Lab 7: Vertebrate Anatomy

BISC 111/113: Introductory Organismal Biology

 Introduction to Organismal Biology Lab  Calendar and Assignments

 Statistics and Graphing  Science Writing Guidelines

 Lab 1: Biodiversity  Lab 2: Population Growth  Labs 3-6: Plant Biology  Labs 7-9: Animal Biology  Lab 10: Laboratory Exam  Lab 11: Population Growth 2  Lab 12: Beetle Presentations

Objectives

 * 1) To explore digestive, urogenital, circulatory and respiratory systems in a representative vertebrate, the fetal pig.
 * 2) To compare adaptations of these systems in adults of different vertebrate classes.
 * 3) To examine the adaptations of these systems during fetal development.

Lab 7 Overview
I. Document systems anatomy in the fetal pig. a. Digestive systemm b. Urogenital system c. Circulatory system <dd>d. Respiratory syste </dl> II. Compare systems anatomy in mammals with those of other vertebrate classes. III. Contrast adaptations for fetal and adult oxygenation in mammals.

Vertebrate Anatomy: Background
The next three labs will examine the design and structure of animal anatomy through dissection, comparative anatomy, and physiological processes. The identification of anatomical structures, their function, and evidence for structural changes in organs and systems as animals diversified and adapted to different environments and habits are emphasized. <BR>

The vertebrate body can be conveniently divided into a series of organ systems, each of which contributes to a major physiological function of the body. However, all the systems interact to maintain the nearly constant environment (homeostasis) of the body. Some organs have a variety of functions and can be placed in more than one system.

Anatomical terminology. Before beginning your exploration of vertebrate anatomy, please familiarize yourself with the following words and learn to use them in referring to the location of the body parts of your specimen.



Digestive, Urogenital, Circulatory and Respiratory Anatomy of the Fetal Pig
External Anatomy and Opening the Abdominal Cavity. A preserved pig wrapped in cheesecloth will be available for each pair of students. Three dissection manuals are available as references to aid in finding structures. DO NOT REMOVE THESE MANUALS FROM THE LABORATORY. Bohensky, F. 1978. Photomanual and Dissection Guide of the Fetal Pig, Avery Publishing Group Inc, Wayne NJ Smith, D. 1998. A Dissection Guide and Atlas to the fetal pig. Morton Publishing Co. Englewood, CO Walker, W. F. 1988. Dissection of the Fetal Pig. W H Freeman and Co, New York, NY

The body of the pig is divided into three major regions: the head, neck and trunk. At the caudal (posterior) end of the trunk is the tail. Note that the trunk is divided into two regions: the thorax and the abdomen  and that the limbs of the pig are directed ventrally. Determine the sex of the pig by examining the external genitalia, see p. 9 in Bohensky (1978).

To open the abdominal cavity and the posterior part of the thoracic cavity, begin your incision at the level of the forelimbs and make a vertical cut down to the level of the umbilicus. Initially cut only the skin and leave the abdominal muscles intact. When you reach the umbilicus, cut around it. Stop at the hind limbs and then make two horizontal cuts: one below the forelimbs and the other above the hind limbs, see p. 61 in Bohensky (1978).

Following the same incision lines made for the skin, cut through the muscle layers to expose the contents of the abdomen. In order to access the abdominal cavity you will need to transect the umbilical vein, make the cut midway so you can easily find both ends of the vein later. Also break the connection of the diaphragm with the ventral body wall. Pin down the lateral flaps to examine the contents of the abdomen.

The Digestive System of the Fetal Pig (see pp 31-41 in Smith, 1998).

You may need to drain and/or rinse extraneous fluids from the abdominal cavity.<BR>

Food enters the mouth, travels through the pharynx, and when swallowed enters the esophagus. The esophagus is an easily compressible tube that lies immediately dorsal to the trachea. The esophagus traverses the thoracic cavity, and then passes through an opening in the diaphragm to enter the abdominal cavity. You will find these organs later in the dissection.

In the abdominal cavity the liver is the large, dark organ that dominates the anterior abdomen. It is divided into five lobes, and is the primary site of conversion of glucose into glycogen (a carbohydrate storage molecule) and the reverse of this process, the breakdown of glycogen into glucose. It is also the site of the synthesis of bile, a product that aids in the digestion of fats by emulsifying them. Bile is stored in the gall bladder, a sac located beneath the right central lobe of the liver, from which it exits via a series of ducts to the small intestine.

The stomach lies on the left side of the upper abdomen. The esophagus transports food to the stomach, which is a major site of protein digestion. Caudally, the stomach empties the semi-digested chyme into the small intestine. The most anterior part of the small intestine, the duodenum, receives bile synthesized by the liver and digestive enzymes from the pancreas. The pancreas is a lobulated structure somewhat lighter in color than the neighboring intestines. The main body of the pancreas lies in the loop of the duodenum. Carbohydrates, fats, and proteins are all digested in the small intestine. Although the long narrow spleen is not a digestive organ, its position, wedged between the stomach and the diaphragm, make discussion of it appropriate. In the pig and many other mammals, the spleen is quite muscular and can eject large quantities of blood into the circulation to correct sudden blood loss. The human spleen is not muscular so it cannot contract nor store a large quantity of blood. It functions as an important part of the immune system, but is not an essential organ, meaning humans can live without one if it must be removed due to trauma or disease.

Follow the course of the small intestine, which is supported by a membrane, the mesentery. The end of the small intestine joins the large intestine (colon), which is noticeably larger in cross section. At the juncture a short blind sac, the caecum, is formed. The large intestine is a site of water re-absorption. Follow its course through the posterior abdomen to its terminal portion, the rectum. You will not be able to see the rectum until you remove the pubic bone later in the dissection. Wastes leave the digestive system via the anus directly below the rectum.

The Urogenital System of the Fetal Pig (see pp. 71-78 in Smith, 1998) The urinary system (which is responsible for the excretion of urine) and the genital (reproductive) systems have little functional similarity. However, the two systems share some aspects of embryological development as well as some ducts. As a result they are often considered together. To see these structures, you will need to push aside the abdominal digestive organs, in particular the large and small intestine.

Excretory Structures. The paired kidneys lie on either side of the vertebral column, on the dorsal wall of the abdomen. They are the site of urine production, and are supplied with blood by the renal arteries. They lie outside the body cavity (retroperitoneal) and are covered by a layer of connective tissue, which you will need to remove to see them clearly.

Urine is transported from the kidneys by the ureters to the urinary bladder where it is stored. The ureters exit from the center of each kidney, and join the urinary bladder at its base.

Urine exits from the bladder via the urethra. To view the next few organs you will need to remove the pubic bone, ask your instructor if you are not sure what to do. In the female the urethra lies ventral to the vagina and voids urine via the urogenital sinus to the exterior. In the male the urethra enters the penis posteriorly and voids urine via the urogenital canal and the urogenital opening at the tip of the penis. Fig. 7.1. The urogenital system of the fetal male pig: kidneys, bladder and penis.

Reproductive Structures. In the female, eggs mature in the small ovaries, which are located near the kidneys. Mature egg(s) enter the oviducts and are connected to the uterus by two uterine horns. Development of the multiple fetuses occurs in the horns rather than in the main body of the uterus formed from the union of the horns posteriorly. The vagina continues posteriorly from the uterus, lying dorsal to the urethra.

In the male, each testis is located within a pouch, which is enclosed in the scrotum, an exterior sac ventral to the anus. Sperm, which mature in the testis, leave via the coiled epididymus and the delicate vas deferens (also known as the ductus deferens). The vas deferens enters the abdominal cavity via the inguinal canal. The vas deferens from each side, then join the urethra, which leads to the urogenital canal within the penis. Fig.7.2. The male (left) and female (right) reproductive system of the pig in lateral view. Image modified from: Thompson, L. H. Circular 1190 University of Illinois at Urbana-Champaign. http://www.aces.uiuc.edu/vista/html_pubs/pigs/pigs.htm#ill.

The Respiratory System of the Fetal Pig (see pp 65-67 in Smith, 1998). Look at the diaphragm, the musculotendinous sheet that separates the thoracic cavity , containing the heart and lungs, from the abdominal cavity that contains the abdominal viscera. There are three tubes that pass through the diaphragm. Once again, find the esophagus, and then identify the other two: the posterior vena cava and the dorsal aorta ?

Cut through the ribs close to the upper limbs, with your scissors. Cut across the distal end of the ribs, and through the clavicles and remove parts of the ribs that can readily be cut out. Don't remove the sternum connecting the two sets of ribs in the midline lying above the top of the heart since there are blood vessels right below it that we want to preserve. Look at the position of the heart within the thorax and note its relationship to the lung.

Cut in a cranial direction up to the tuft of hairs under the chin and locate the larynx (voice box). You will need to remove some thymus gland, which is very extensive in young animals, to see it clearly. Note the tough cartilaginous larynx. The trachea (wind pipe) is a tube extending from the larynx into the thoracic cavity. The air passage is kept open by C-shaped rings of cartilage along the entire length of the trachea. Note that the rings do not completely encircle the trachea, but are incomplete on the dorsal surface. Note the dark brown thyroid gland on the ventral surface of the trachea. The trachea branches to form two bronchi (you won't see these since they can only be seen if you remove the heart).

Look at the lung and observe its division into several lobes. Remember that fetal pigs have not used their lungs to breathe and that there is no air present in the alveoli (air sacs in the lungs). Look at the lung in relation to the thoracic cage. In an adult this space would be totally occupied by the inflated lungs and heart.

The Circulatory System of the Fetal Pig (see pp. 43-60 in Smith, 1998) Heart. Carefully cut the tough the pericardial sac and look at the heart and the large vessels leaving the heart. The major features of the vertebrate heart will be identified on the larger calf heart in Lab 8. Observe these features as far as possible on the smaller fetal pig heart today.

Veins. Identify the following veins: If needed gradually and carefully cut away at the remaining sternum and remove the thymus gland and the pericardial sac. anterior (superior or cranial) vena cava -major vein draining anterior portion of body brachiocephalic - formed from the union of: subclavian - from the forelimbs external and internal jugulars - from the head posterior (inferior or caudal) vena cava - major vein draining posterior portion of body hepatic portal veins - from the digestive organs to the liver renal vein – drains kidney Fig. 7.3. Veins of the Fetal Pig.

Arteries. To find the arteries at the anterior end of the heart, you will likely have to remove the overlying veins. Also if needed gradually and carefully cut away at the remaining sternum and remove the thymus gland and the pericardial sac. Identify the following arteries: pulmonary artery - brings blood to the lungs aorta - the largest systemic artery brachiocephalic (innominate) - divides into: right subclavian - to the right forearm right and left common carotids - to the head left subclavian - to the left forearm anterior mesenteric - to the small intestine renal - to the kidneys external iliac - to the hindlimbs umbilical - to the placenta Fig. 7.4. Arteries of the Fetal Pig. Fig. 7.5. Organization of the fetal circulation of the pig. The most highly oxygenated blood is indicated by hatch marks.

Circulatory System Adaptations for Oxygenation in the Fetus
While a fetus is in utero the exchange of oxygen, nutrients,and waste products occurs across the placenta. Since the digestive tract, lungs and kidneys of the fetus will not begin to function until after birth the fetal circulation has been modified to adjust blood flow to these organs. Oxygenated blood is brought to the fetal heart through the umbilical cord, umbilical veins, and vena cava. <BR><BR>

Follow the umbilical vein, transected earlier in the dissection, as it seems to disappear in the liver. In Figure 7.5 there is a portion of the vein labeled ductus venosus</U>, that is shunting about ½ the oxygenated blood directly to the vena cava and thus to the fetal heart. This is one of three modifications to the fetal circulatory system. After birth the umbilical vein and umbilical arteries atrophy and the ductus venosus gradually fills with connective tissue and closes (ligamentum venosum). <BR><BR>

Since the lungs are not functioning in the fetus, there are two more modification to the circulation that are found within the heart:  The ductus arteriosus and the foramen ovale. We will look at these two modifications more closely when dissecting the calf heart.

Comparative Adult Anatomy of Systems in Vertebrates
A central argument for the common ancestry of vertebrates is the presence of the same body systems in all subgroups. Despite extensive similarities, body systems in different vertebrate subgroups have been adapted over evolutionary time for survival in different habitats. Dissections of four vertebrates have been prepared before class for comparative study. These vertebrates are: Perch, genus Perca - a member of the Class Osteichthyes (bony fish) Mudpuppy, genus Necturus - a member of the Class Amphibia Rabbit, genus Sylvilagus - a member of the Class Mammalia Boa constrictor, genus Boa - a member of the Class Reptilia Working in groups of four, examine one of the physiological systems below, preparing comparative information for class discussion. Digestive System How do digestive tracts reflect the type of food that an animal ingests and processes? Is plant material or flesh easier to catch? digest? Which food type is consumed in greater volume? Which animals have storage structures as part of the digestive tract? Respiratory System How do respiratory structures reflect the medium in which animals live? Identify which parts of the respiratory system are actually locations of gas exchange. What traits are typical of structures where gas exchange occur? What structure(s) in addition to gills and lungs can be considered gas exchange tissues? How might metabolic rate affect the relative size of an animal's respiratory structures? Urogenital System Which animals bear eggs and which bear live young? How might this difference be reflected in the number of eggs that are produced? If animals bear live young, where are they "housed" while they undergo embryological development? What is the source of nutrition during development? How is mode of fertilization tied to site of embryological development. Circulatory System How many chambers does the heart possess? What is the functional result of anatomical subdivision of the heart? At what point in a circulatory circuit does blood reach the respiratory surface(s)? 2-chamber vs 4-chamber hearts

Assignment
Material in this lab will be part of the Lab Practical in Lab 10. Make sure that you can find and identify each of the structures underlined in Lab 7, know their function(s) and understand the adaptations in the comparative organisms. Your lab instructor will describe the format of the lab practical, which will require detailed identifications and functions.

Other Labs in This Section
Lab 8: Vertebrate Circulation and Respiration Lab 9: Conduction Velocity of Nerves