|
Facial and Masticatory Muscles |
Photo and text by Lazaro Gonzalez, Jr. Introduction: The size of the fetal pig’s internal organs will vary from that of an adult pig due to its early developmental stage. Medical students commonly use pigs as subjects for practicing thoracic surgery due to the size and arrangement of the pig’s internal organs. The circulatory system is often the most studied aspect of a fetal pig. The venous and arterial systems of my specimen were injected with latex, blue and red respectively, which made identification of the fetal blood supply easier. General Dissection: Any identification or documentation of chest musculature was done before cutting to get to the thoracic cavity. I made a transverse incision approximately an inch below the end of the sternum, being careful not to cut too deeply. At the edge of this incision (approximate a half inch ventral to the forelimb) I used blunt scissors to cut cranially through the cartilaginous ribs to the top of the chest. I made the same cuts laterally and medially and cranially and caudally. These cuts formed a rectangular “door” to the thoracic cavity. I pulled this piece up with little difficulty, only getting caught on some fascia that tore easily. The lungs are located on either side of the heart and extended deeply caudally. I severed the pulmonary artery to free the left lung from the pleural cavity and removed it. This revealed the descending aorta and clear view of ductus arteriosus along with some innervation. The thin pericardial sac cut open very easily. I used scissors to cut most of the sac out to make the area less cluttered. The heart was difficult to remove because the major vessels holding it in place were filled with thick latex. I severed the precaval, postcaval, and aorta with the scalpel and removed the heart from the thoracic cavity. Discussion: This lab’s focus is the circulatory system of the fetal pig. However, the major organs of the thoracic cavity are briefly discussed. The lungs are the first accessible organs as the chest cavity was opened. They are covered in a thin film called visceral pleura [1]. The right lung has an apical, a cardiac, an intermediate, and a diaphragmatic lobe, whereas, the left lung has a cranial, a cardiac, and a diaphragmatic lobe [1]. The space they occupy in is known as the pleural cavity [1,2,3,4,5]. The thymus is also visible upon opening to the thoracic cavity, which plays a major role in the immune system because it releases thymosin, a hormone for development of the immune system [2]. It is much larger relative to body size in fetal pigs than adults due to this important role in development. After removal of the lungs, the aorta and esophagus were visible in the left pleural cavity. Phrenic nerves can be seen innervating the diaphragm, passing lateral to either side of the pericardium. The heart lies inside the fluid-filled pericardial sac. The major vessels of the heart are: aorta, precaval, postcaval, pulmonary arch, and pulmonary veins. The aorta carries some oxygenated blood from the left ventricle to the body, but also carries semi-oxygenated blood back to the mother via the umbilical arteries. Deoxygenated blood from the body is brought back to the right atrium through the postcaval and the precaval. The internal thoracic vein drains blood from the armpit into the precaval at a ninety-degree angle. Coming into the precaval at the same angle are the left and right axillary veins, which are carrying blood from the forelimbs back. The subclavian vein, made up of the subscapular and axillary vein, also drains into the precaval. The cephalic vein is the last of the three veins that brings blood back from the forelimb. Blood drainage from the head and neck come via the internal and external jugular veins, respectively. The pulmonary arch is the pathway that carries semi-oxygenated blood from the right ventricle via the pulmonary artery to the aorta and upper extremities [2]. Because the fetal pig gets most of its nutrient-rich oxygenated blood from its mother little use is made of the pulmonary artery. A structure known as ductus arteriosus connects the pulmonary artery and the aorta, bypassing the inactive lungs of the fetus [2,3,4,5]. The heart was then cut out of the thoracic cavity by severing the major vessels. The left and right auricle, left and right ventricle, and left coronary artery and vein are visible externally on the heart. The atrial chambers have thin walls and are separated by the atrial septum. The foramen ovale is another fetal structure that helps reroute blood to bypass the lungs. It is located in the atrial septum and closes off at birth [2,3,4,5]. These chambers are holding areas for blood traveling through the heart. The ventricular chambers have muscular walls in order to pump blood. These chambers are also separated by a wall of tissue called the ventricular septum. If the foramen ovale does not close completely after birth it can cause blood flow complications, commonly called a “heart murmur.” Comparative Anatomy: The circulatory system pathway of the fetal pig is different from that of adult mammals. It shares a blood supply with its mother via the umbilical artery and vein in order to get nutrients while attached by the placenta. The fetal pig uses the ductus arteriosus and foramen ovale while these structures are atrophied and closed in mammals with independent circulatory systems. The thymus is larger in proportion to the fetal pig’s body size due to its importance in immune system development. Pigs have 1 precaval vein, which differs among other mammals; the opossum has two precavals. The thoracic cavity in the fetal pig is average in relation to body size; I noticed that in the beaver the thoracic cavity is much smaller in relation to its body length. References: 1. Odlaug, T.O. 1980. Laboratory Anatomy of the Fetal Pig. Wm. C. Brown Company Publishers, Dubuque, Iowa. 2. Smith, D.G. 1998. A Dissection Guide and Atlas to the Fetal Pig. Morton Publishing Company, Englewood, CO. 3. Walker, W.F. 1980. Dissection of the Fetal Pig. W.H. Freeman and Company, San Francisco, CA. 4. Field, H.E. 1944. The Fetal Pig: An Introduction to Mammalian Anatomy. Stanford University Press, CA. 5. Holstad, G.E. 1959. The Fetal Pig: An Introduction to the Anatomy of the Fetal Pig. Burgess Publishing Company, Minneapolis, MN.
|
Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
