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Facial and Masticatory Muscles |
Photos and text by Ana Panduro and Yu Jin Kim Introduction: Muscles in the antebrachium (forearm) mostly control the movement of the wrist and fingers, and can be divided into two groups based on their movements. Those that extend the joints are called extensors. Conversely, those that flex the joints are called flexors. Extensors straighten the joints by moving the base of the phalanges of the manus away from the palm of the manus, thus increasing the angle between the wrist and the palm. These muscles are located on the dorsal side of the forearm [1]. The extensor group is comprised of eleven muscles, seven of which are superficial muscles (extensor carpi ulnaris, extensor digitorum lateralis, extensor digitorum communis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digiti quarti, and extensor digiti quinti proprius) and four deep muscles (abductor pollicis, extensor pollicis brevis, and extensor pollicis longus) [1,2]. Unlike some other mammals, Rattus norvegicus lacks brachioradialis [1], an extensor muscle that in humans originates in the lateral supracondylar ridge of the humerus, inserts in the styloid process of the radius, and is used to flex the elbow and supinate the hand. Instead, Rattus norvegicus supinates using the supinator muscle, which arises from the dorsal region of the lateral epicondyle and inserts into the base of the fifth metacarpal [1]. Flexors bend the joints by moving the base
of the phalanges toward the palm of the manus, resulting in a decrease
of the angle between the two[1]. They are located on the ventral side
of the forearm, most originating from the medial epicondyle of the humerus.
The flexor group is comprised of seven muscles, five of which are superficial
(pronator teres, flexor carpi radialis, palmaris longus, flexor carpi
ulnaris, and flexor digitorum superficialis), and two that are deep (pronator
quadratus and flexor digitorum profundus) [1,2]. Dissection notes: The dissection began with skinning the forearm and wrist of the rat. Though the skin was not difficult to remove, it became a tedious task, as there was much subcutaneous fascia connecting the skin to another layer of fascia that tightly hugged the antebrachial musculature. The second layer of fascia (antebrachial fascia) was tough to remove because it adhered tightly to the muscles. Once this antebrachial fascia was removed, the muscles were easy to separate and distinguish. Most of the extensor muscles that originate from the lateral epicondyle of the humerus are of comparable size and thickness. The same held true for the flexors that originated from the medial epicondyle. Similarly, in both groups muscle thickness decreases
distally, with fleshy bellies near the elbow extending as long tendons
over the wrist and digits. A third, and more adhesive layer of fascia,
known as a retinaculum, wraps around the wrist. Because flexors and extensors
have long tendons, the retinaculum prevents the relaxed tendons from shifting
out of place. The dorsal portion of the retinaculum holds the extensors
in place while the ventral portion of the retinaculum holds the flexors
in place. We cut the retinaculum parallel to the tendons. Rattus norvegicus is a pentadactyl mammal with a reduced pollex. Unlike the other digits of the manus, the pollex has a fully developed, flattened nail instead of a sharpened claw. The rat has walking pads on the ventral aspect of the manus. There are “five apical pads on the tips of the digits, three interdigital pads on the palm, and two on the base of the palm” [p. 2-3, 1]. The dorsal surfaces of the manus are sparsely covered with short fur while the palms lack fur completely.
References: 1. Greene, Eunice C. 1935. Anatomy of the Rat. American Philosophical Society, Philadelphia. pp 47-53. 2. Hebel, Rudolf and Melvin W. Anatomy and Embryology of the Laboratory Rat. Stromberg. 1986. BioMed Verlag, Germany. pp. 34-39.
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
