the skin and deep fascia have been removed. However, it may be necessary to supinate the arm slightly to get a clear view of the brachioradialis muscle. For each layer I will describe the muscles in a radial to ulnar direction based on the order of their muscle bodies. Assume that the pollex is lateral, hand is supinated, for all directions involving the terms medial and lateral. In Figure 1 the extensors of the right arm are seen with the radial side up.
Brachioradialis - The most radial of the dorsal muscles takes its origin from the lateral edge of the proximal humerus and inserts onto the lateral edge of the proximal radius. Extends at the elbow and supinates. Note that brachioradialis cannot be seen in Figure 1 because of its position between the flexors and extensors on the far lateral side of the forearm.
Extensor carpi radialis longus - Arises just proximal to brachioradialis and inserts dorsally on the second metacarpal. Extends and abducts the wrist.
Extensor carpi radialis brevis - Medial to extensor carpi radialis longus, this muscle arises from a common tendon with extensor carpi radialis longus and inserts dorsally on the third metacarpal. The muscle body is small and tightly bound to extensor digitorum communis. Extends and abducts the wrist.
Extensor digitorum communis - Medial to extensor carpi radialis brevis, this muscle arises from the lateral epicondyle and inserts into distal phalanges of all digits except the pollex. Extends all digits except pollex.
Extensor digitorum ulnaris - Medial to extensor digitorum communis, this muscle arises from the lateral epicondyle and inserts into the distal phalanges of IV and V. Extends digits IV and V.
Extensor carpi ulnaris - Arises from the lateral epicondyle and inserts dorsally into the fifth metacarpal. The muscle body was very tiny relative to the others in the dorsal antebrachium. Extends and adducts the wrist.
Cut the tendons of the superficial layer. Stagger the cuts and make the
incisions proximal to the muscle bodies so that muscle insertions can still
be distinguished. Leave the superficial layer of one arm uncut if possible.
Now observe the following three muscles:
Extensor digitorum radialis - Arises from the lateral epicondyle and inserts
into the distal phalanges of digits I, II, III. Extends digits I, II, III.
Supinator - Arises from the lateral epicondyle and inserts into the lateral edge of the proximal radius. Most of the muscle body is medial to extensor digitorum radialis. Rotates the radius around the ulna medial to lateral, supination.
Abductor pollicis longus - Arises from the radius and ulna and inserts into the metacarpal of digit I. Overlaps and can be difficult to separate from supinator. Look for abductor pollicis longus to be slightly distal to supinator. Also look for a slight difference in muscle direction (APL is more proximal-distal oriented), and the insertion for APL should be fairly distinctive. Abducts the pollex, first digit.
The extensor pollicis brevis is usually absent in tree shrews, but was reported in Tupaia picta [1].
Muscles of the Ventral Antebrachium
The following muscles are all visible on the ventral side of the antebrachium once the skin and deep fascia have been removed. As with the dorsal antebrachium, I will describe the muscles in a radial to ulnar direction based on the order of their muscle bodies. In Figure 2 the flexors of the right arm are seen with the radial side down.
Pronator teres - Arises from the medial epicondyle and inserts on the lateral side of the mid-radius [1]. Rotates the radius about the ulna in a lateral to medial direction, pronation.
Flexor carpi radialis - Arises from the medial epicondyle and inserts on the metacarpals of digits II and III. Flexes and abducts the wrist.
Palmaris longus - Arises from the medial epicondyle and inserts on the palmar aponeurosis. Flexes at the wrist and tightens palmar aponeurosis.
Flexor digitorum superficialis - Arises from the medial epicondyle and inserts on the middle phalanges of digits II-IV. The muscle's belly can be seen with the superficial layer but is slightly deep to palmaris longus and flexor carpi ulnaris. Flexes middle phalanges of digits II-IV at the proximal interphalangeal joint.
Flexor carpi ulnaris - Arises from the medial epicondyle and inserts into the pisiform and fifth metacarpal. Flexes and adducts the wrist.
Cut the tendons of the superficial layer. Make the incisions proximal to the muscle bodies, but distal enough that muscle insertions can still be distinguished. Leave the superficial layer of one arm uncut. Also, cut the flexor retinaculum, a tough ligament running mediolaterally across the distal end of the antebrachium. Follow the tendons of each muscle into the manus. Now observe the following two muscles:
Flexor digitorum profundus - Arises from the medial epicondyle and inserts on the distal phalanges of digits I-V. Flexes the distal phalanges and some flexion of the wrist.
Pronator quadratus - Arises from the lateral side of the distal ulna and inserts into the medial side of the radius. Cut the tendon of flexor digitorm profundus and reflect it distally. It was very hard to separate pronator quadratus from flexor digitorum profundus. Much of the muscle body was torn making it difficult to distinguish this muscle. It is important to take much care delineating and separating these muscles before reflecting flexor digitorum profundus. Pronates the forearm.
In animals that use their hands to manipulate objects, the musculature of the
forearm and manus are responsible for providing the force required by grasping
movements. The grasping abilities of tree shrews have been in question for many
decades now. This is a particularly interesting aspect of tree shrew biology
because it is convergent upon, or perhaps distantly related to, the highly specialized
grasping abilities of Primates.
While many animals possess some degree of manual dexterity, primates are characterized by prehensility, the ability to grasp an object with one hand. Some primates are also able to bring the hallux or pollex, thumb or big toe, into opposition with the other digits through abduction, medial rotation, and flexion at the carpo-metacarpal joint [2]. Some investigators have argued on the basis of morphological evidence, such as lack of a divergent pollex, and behavioral evidence, such as lack of grasping performances, that tree shrews do not resemble primates in their hand use [3]. However, such studies are often either biased by a reliance on a single species, Tupaia glis, or fail to examine the more arboreal Ptilocercus lowii and Tupaia minor. In a comparison of seven arboreal Tupaia minor and five terrestrial Tupaia tana, Sargis concludes that there are significant differences in grasping abilities between the two and that T. minor is more primate-like [4]. As might be expected the more arboreal T. minor exhibited a higher frequency of climbing, leaping, and suspensory behaviors, while T. tana relied mostly on quadrupedalism. Additionally, T. minor preferred an arboreal substrate 96% of the time, while T. tana was found on the ground 78% of the time. During climbing T. minor was more likely to reverse its feet at the ankle joint, and also used a semi-plantigrade posture as opposed to the digitigrade posture adopted by T. tana. These behavioral observations demonstrate some differences between these two species related to their substrate preference, but do not necessarily imply differences in grasping ability. However, T. minor also showed the ability to eat fruit, mealworms, and crickets with one hand, while T. tana always used two hands. Furthermore, T. minor was observed grasping dowels as it crossed them 84% of the time, while T. tana never grasped branches or dowels. Additionally, T. minor was often observed with a divergent hallux during locomotion, while T. tana kept its hallux in line with the remaining digits. This evidence lends overwhelming support to the hypotheses that Tupaia minor is capable of primate-like grasping performances. Scattered evidence from many other sources also suggests that Ptilocercus lowii is capable of prehensile movements [5,6].
While tree shrews are not now thought to be close relatives of primates, their
striking resemblances in grasping capabilities provide a unique look into what
an early primate ancestor may have looked like. Even if these abilities are
convergences, they still yield a great opportunity to tease apart which ecological
and behavioral variables are correlated with certain morphological features.
Other studies have shown remarkable similarities in locomotion and grasping
between didelphid marsupials and primates, which seem to be related to a small
branch niche [7]. Similar studies involving tree shrews will greatly enhance
our understanding of primate adaptation and evolution.
1 George, R.M. (1977). The limb musculature of the Tupaiidae. Primates 18:1-
34.
2 Napier, J.R. 1961. Prehensility and opposability in the hands of primates.
Symp. Zool. Soc. Lond. No. 5: 115-132.
3 D'Souza, F. 1974. A preliminary report on the lesser tree shrew Tupaia
minor.
In Prosimian Biology:167-182. Martin, R.D., G.A. Doyle, and A.C. Walker, eds.
London: Duckworth.
4 Sargis, E.J. 2001. The grasping behaviour, locomotion, and substrate use
of
the tree shrews Tupaia minor and T. tana (Mammalia, Scandentia).
J.
Zool. Lond. 253:485-490.
5 Gould, E. 1978. The behavior of the moonrat, Echinosorex gymnurus
(Erinacediae) and the pentail tree shrew, Ptilocercus lowii (Tupaiidae)
with
comments on the behavior of other Insectivora. Z. Tierpsychol. 48:1-27.
6 Haines, R.W. 1955. The anatomy of the hand of certain insectivores. Proc.
Zool. Soc. Lond. 125:761-777.
7 Lemelin, P. 1999. Morphological correlates of substrate use in didelphid
marsupials: implications for primate origins. J. Zool. Lond. 247:165-175.
Additional anatomical resources
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale.
Field
Mus. Publ. Chicago Zool. 20:383-405.