Tree Shrew brachium
Text and photos: Terence Mitchell
Removing deep fascia - The deep fascia adheres strongly to the musculature of the brachium, and obscures the view of smaller muscles and origins/insertions. Create a small hole with the sharp probe, and then insert the probe between the fascia and muscle to 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.
Dorsal Brachium
Triceps - The triceps is a large muscle group composed of three heads - medial, lateral, and long. The medial head arises from the medial humeral shaft with some connection to the teres major. The long head arises from the axillary border of the scapula, and the lateral from the lateral humeral shaft. All three heads insert into the olecranon process of the ulna as a common tendon. Note that in Figure 1 the medial head can be seen attaching to the muscle body of teres major just before it dives deep to the long head. Look for two other large heads on the dorsal brachium - the long head is between the lateral and medial. Triceps extend at the elbow joint.
Ventral Brachium
Biceps brachii -Arises from the supraglenoid tubercle of the scapula and inserts on the proximal radius and ulna. In this case the short head had completely fused to the long. However, in some specimens the short head is present. Notice in Figure 2 the larger biceps brachii is medial to the smaller coracobrachialis. Biceps brachii flexes at the elbow joint and supinates.
Brachialis - Arises from the lateral humeral shaft and inserts onto the ulna with the biceps. Look for this muscle adjacent and lateral to the biceps brachii. Brachialis flexes at the elbow.
Coracobrachialis - Arises from the coracoid process of the scapula and inserts along the humeral shaft. It is necessary to cut the distal attachment and reflect biceps brachii to get a clear view of this muscle. Coracobrachialis extends the shoulder, thereby rotating the humerus cranially.
Le Gros Clark described several characteristics of Tupaia minor myology that he thought supported a close link between tupaiids and primates to the exclusion of the Insectivora [1]. It is important to note that Le Gros Clark and others lumped many generalized small mammals that eat primarily insects into wastebasket taxon known as Insectivora, and referred to these animals as insectivores. These mammals were thought to be the represent the most primitive extant condition. Tree shrews are interesting because they are very similar to members of the Insectivora, but differ in ways that Le Gros Clark thought made them more closely related to Primates. The debate over the use of these characters is treated more fully in this dissector under the shoulder section. Here I want to briefly mention three of these characters found in the brachium and antebrachium. First, Le Gros Clark claimed that the short head of the biceps brachii muscle was present in tree shrews and prosimians, but not in insectivores. However, tree shrews are actually variable in possession of one or two heads, and insectivores have only one head, making it difficult to tell what structure is present in insectivores [2]. Second, he claimed that the coracobrachialis muscle was fully developed in tree shrews and prosimians, but lacking in insectivores. In light of the fact that many marsupials have a coracobrachialis muscle, it should be classified as a symplesiomorphy between primates and tree shrews. Third, Le Gros Clark claimed that a brachioradialis muscle is present in tree shrews and not in insectivores. This character is similarly present in marsupials and therefore probably primitive for therians [3]. The above traits must therefore be rejected as synapomorphies of a primate/tree shrew clade.
As noted above, the muscles of the brachium are responsible for extension, flexion, abduction, adduction, and rotation of the forelimb. Therefore, these muscles are critical to the proper functioning of the brachium during locomotion. Tupaiid locomotion was studied intensely because of its potential usefulness as a model for early therian biology [2]. Such studies use cineradiographic techniques to investigate the movement of skeletal elements during locomotion on treadmills. In a recent study Schilling and Fischer examined the kinematics of walk, trot, and gallop gaits in a male and female common tree shrew, Tupaia glis [3]. The data show that tree shrews increase speed from walk to trot gaits by increasing step frequency. Due to a lack of data for gallops, and the animals' inability or unwillingness to perform bounds on the treadmill, it could not be determined how speed increases for the in-phase, bound and gallop, gaits. This is a fairly common result as many animals fail to perform normally on treadmills [4]. Tree shrews resemble other small mammals in that their humerus and tibia are horizontal to the ground when their foot is up, and their femur is horizontal when their foot is down [3]. Additionally, tree shrews maintain their ulna in a horizontal plane when their forefoot is down. Such limb geometry during symmetrical gaits allows for the full length of the structure to contribute to extension, and allows for a crouched posture. Crouching gives animals a greater ability to maneuver and increases stability. T. glis is similar to other small mammals in the degree to which the scapula contributes to overall body propulsion, total length that a fixed point on the animal moves over one stride. The scapula contributes a surprisingly high 40-60% of total propulsion length. This large contribution seems to be part of a general trend in small mammals towards dominance of proximal limb segments to stride length. The contributions of the humerus and ulna are highly variable and dependent on gait. The hand contributes significantly more to overall body propulsion relative to other small mammals. The contributions of hindlimb segments are also variable across gaits due to the added extension from the lumbar vertebrae during galloping. The movement of the femur and foot contribute significantly more to total stride length relative to other small mammals. While there are minor differences all tree shrews share important locomotor traits such as contributions of the spine to propulsion, flexed limb posture, and dominance of proximal segments to overall propulsion. In contrast they show few locomotor similarities to primates [5].
1 Le Gros Clark, W.E.1924. The myology of the tree shrew (Tupaia minor).
Proceedings of the Zoological Society of London 1924:559-567.
2 Reed, C.A. 1951. Locomotion and appendicular anatomy in three Soricoid
insectivores. Amer. Midl. Nat. 45:513-671.
3 Barbour, R.A. 1963. The musculature and limb plexuses of Trichosurus
vulpecula. Aust J. Zool. 11:488-610.
Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale.
Field
Mus. Publ. Chicago Zool. 20:383-405.
George, R.M. 1977. The limb musculature of the Tupaiidae. Primates 18:1-
34.
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.