|
Facial and Masticatory Muscles |
Photos and text by Sarah Ogburn and Linda Brogdon Dissection: We cut the remaining body wall musculature away to expose the abdominal cavity. The liver is large, and situated cranially over the stomach. It has five distinct lobes. We were unable to find the hepatic portal vein. The gall bladder is small and is located caudally, between the second and third lobes of the liver. The stomach is about half the size of the liver. It is caudal to the liver and located to the left of the midline. It is simple and globular. The cardia (area where the esophagus meets the stomach) is situated very close to the pylorus (area where the small intestine meets the stomach). The spleen is located caudolaterally to the stomach on the specimen’s left side. The spleen is tri-radiate, fairly large, and lacks the venous sinuses present in many eutherian mammals. The pancreas runs caudally to the stomach; it is large relative to body size and when compared to those of other animals in lab. The pancreas is compact and within the mesentery of the small intestine. The small intestine of M. domestica is relatively much longer than the colon, but the digestive tract in general is relatively short when compared to other animals of similar size (rat) and similar diet (pig). The small intestine measures 170 mm, not even the length of the specimen’s body. The caecum is simple and of intermediate size and the colon is relatively very short. Since M. domestica is omnivorous, the combination of a gall bladder, caecum, and long (compared to other parts of the digestive tract) small intestine must allow for ready digestion and absorption of all the proteins, lipids, and carbohydrates that it ingests. Since M. domestica doesn’t rely solely on vegetation, its digestive tract isn’t as specialized for fermentation as those of folivorous animals such as the goat. However, some fermentation takes place in the opossum’s caecum. Discussion: The guts of different animals vary according to diet. Many specializations of the gut have evolved to digest the wide range of mammalian diets. The gut can be divided into seven parts: mouth, esophagus, stomach, small intestine, caecum, gut outpouches/diverticuli, and the large intestine or colon. Based on the diet of the animal, changes to the different parts are mainly responsible for maximizing nutrient yield [2]. Food is ingested into the mouth where it is mechanically broken down by the teeth and chemically broken down by saliva produced in the salivary glands. The esophagus serves to transport the bolus of food from the mouth to the stomach. The stomach is where food may be temporarily stored, and enzymes—such as gastrin digest it. The small intestine continues the breakdown of the food and absorbs nutrients. The caecum is a “fermentation chamber”. The symbiotic microorganisms in the caecum break down the cellulose in ingested plant materials. Gut outpouches include the liver (produces bile, which emulsifies fat), the gall bladder (stores bile), and the pancreas (secretes digestive enzymes used by the small intestine). The large intestine or colon absorbs water and electrolytes, the final stage before the waste materials are released from the organism. The guts of several mammals were examined and by comparing the relative lengths of the gut segments and looking at the specializations, much could be determined about the diet of the animal. Five animals were displayed that were not being dissected in class: a lemur, goat, sloth, dog, and a pangolin. The lemur had a very simple stomach with a relatively long small intestine and a short colon. This combination is characteristic of an omnivorous animal. The goat had a very complexly sacculated stomach, with relatively long intestines, and the caecum was very large. These specializations are common to folivores, animals that eat vegetation. The domestic goat Capra hircus is a special type of folivore, a ruminant, which means it “chews its cud.” The goat has symbiotic microorganisms in the first three stomach chambers. These microorganisms break down the cellulose in plant material. The animal regurgitates food and “chews its cud” before re-swallowing it for another round of breakdown in the stomach. Then the food moves on to the small intestine where the products of fermentation are absorbed. Ruminants have four stomach “chambers”: the rumen, reticulum, the omasum, and the abomasum or true stomach [3]. The walls of the four chambers are also different. The rumen is used for fermentation, the honeycombed walls of the reticulum catch foreign objects, the omasum absorbs water, and the abomasum absorbs nutrients. All of this is necessary to break down vegetation because it is so tough and low in nutrients. The sloth, Bradypus, also has a sacculated stomach and a caecum; it too is a foregut fermenter. The dog, Canis familiaris, is a carnivore and has a small caecum with a relatively long small intestine and a short colon. A long small intestine is necessary in carnivores, and also omnivores, to absorb all the nutrients present in the proteins, lipids, and carbohydrates of the diet. The last animal presented was the pangolin, or scaly anteater, a member of the genus Manis. The stomach is sacculated, with very thick walls, and the large intestine is relatively long. Pangolins eat ants and their stomachs have specialized “teeth” to break down the insects [4]. These features have encouraged discussion about the evolution of Manis because faunivores usually have long small intestine but a noncompartmentalized stomach. Of the animals that are being dissected in the lab the rabbit, the rat, and the beaver are hindgut fermenters. Fermentation takes place in the caecum where microorganisms break down the food. Because this takes place after food has already passed through the main site of absorption—the small intestine—not much of the products of this fermentation can be absorbed. Hindgut fermenters will often eat their own feces (coprophagia) in order to get more nutrients out of their nutrient-poor diets. The pig is an omnivore that has a long colon and a caecum. The cat, Felis catus, is a faunivore and has a long small intestine for fat and lipid absorption from the meat diet, and a small colon. Our specimen of Monodelphis does not really resemble any of the other animals. The proportions of the gut are radically different—all of the other animals being dissected have small intestines that are considerably longer than the body length of the animal. The small intestine of M. domestica is shorter than the body length of the animal. The colon is also very short. We believe that these represent phylogenetic differences rather than dietary adaptations, as the diets of the pig and opossum are very similar. We also had the opportunity to examine the gut of another didelphid, Didelphis virginiana—the North American opossum. D. virginiana is much larger than M. domestica, but the proportions of the gut are nearly identical. They share similar diets, with D. virginiana typically eating 60% animal material (insects, worms, small rodents, birds, crayfish, snails, snakes, frogs, and lizards) and 40% plant material (fruit, leaves, acorns, seeds, berries, agricultural crops, and plant litter) [5]. These findings did not allow us to determine if a phylogenetic component, dietary component, or combinations of the two is responsible for the anatomy of the digestive tract in these genera. However, marsupials have—on average—a lower basal metabolic rate than placental mammals. These differences are phylogenetic. This means that marsupials have lower nutrient requirements than placental mammals, which could possibly account for the reduced proportions of the digestive tract [5]. References: 1. Nowak, R. 1997.“Short-tailed Opossums”. Walker’s Mammals of the World Online 5.1. The Johns Hopkins University Press. 2. Hildebrand, Milton. 1995. Analysis of Vertebrate Structure. John Wiley & Sons. pg 211-228. 3. Raupp, Suzanne. 1999. The Digestive System of the Goat. Ithaca: New York. Accessed 2004 at http://www.ansci.cornell.edu/4H/meatgoats/meatgoatfs14.htm 4. Nowak, R. 1997.“Pangolins, or Scaly Anteaters”. Walker’s Mammals of the World Online 5.1. The Johns Hopkins University Press. 5. Hume, I. 1982. Digestive physiology and nutrition of marsupials. Cambridge University Press, Cambridge.
|
Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
