dissection of beaver antebrachium, BAA 289L 2004

The traits of the beaver antebrachium are distinctive and reflect its varying functions. The musculature and skeletal elements of the brachium and antebrachium are characteristic of a digging rodent. The olecranon process of the ulna is relatively large and long, supporting large and well-developed triceps. The humerus, radius, and ulna are all relatively short and stout, making the upper limb proportions small both in comparison to the body size as a whole, and in comparison to the highly specialized lower limbs. The beaver, which is semi-aquatic, does not use its arms/forearms for locomotion in water. In fact, in the water, beavers hold their arms tightly against the thorax to create a more streamline shape. On land, the arms are used quite effectively for burrowing, and also for more dexterous jobs such as holding branches, food, or other items for consumption or construction. The manus can be pronated and supinated, allowing for a larger variety of uses for the phalanges.

We began the dissection by making a proximal to distal cut along the dorsal side of the antebrachium. Large pockets of fat were encountered in the proximal portion of the antebrachium, just distal to the elbow. Dissection of this area proceeded with relative ease. The physical nature of the forearm, which as previously mentioned is relatively short and stout, made dissection of the distal antebrachium more difficult. Incisions in the distal portion of the forearm had to be made very carefully, due to a large number of tendons found in this portion of the antebrachium. Removal of the fascia was done slowly in order to avoid severing the tendons. Dissection of the ventral side of the forearm was also somewhat difficult, due to the limited flexibility of the shoulder joint. We abducted and extended the limb, and then proceeded to remove the outer layers of skin and fascia to expose the muscles and tendons. After all the superficial muscles of the antebrachium were exposed, we then proceed to dissect the manus. We first removed the skin from the dorsal aspect of the manus, making sure not to sever any tendons below the thin fascia. We proceeded to separate out each tendon, and then attempted to determine the identity of each. The palmar dissection proved to be difficult, as the skin of the palm is very thick and the muscle fibers are not well defined.

Muscle	Origin(s)	Insertion(s)	Action(s)
Antebrachium Extensors:
Extensor carpi ulnaris	Distal aspect of the lateral epicondyle of the humerus, lateral surface of olecranon	Converges on one tendon, which inserts into the shaft of the fifth metacarpal	Extends the wrist
Extensor digiti quinti	Lateral epicondyle of the humerus, and intermuscular septa formed by surrounding musculature	Becomes tendinous at the mid-forearm and inserts in the dorsum of the first phalanx of digit V	Extends the fifth digit
Extensor digitorum communis	Lateral epicondyle of the humerus	Converges on three tendons, which insert in the base of the terminal phalanx of digits II - V	Extends digits II - V
Brachioradialis	Proximal aspect of the lateral epicondyle of the humerus	Converges on one tendon at the mid-forearm, tendon inserts on base of the second metacarpal	Supinates the forearm
Extensor carpi radialis	Lateral epicondyle of the humerus	Becomes tendinous at the mid-forearm, tendon inserts into multangular major	Extends the wrist
Extensor carpi obliquus	Upper half of lateral radial shaft, and upper half of lateral ulna	Converges on one tendon, which inserts into the accessory carpal bone and the greater multangular	Extends the wrist, and may supinate the forearm
Extensor indicis proprius	Upper half of the posterolateral ulnar shaft	Converges on one tendon, which inserts into the base of the terminal phalanx of digit II	Extends the wrist
Supinator	Anterodistal aspect of the lateral epicondyle of the humerus	Upper half of the shaft of the radius	Supinates the forearm

Antebrachium Flexors:
Pronator teres	Anterior aspect of the medial epicondyle of the humerus	Medial surface of the upper half of the radial shaft	Pronates the forearm
Pronator quadratus	Palmar surface of the middle third of the shaft of the ulna.	Postero-palmar surface of the middle third of the radius	Pronates the forearm
Flexor carpi radialis	Medial epicondyle of the humerus, and intermuscular septa formed by surrounding musculature	Converges on one tendon, which inserts onto the second metacarpal	Flexes the wrist
Palmaris longus	Medial epicondyle of the humerus, and proximal aspect of the olecranon	Forms a tendon two-fifths down the forearm and inserts into palmar pad and aponeurosis	Flexes the digits, and tightens the palmar aponeurosis
Flexor digitorum sublimis	Medial epicondyle of the humerus	Converges on one tendon, which divides into three branches that insert in digits II – IV at the second phalanges	Flexes digits II – V at the proximal interphalangeal joint
Flexor digitorum profundus	Medial epicondyle of the humerus	Divides into five tendinous branches which insert at the palmar side of the base of the terminal phalanges	Flexes digits I – V at the distal interphalangeal joint
Flexor carpi ulnaris	Posterior margin of the proximal portion of the olecranon	Tendon attaches at the pisiform	Flexes the wrist

Manus:
Flexor digiti quinti brevis	Palmar aspect of the contiguous portions of the bases of the fourth and fifth metacarpals	Fibers converge into a short tendon which is inserted onto the first phalanx of the fifth digit	Flexes the fifth digit
Abductor digiti quinti	Pisiform bone	Ulnar side of the base of the first phalanx of the fifth digit	Abducts the fifth digit
Abductor pollicis brevis	Accessory carpal bone	Radial sesamoid bone and onto the base of the first phalanx of the first digit	Abducts the pollex (digit I)
Lumbricals
a) Lumbrical 1	Medial surface of the deep flexor tendon of the second digit	Passes to the radial side of the corresponding digit and inserts into the digital sheath; connected with extensor digitorum communis	Extends the proximal and distal interphalangeal joints, and may flex the metacarpophalangeal joints
b) Lumbrical 2	Contiguous sides of the tendons of the second and third digits	See insertion for Lumbrical 1	See action for Lumbrical 1
c) Lumbrical 3	Contiguous sides of the tendons of the second and third digits	See insertion for Lumbrical 1	See action for Lumbrical 1
d) Lumbrical 4	Palmar surface of the tendon to the fifth digit	See insertion for Lumbrical 1	See action for Lumbrical 1
Interossei
a) Interosseus 1	Palmar surface of the base of the metacarpus of the digits	Sesamoid bone on the radial side of the second digit	Extends the proximal and distal interphalangeal joints
b) Interosseus 2	Palmar surface of the base of the metacarpus of the digits	Sesamoid bone of the third and fourth digits	Extends the proximal and distal interphalangeal joints
c) Interosseus 3	Palmar surface of the base of the metacarpus of the digits	Sesamoid bone of the third and fourth digits	Extends the proximal and distal interphalangeal joints

Reference:

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