Dissection of beaver arm, BAA 289L 2004

C. canadensis employs two different forms of locomotion depending on whether it is moving aquatically or terrestrially. When swimming, the forelimbs are held against the thorax and are not employed in locomotion. The small size of the forelimbs and the ability of the beaver to hold them tightly against the body are important to maintaining a streamlined shape in the water. On land the beaver is quadrupedal, walking on all four limbs in a plantigrade fashion. However, the forelimbs are less specialized for locomotion (as the beaver’s form of quadrupedality requires few specializations) than they are for digging.

The beaver also uses its forelimb to manipulate objects with great dexterity. The beaver can pronate and supinate its forelimb, though movement is somewhat restricted due to limited rotation of the radius at the elbow joint. The hands are used for grasping food and manipulating objects such as branches or other materials used in construction of dams and lodges.

In comparison to the hindlimb, the forelimb is relatively short. Indeed, it is short in proportion to the body size as a whole. The major differences in the anatomy of the beaver forelimbs and hindlimbs are related to their different functions; the beaver forelimbs are used in digging and manipulating objects, while the beaver hindlimbs are used mainly for aquatic locomotion. The functions of the forelimb are reflected in its dexterity (for manipulating objects) and by the large, well-developed triceps and long olecranon process (for digging).

Dissection of the brachium proceeded quickly, as the brachium itself is relatively short. We began in the neck region by making a curved incision from the dorsal aspect of the neck, down toward the ventral aspect. The panniculus carnosus is especially thick in this region (upwards of 1-2cm), so extra force and deeper cuts were required in this area. We then proceeded to make an incision in the axillary region, reflecting the remaining cutaneous musculature in this area. There are large fat deposits in this area, which made separation of the surrounding muscles difficult. We were able to remove most of the fat by pulling on the tissue (thus putting tension on the underlying fascia) and then using the scalpel to make small, precise cuts to separate it from the muscle. However, small amounts remained because we were unable to remove all of the fat without damaging the underlying muscle. We next made an incision at the proximal end of the brachium, and proceeded to cut down its midline until the reaching the proximal end of the forearm. We located the olecranon process immediately, as it was relatively superficial and large in proportion to the other bony elements of the brachium. After locating the triceps musculature, we dissected out each of the five heads. The long, lateral, and two medial heads were easily located, but anconeus proved more difficult to identify. Anconeus is quite small and mostly tendinous. Throughout the dissection, we removed portions of the remaining fascia covering different muscles to provide a better view of the arm musculature.

Muscle	Origin(s)	Insertion(s)	Action(s)
Elbow Extensors:
Triceps
a) Lateral Head	proximal shaft of the humerus	antero-lateral aspect of the olecranon	extends forelimb at the elbow joint
b) Long Head	posterior border of scapula	proximal aspect of the olecranon	extends forelimb at the elbow joint, and retracts the arm at the shoulder joint
c) Medial Head, major	postero-medial surface of humerus, crest of the lateral epicondyle	olecranon, deep to long and lateral heads	extends forelimb at the elbow joint
d) Medial Head, minor	medial epicondyle of humerus	medial margin of olecranon	extends forelimb at the elbow joint
e) Anconeus	distal aspect of the lateral humeral epicondyle	postero-lateral border of ulna	extends forelimb at the elbow joint

Elbow Flexors:
Brachialis	inferior aspect of the greater tuberosity, humeral shaft	lateral to insertion of biceps brachii on the ulna	flexes forelimb at the elbow joint
Biceps brachii	supraglenoid tubercle of scapula	bicipital tuberosity of ulna	flexes forelimb at the elbow joint, and extends arms at the shoulder
Coracobrachialis	coracoid process of scapula	antero-medial and medial border of humerus	flexes forelimb at the elbow joint

The large triceps (and surrounding dense musculature), the large olecranon, and the relatively short length of the antebrachium of the beaver are all characteristics of the forelimbs of digging mammals. The triceps musculature is quite thick and well developed and the skeletal elements are robust. The olecranon process of the ulna is proportionally quite long and robust, as is necessary to provide insertion for the large triceps. Three of the triceps heads are located on the lateral aspect of the brachium: long head, lateral head, and anconeus. The long head inserts on the most proximal aspect of the olecranon process, with the lateral head inserting more distally. Anconeus is the only head of triceps that does not attach to the olecranon process, inserting instead on the posterolateral aspect of the ulna. The two medial heads are located in the medial compartment and axillary region of the brachium, and cannot be viewed laterally.

The biceps brachii of the beaver is unusual in that it actually possesses only one head. In the general mammalian form, the long head of biceps crosses the shoulder joint, giving the muscle the ability to extend the arm at the shoulder in addition to flexing the elbow, while the short head of the biceps does not cross the shoulder joint. In the beaver, the short head is not present.

1. Young, F.W. (1937). Studies of Osteology and Myology of the Beaver (Castor canadensis). Michigan State College, East Lansing. pp. 73-75.