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Facial and Masticatory Muscles |
Photos and text by Ana Panduro and Yu Jin Kim Abdominal hypaxial muscles: The body wall is composed of four layers of hypaxial muscles that are present in the neck, thorax, and abdomen. In the abdomen, these four layers are: rectus abdominis, external oblique, internal oblique, and transversus abdominis. Rectus abdominis runs caudo-cranially from the symphysis pubis to the sternum, inserting into the first rib[1]. The remaining three layers of hypaxial muscles are laterally attached to the rectus abdominis on each side. The innermost layer, transversus abdominis, runs horizontally around the abdomen for the most part, but slants ventrally downward near the aponeuroses with rectus abdominis. The layer superficial to transverse abdominis, but deep to the external oblique, is the internal oblique. This muscle is the middle layer and extends ventro-anteriorly, starting from the lumbodorsal fascia and the crest of the ilium, and inserting into the linea alba [1,2]. The most superficial layer is the external oblique, which runs caudo-cranially, like the rectus abdominis. In most mammals, the fibers of this muscle slant ventrally downward [p. 68, 1] making the striation pattern for our specimen unique among those we saw in the lab. To observe the four layers of hypaxial muscles in the abdomen, we made one cranial-caudal incision with a scapula starting about 2 centimeters from the xiphoid process and ending about 8 centimeters anterior to the vagina. We then made one medio-lateral incision at the starting and ending points of the last incision. Each incision was about 4-5 centimeters long. Finally, we retracted the flap of muscles and began separating them with a probe. This proved to be a difficult task because the muscles are tightly adhered. Fortunately, the striations in the muscles of our rat are pronounced enough that we were able to use them as guidelines. The striations of the hypaxial muscles are less noticeable in the fetal pig, because it has not yet developed, and the opossum, whose hypaxial muscles are paper-thin. Thoracic hypaxial muscles: The layers of hypaxial muscles in the thorax include the external intercostals, internal intercostals, innermost intercostals, serratus dorsalis, and the scalenes. The external and internal intercostals are homologous with the external and internal obliques [3]. The intercostals extend between adjacent ribs, with superficial external intercostals slanting ventrally downward and the deeper internal intercostals slanting dorsally downward [3]. We observed the relationship of these muscles on the part of the rib cage that had been cut out three weeks before during the thoracic cavity lab. The innermost intercostals are the deepest of the hypaxial muscles found in the ribs. We were unable to distinguish the direction of their striations because of how thin they were. We were also unable to locate the scalene muscles because the our specimen’s dorsal neck and head regions had been damaged before we got them. The serratus dorsalis muscles originate in the spinous process of the thoracic vertebrae and insert into the ribs. The cranial portion of the muscles runs caudoventrally and the caudal portion runs cranioventrally [2]. Epaxial musculature: To locate the deep epaxial muscles of the back, we first removed the lumbodorsal fascia using scissors, tweezers, and a scalpel. We failed to remove the entire fascia because it is very tough and tightly adhered to the longissimus dorsi muscle. Multifidus is the most medial muscle, originating in the articular process of the lumbar and thoracic vertebrae. As it extends more cranially, its name changes to spinalis. Lateral to the multifidus is the longissimus dorsi, which is a thick and broad muscle. In the anterior dorsal thoracic region, iliocostalis is lateral to the longissimus and deep to serratus dorsalis.Abdominal hypaxial mm. [1,2,4]
Thoracic hypaxial mm [1,2, 4]
Epaxial musculature [1,2, 4]
References: 1. Greene, Eunice C. 1935. Anatomy of the Rat. American Philosophical Society, Philadelphia. 2. Hebel, Rudolf and Melvin W. Anatomy and Embryology of the Laboratory Rat. Stromberg. 1986. BioMed Verlag, Germany. 3. Cartmill, Matt et al. Human Structure. Cambridge: Harvard University Press, 1987. 4. 2002 Comparative Mammalian Anatomy class. Ferret Body Wall Lab. Retrieved on April 26, 2004 at http://www.baa.duke.edu/companat/mainpagemammals.htm
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
