Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Antebrachium of the Rabbit (Oryctolagus cuniculus)

Photos and text by Janelle Cooper and Alexandra Sardi

Introduction:

The rabbit is a plantigrade cursorial mammal but has a very unique style of locomotion: half-bounding (1).  In half-bounding, the two hind feet touch the ground almost in unison but the forefeet alternate (3).  The time the forefeet spend on the ground is shorter at all speeds.  This creates a gait in which the hind legs are much more important in providing the force than the forelimbs (3).  The rabbit has four digits and a reduced pollicis that has very little functional significance in locomotion and other movements of the manus.  The rabbit has non-retractile front claws that it uses in digging, grooming and self-defense.

Dissection Notes: 

We began the dissection by removing the skin from the elbow joint down to the digits.  This was relatively easy since the fascia attaching the skin to the muscle was relatively thin.  We then began removing the thick fascia that surrounded the muscles.  Removing this fascia was much more difficult because it was attached very close to the muscle.   The tendons originating from extensor carpi ulnaris and extensor digitorum lateralis were compartmentalized in their own thick surrounding of fascia.  We also found the dorsal carpal ligament tightly bound across the wrist of the rabbit.  We had to cut this ligament to separate the muscles and tendons.  Due to this compartmentalization and the presence of tendinous insertions, the muscles were very easy to separate and distinguish from each other.  The manus was particularly difficult to dissect due to the presence of a fleshy pad on the palmar side.  It was hard to separate from the tendons and muscles beneath it.  Furthermore, the pad had to be shaved because the hair growing directly from it and from beneath the nails made it extremely difficult to see.

Once the skin was removed, the superficial muscles of the antebrachium were visible.  On the lateral side, we identified extensor carpi radialis, flexor digitorium superficialis, extensor digitorium communis, extensor digitorium lateralis, extensor carpi ulnaris, and flexor carpi ulnaris.  On the inner or medial side we also identified flexor digitorum profundus and flexor carpi radialis, as well as abductor pollicis longus and extensor digitorium communis.  It is interesting to note that the rabbit does not have a brachioradialis muscle (2).  Instead, it has an extensor carpi radialis muscle with a higher insertion point.  Compared to the other specimens in class, the only other animal that lacks brachioradialis is the rat.

 In the manus, extensor digitorum communis separates into four tendons, extending digits 2, 3, 4 and 5.  Extensor digitorum lateralis divides into two tendons, extending digits 4 and 5.  The pollex, abducts separately from the rest of the digits.  On the flexor side, flexor digitorum superficialis extends to digits 2-5, as does flexor digitorum profundus (1). The rabbit has two interosseus muscles between each of the digits (2).

The median, radial and ulnar nerves innervate the forearm of the rabbit.  The median nerve arises from the 6^th and 7^th cervical nerves and runs across the caudal side of the humerus to the lateral surface of the forearm.  The radial nerve is formed by the 7^th and 8^th cervical nerves and innervates the extensor muscles of the forearm.  The ulnar nerve comes from the 8^th and 1^st thoracic nerves and innervates the flexor muscles of the forearm (1).  The nerves are not visible in the pictures because they were destroyed in the dissection.

Adapted from [1]

Discussion:

The antebrachium of the rabbit does not contain pronating or supinating muscles, which makes sense when one considers what the rabbit does with its forelimb.  The rabbit uses the half-bound as a means for fast locomotion. The forelimb is not as important as the hind limb, yet it still bears a lot of the animal’s weight.  Pronation and supination would probably lead to more injuries than benefits.  Furthermore, it would be detrimental to the rabbit while burrowing and is unnecessary for eating since the rabbit does not use its forelimbs to eat but grasps the grass with its lips.  The elimination of these muscles makes the antebrachium lighter and more adapted for speed.  In contrast, the cat retains them, and pronates and supinates to catch prey.  The rabbit retains the pollex but it is small and does not touch the ground when it walks.  Movement in the pollex is extremely limited.  The rabbit does, however retain movement through the abductor pollicis longus (1).  

The muscles of the manus are also restricted since rabbits do not require much dexterity to half-bound and burrow.  Most of the muscles extend or flex all of the digits in unison, the only exception being the pollex, which has its own abductor (2). 

References:

1. McLaughlin, Charles A.  Laboratory Anatomy of the Rabbit 3^rd Edition, McGraw-Hill, 1990: United States.

2. Popesko, P., Rajtova V., Horak J., A Colour Atlas of Anatomy of Small Laboratory Animals, V1.  Wolfe Publishing Ltd. London, England: 1992.

3. Kardong, Kenneth V., Vertebrates: Comparative Anatomy, Function, Evolution. McGraw Hill, New York: 2002.