PHYLUM CHORDATA – THE FETAL PIG
The pig, Sus scrofa, will be studied in detail. The fetal pig is a very favorable animal for dissection and a study of the pig will give considerable information about the morphology of mammals as well as that of chordates. It was a comparison of studies of a large number of animals made in this way that furnished considerable impetus for the formulation of evolutionary theory. Studied at a very late stage of embryonic development, the pig furnishes an understanding of a number of fundamental changes that occur in all mammals at birth.
As a chordate, the pig will be found to have a dorsal tubular nervous system with supporting structures, an internal skeleton, a closed circulatory system with ventral heart, and a metameric body plan.
As a mammal, the fetal pig exhibits the distinguishing characteristics of the class mammalia. The presence of hair, mammary glands, teeth specialized for several functions, an external ear, a muscular diaphragm, a placenta establishing nutritive communication between mother and embryo (in all higher mammals), a four-chambered heart connected to a single left aortic arch, (the fourth), great expansion of the cerebral hemispheres, etc.
Turning to a more direct consideration of the fetal pig, it should be realized that at the time of killing, it was in an advanced stage of development in the maternal uterus, and by definition is therefore a fetus; an earlier stage in more aptly termed an embryo. The period from conception to birth is about seventeen weeks for the pig, as compared with nine months for the human being. The number in an average size pig litter is about eight, although occasionally more than twice as many may be present. A newborn pig generally weighs 1 ½ to 2 ½ pounds and measures up to eleven inches in length.
I the account that follows, a series of exercises is devoted to a survey of the several parts and organ systems of the pig. For success, it is necessary to follow directions accurately, step by step, to proceed carefully with the dissection and to make comparisons with frog and elementary human anatomy. All students will be held responsible for an adequate knowledge of both the frog and the pig.
Since the pig will be studied over a period of several weeks, it is necessary to use special precautions to keep the pig in such condition that the study remains a pleasure. Plastic bags will be furnished in which the pig should be kept when it is being dissected. If the specimen becomes moldy, it should be wiped with a cloth soaked in a mixture of phenol, glycerin and water. The pig should not be dripping with liquid when returned to the bag. The pigs should be kept in the small lockers.
The pig is covered with a rather firm layer of skin underlain by and attached to the body by sheets of connective tissue, called fascia. In some sections of the body there is prominent layer of fat. These layers and muscles associated with the skin will have to be cut and laid back to expose the internal organs. To do this, first make a shallow incision with scissors, then stretch the skin away from the cut, and keeping the scalpel close under the skin, break the connective tissue and peel skin back. It is preferable to use the dull side of the scalpel or a heavy probe for much of this work to avoid cutting deep internal structures.
It must be stressed that great care must be used in following directions and that structures, especially blood vessels, must not be torn or cut until specifically directed. A scalpel should not be used at all in dissecting the blood vessels, but, rather, structures should be separated by use of probes, forceps or other blunt tools. A greater amount of time will be spent in the actual dissection than in examining and learning the parts.
A.External Anatomy
1. Four distinct body regions are evident: a relatively large head, (characteristic of fetuses in general), a short compact neck, a compressed trunk bearing the paired appendages, and a short flexible tail. Traces of hair may already be present, especially in the form of bristly vibrissae, (function?). On the head identify the mouth and strong jaws bearing teeth; the prominent snout with its paired nostrils, (external nares) and the tough dorsal rooter; the eyes, (which may still be sealed; if not, look for the nictitating membrane at the medial corner and describe its nature); and the large flaps, (pinnae), of the external ears.
2. Behind the short muscular neck, the trunk is divisible into three regions: (a) an anterior thorax marked externally by the shoulders, forelimbs and ribs, and internally by the heart and lungs; (b) a middle abdominal or lumbar region containing the body cavity and viscera. Observe the ventral umbilical cord and its attachment, the umbilicus, (significance and relationship to navel?); also the two rows (milk lines) of nipples, (how man?); (c) a posterior rigid sacral region marked by the pelvic girdle, hind limbs and the posterior portions of the digestive and urogenital systems. Locate the anus just below the tail.
3. Identify and compare the various parts of fore and hind limbs. Examine the digits; not the structure and number. Be able to compare these limbs in detail with those of the frog and the human being.
4. The sex may be determined by noting that in females an external orifice, the vulva, lies just ventral to the anus; while in males the urogenital orifice is just posterior to the umbilicus, and the scrotal sacs containing the testes appear as outpocketings ventral to the anus in older fetuses.
B.Internal Anatomy
The Digestive System
The salivary glands and associated structures.
1. The salivary glands are associated with the oral cavity, and their secretions aid in lubricating the food during chewing and in digestion. The glands are cushioned by fat, and some effort must be expended to avoid removing the glands with the skin during dissection. Start the dissection by removing a triangular piece of skin on the left side of the body as follows. Make an incision along the midline of the ventral surface, starting at the level of the shoulders, and cutting forward to the tip of the jaw. From the posterior end of this cut, make a new cut to the base of the left ear. Now turn back the triangular flap of skin, carefully observing the direction given at the beginning of this section. Dissect away the thin layer of facial muscles just beneath the skin.
a. The large and very loose parotid glad is located under the superficial fascia (layer of connective tissue). It consists of masses of small reddish lobules that extend from the midventral line of the throat to the ear, and from the massive jaw muscle (the masseter muscle) in front to the level of the shoulder posteriorly. With a probe, free the gland of fatty tissue. The duct of the parotid glad passes around the anteroventral border of the masseter muscle, appearing as a whitish cord, imbedded in the superficial fascia. It lies in a distinct groove. Gently oft the fascia with forceps and free the duct, cleaning off the fascia as you proceed toward the anterior. Trace the duct to the angle of the jaw where it turns inward to open into the mouth cavity close to the last premolar or first molar tooth. Note that an artery and a vein (injected with colored material) run closely parallel to the duct in the groove mentioned above. Trace the duct back to its origin in the deep face of the gland and note the smaller ducts contributing to it from lobes of the gland. In man, mumps is an infection of the parotid glad.
b. The submaxillary gland is considerably more compact than the parotid. It is directly posterior to the large jaw muscle and is exposed by turning back the parotid glad. It is usually dark, smooth and triangular. Expose it and, by cutting the overlying muscle and the minimum number of blood vessels, find its duct (submaxillary duct) which leaves the anterior tip of the gland and immediately runs under a muscle attached to the edge of the jaw. Trace the duct by splitting this muscle away from the jaw with a probe. It continues forward and opens into the mouth cavity under the tongue. (You can see the openings of your own submaxillary and sublingual ducts under the tongue by using a mirror).
c. The submaxillary duct is surrounded by an elongate mass along most of its length. This is the sublingual gland. The ducts of this glad open near those of the submaxillary gland but are small and may not be seen.
2. Continue your medial cut posteriorly from shoulder level to the edges of the rib basket and turn back the skin to either side. Separate the long muscles of the neck along the midventral line. Just internal to the muscles observe lobes of the thymus gland which extend toward the anterior to a point ventrolateral from the submaxillary gland. The posterior part of the thymus straddles the heart and is hidden by the ribs and thoracic muscles. The thymus is a mass of lymphoid tissue which decreases in size as the mammal grows. Its function is not understood, but it may have some endocrine function.
3. Turn the thymus to one side by cleaning out the connective tissue along its medial borders. The thin, flat muscles may be removed, but be sure not to remove any blood vessels (colored) or nerves (white). Directly on the midventral line, deeper than the surrounding lobes of the thymus, is an oval reddish body, the thyroid gland, an important endocrine gland. Read about its function in the text. The thyroid lies against the trachea, which is directly dorsal from it. The trachea, or windpipe, is a tube which has cartilaginous supporting rings which keep it from collapsing under the pressure imposed on it by actions of the throat muscles and of swallowing. The anterior rings are expanded to form a large cartilaginous voice box, or larynx. This structure contains the vocal cords, which will be seen later.
4. Dorsal to the trachea is the esophagus, a muscular tube leading to the stomach. It will appear flat and collapsed.
The digestive tract.
1. Turning now to the abdominal region, make a median incision through the midventral body wall from the lower tip of the sternum (the breast bone) posteriorly to within one-quarter inch of the umbilicus. Cut around the umbilicus laterally and continue the cuts to the pubic symphysis (ventral junction of the pelvic girdle), leaving a strip about one-half inch wide. Be careful in dissecting a male pig that you do not cut through the terminal portion of the reproductive system, as it lies just below the skin posterior to the urogenital orifice. Make two transverse cuts about two inches long on either side of the midline, one in front of the most anterior teat the other just anterior to the symphysis; turn back these lateral flaps. Before turning back the narrow umbilical flap, notice the large umbilical vein leaving at the anterior margin. Where does it proceed anteriorly? Cut the umbilical vein, leaving a stump for future reference, and turn the flap back.
2. this operation has opened the coelom. In mammals, the coelom consists of an abdominal cavity lined by peritoneum, which will be seen as a glistening sheet, a pericardial cavity around the heart lined by the pericardium, and a pleural cavity around the lung lined by the pleura. These linings extend out from the dorsal part of the body wall as mesenteries and cover the viscera, heart, and lungs. These organs are therefore separated from the coelom by these epithelia. The anterior boundary of the abdominal cavity is formed by a muscular sheet, the diaphragm which is used in respiratory movements. (Does the frog have a diaphragm?)
3. Identify the major organs of the abdominal cavity.
a. The liver, a large, dark, lobed organ, one of the most prominent organs in the abdomen.
b. The gall bladder, a round, dark sac embedded in the dorsal wall of the right central lobe of the liver. This organ stores bile, received from the liver.
c. The stomach, a large organ lying under the left lobes of the liver and receiving the esophagus near its anterior or cardiac end. Note that the esophagus enters the abdominal cavity through an opening in the diaphragm. The major part of the curved stomach at the left of the esophagus is the fundus. The end opposite the cardiac end is the pyloricend.
d. The small intestine originates at the pyloric end of the stomach; the opening between the two is the pylorus, and a sphincter muscle in the wall acts as a valve in controlling the flow of material from the stomach. The portion of the small intestine nearest the stomach is called the duodenum.
Spread apart some of the coils of the small intestine and observe that it, along with the stomach and large intestine, is suspended by a dorsal mesentery, rightly supplied with blood vessels. Accompanying the blood vessels are very think lymphatic vessels and nerves. The small round or oval bodies scattered singly or in clusters throughout the mesenteries are lymph nodes, through which lymph is filtered.
e. The pancreas, an irregular light-colored, lobed organ that lies posterior and slightly dorsal to the stomach, and is embedded in the mesentery between it and the first loop of the intestine. This is a very important organ with two functions: it secretes part of the digestive juice, and it makes the hormone insulin. In connection with the pancreas, look for the common bile duct, a greenish tube passing from the gall bladder to the duodenum; this duct is closely appressed to and may be partly buried in the pancreatic tissue. The pancreatic duct is not easily seen.
f. The large intestine is found by tracing back the small intestine. At the juncture of the two, on the left, is a small sack-like blind pocket, the caecum, part of which is homologous with the human appendix. Most of the large intestine is the tightly packed and coiled colon. The shorter, posterior part, rectum, will be seen later.
g. The spleen, an elongate, dark red structure lying along the greater curvature of the stomach, is an important blood forming and storing organ.
h.Kidneys, dark bodies against the dorsal body wall, and ovaries, if the pig is a female, will be seen. Do not destroy them in subsequent dissection.
A small piece of small intestinal lining in water may be inspected with a dissecting microscope. In the large intestine, note the greenish material which is composed of sloughed-off digestive epithelium, swallowed hair, and oily embryonic secretions. The color is derived from bile. This material is sterile, in contrast to the feces in the large intestine of adult animals, which contain great numbers of bacteria.
The Circulatory System
The major arteries and veins have been injected with colored substances so that they may
be easily located and identified. Veins are injected in blue, arteries in red. The vessels will be described regionally, in that arteries and veins in each major region of the body will be treated together. As specific arteries and veins are dissected and identified, learn what function they serve; that is, from and to what organs they conduct blood. By the time the dissection of the circulatory system is completed, it should be possible to trace mentally the course of blood from any organ or region of the body to any other organ or region, in the fetal and the adult pig. Most of the structures seen here have their homologues in the human body, and much of the nomenclature is the same in the two organisms. The diagrams in the two texts listed above will be considerable help during the dissection. Nevertheless, there are considerable individual variations in the exact arrangement of blood vessels, especially in veins. It will be necessary to use intelligence in interpreting the specimens. Major aberrations noticed by the instructor will be called to the attention of the entire class.
During the dissection, notes should be made so that you will have a record of what you saw. Many of the structures will be destroyed by further dissection, and your notes will be the only reminder you will have. The most effective notes on the circulatory system are in the form of rather large diagram drawn to show the origin and relation of vessels as you dissect them.
In the dissection of blood vessels, by far the most useful instrument is a blunt probe. Work back and forth along the length of the vessel being dissected to free it, a short length at a time, from the connective tissue that surrounds it.
Veins anterior to the heart
1. Open the thoracic cavity by cutting carefully along the midventral line of the chest from the diaphragm anteriorly to, but not through, the first rib. Gently spread the cut edges to expose the large heart, covered by a membranous sac, the pericardium, and lying in the pericardial cavity, a portion of the coelom. Make a small cut through the pericardium to either side of the heart to enter the two pleural cavities, also part of the coelom, in which lie the lungs. Now, working very carefully with a scalpel, cut just through the midventral juncture of the first ribs at the anterior end of the thoracic basket. Too deep a cut will destroy the large blue veins that lie just dorsal to this point.