Anatomy of the Rat
I. External, Skeletal, Muscular, and Digestive Systems
Classification of the Rat
Although most of us do not like to share our dwellings with the rat, we are not too distantly related to it. The classification of both the human and the rat are identical to the level of class. We both belong to Phylum Chordata, because we possess bilateral symmetry, a single dorsal hollow nerve cord, and both pharyngeal gill slits and a notochord at some stage of development. The notochord is the first supporting rod in the embryo. It is later replaced in both humans and the rat by a series of vertebrae. For this reason, we are both members of Subphylum Vertebrata.
As mammals, we share characteristics that make us particularly well suited to life on land. Our warm-bloodedness allows us to maintain a constant body temperature in spite of environmental extremes. A coat of hair, present on all mammals, decreases the loss of heat, while sweat glands aid in cooling the body. In addition, we have some important modifications enabling a sufficient amount of oxygen to reach the cells of our fairly large bodies. A four-chambered heart prevents the mixing of oxygenated and deoxygenated blood. Furthermore, only mammals have a diaphragm, a muscle that aids in ventilating the lungs. Certain adaptations for the care of young have also arisen in Class Mammalia. Fertilization is internal. Therefore, fewer gametes must be produced to ensure their union. The probability of continued development of the zygote and embryo is increased in mammals because, with very few exceptions, the eggs are retained within the uterus and nourished through a placenta. The female is able to continue providing food for her young after they are born with her milk-producing mammary glands.
However, at this point the classification of human and rat diverge. The rat, along with mice, squirrels, and similar gnawing animals, belongs to the Order Rodentia. The members of this order have conspicuous incisor teeth that enable them to gnaw through hard materials to reach their food. Unlike teeth in other mammals, a rodent’s incisor teeth continue to grow throughout life. This is advantageous because gnawing tends to wear these teeth away. The Norway rat (the common lab rat) is further classified as Family Muridae, Genus Rattus, and species Rattus norvegicus. The classification of the rat is:
Phylum Chordata Subphylum Vertebrata Class Mammalia Order Rodentia Family Muridae Genus Rattus
You will find it helpful to understand the following terms used to describe position or direction (see Figure 1):
- Dorsal - the back or upper surface
- Ventral - the belly or lower surface
- Lateral - the side
- Anterior - the front or head end
- Posterior - the hind or tail end
- Medial - toward the midline of the animal
- Proximal - closer to the midline of the body
- Distal - farther from the midline of the body
- Superficial - near the surface
- Deep - a distance below the surface
As you study the external anatomy of your specimen, identify the following features and compare with Figures 2 and 3:
1. Hair - Remember that this is a characteristic of mammals in general. The fur color is white because an albino lacks pigment.
2. Body regions - The rat’s body consists of several regions: the head, trunk, appendages, and tail. Compare the length of the tail and the rest of the body. The tail is used for balance and support, particularly when the animal is sitting erect and using the forelimbs in a manner more or less similar to the way the arms are used by humans. Notice that the appendages are adapted for walking.
3. Vibrissae - These are groups of very long hairs found just behind the nose and above the eyes. They are tactile organs very useful to a nocturnal animal. While rats scurry for food, often in total darkness, these bristles enable them to maintain contact with walls and other solid objects that will guide their search.
4. Ear - The long flexible fold of the ear is called the pinna. It is composed of cartilage covered with skin. In the living animal, the pinna can be rotated to catch the slightest sound from almost any direction. This is useful for an animal whose activity is mainly confined to darkness. The sound caught by the pinna is directed to the external auditory canal. The sound waves move through this conduit and impinge on the tympanic membrane, causing it to vibrate in synchrony with the wave. The movements of the tympanic membrane cause the bones of the middle ear to move, thus converting the sound wave into mechanical energy and transmitting it to the inner ear, where it is detected by branches of the auditory nerve.
5. Eyes - Notice that the eyes are placed anteriorly in the skull. The central area through which light enters the eye is the pupil. It is surrounded by a circular structure called the iris. In most animals, the iris is pigmented. By its contraction, the iris can regulate the size of the pupil and, therefore, the amount of light that enters the eye. However, the albino rat lacks pigments except for hemoglobin, the red pigment of the blood. Because the rat has no pigment in the iris, it cannot regulate the amount of light entering the eye. In the living animal, the eyes look pink because the blood vessels are visible through the nonpigmented iris. This is a characteristic of all albinos, including humans.
6. External nares - Look for this pair of openings near the tip of the snout. Air can be drawn through these openings into the respiratory system. While passing through the nasal passages, gaseous substances in the air can stimulate the very sensitive olfactory receptors.
7. Mouth and incisors - Notice the way in which the lower incisors shear against the upper ones.
8. External reproductive structures and openings - Determine the sex of your rat (see Figure 3). Compare it with one of the opposite sex. In the male, the scrotal sac projects posteriorly between the thighs beyond the base of the tail. Within this pouch lie the testes. In the midventral line, anterior to the scrotum, find a small sheath of loose skin, the prepuce. Look for the opening in the apex of the prepuce. During sexual excitement, the penis can be protruded through this opening. In the female, find the vaginal opening. This will be seen between the anus, at the base of the tail, and the opening of the urethra, which is more ventral. Look for the mammary glands. There are usually six pairs–three in the thoracic region and three in the abdominal region. They will be small and difficult to see in both immature females and in males.
9. Opening of the digestive and excretory systems - The posterior opening of the digestive system is the anus, found at the base of the tail. The excretory system opens via the urethra. In the male, the urethra runs through the penis. The urethral opening of the female is anterior to the vaginal opening and the anus.
The skeletal system supports the body against gravity and provides a protective encasement for some of the vital organs such as the brain and heart. There are two main parts to the vertebrate skeleton: the axial skeleton, which is composed of the skull, mandible, vertebral column, ribs, and sternum; and the appendicular skeleton, which is composed of the limbs and the pectoral and pelvic girdles.
1. Skull - This is composed of several firmly connected bones. Note the large opening at the posterior and ventral part of the skull. This is the foramen magnum, through which the spinal cord enters the skull. The mandible is the lower jaw.
2. Vertebral column - This is essentially a beam that acts to support the head and body and to transfer the weight to the appendages. The five regions of the column have vertebrae that are modified to suit the various requirements of support and movement. Most mammals have a freely movable neck with 7 cervical vertebrae. Look carefully at the first two cervical vertebrae–the atlas and the axis. Notice how they are modified to form a sort of universal joint for head movements. Rats and mice have 13 thoracic vertebrae to which the ribs are attached. Notice the large transverse process of the lumbar vertebrae. There are 3 or 4 sacral vertebrae that are fused to form the sacrum. The sacrum articulates with the pelvic girdle and transfers the weight of the body to the hind legs. Weight is transferred to the front legs by a muscular sling extending from the distal end of the ribs to the pectoral girdle. Rats have 26 to 30 caudal vertebrae in their tail.
1. Pectoral girdle - This is the arch that supports the front limbs. Only muscles and ligaments connect it to the axial skeleton. The largest component of the pectoral girdle is the blade-shaped scapula, commonly called the shoulder blade. This articulates with the clavicles, commonly called the collarbones.
2. Front limb - The humerus is the bone of the upper forelimb. It connects to the scapula with a “ball and socket” type of joint. Swing your arm around in a circle. Notice the freedom of movement this type of joint allows. The bones of the forearm are the radius and ulna. The wrist is composed of small carpal bones. The metacarpals are the bones of the hand and the phalanges (singular, phalanx) are the bones of the fingers.
3. Pelvic girdle - This arch supports the hind limbs. It consists of three bones: the ilium, ischium, and pubis. These three bones meet and fuse at the hip socket.
4. Hind limb - The thigh bone is the femur. It connects to the pelvic girdle with a ball and socket joint. The bones of the lower hind leg are the tibia and fibula. The tarsals, metatarsals, and phalanges are the bones of the ankle, foot, and toes, respectively.
A Comparative Study
Specialized structural characteristics of animals are related to their environments. Striking skeletal modifications can be seen among vertebrate animals and are often indications of their mode of life or ecological niche.
Study the fish skeleton and determine how it is adapted to its way of life. The head and trunk of a fish usually move as a unit. Are the first two vertebrae the same as in the skeletons we have looked at that permit free movement of the head and neck? Notice that a sternum is missing. Also, the appendicular skeleton is simple and involved with fin movement.
The snake is specialized for slithering along the ground. A snake skeleton has no pectoral or pelvic girdles. This is an example of degenerate specialization. What has happened to the limbs of the snake?
The skeleton of the bird provides an opportunity to study some adaptations for flight. The bones are hollow and thin, making the skeleton lighter relative to the rest of the body weight than it is in most mammalian skeletons. According to physical principles, a tube is more resistant to certain types of stress than is a rod of equal weight. Most of the bony substance of the bird's bones is at the periphery, forming a tubelike structure and providing better structural support. Further strength is added to these bones in the form of internal struts. Similar to struts found inside the wing of an airplane, these provide strength without adding too much additional weight.
Compare the length of the neck and the articulation between the skull and the vertebral column of the bird, rat, and human. The neck of the bird is long and there is a good deal of freedom of movement between the head and neck. This movement is important because the bill of the bird is used for such varied activities as feeding, nest building and defense.
Now compare the trunk regions. Notice that, in the bird, this region is shortened and the vertebrae are firmly united, providing a strong fulcrum for the action of the wings. This area also provides a strong point for the attachment of the pelvic girdle and the hind legs. This is important because, when the bird is on the ground, its hind limbs must bear the entire weight of the animal. What structural modification has occurred in the sternum of the bird? Notice the large ventral keel that provides a broad area for the attachment of flight muscles.
The appendages have also been modified. The bones of the wing are homologous with those of the arm of other vertebrates. The fifth toe of all birds, and the fourth of some, have been lost. In most species of birds the first toe is turned back. This serves as a prop that increases the grasping action of the foot during perching. Notice that the limb bones have become fused in some places. This decreases the probability of dislocation and injury when the bird is landing. The bat is a mammal adapted for flight. Notice the modified forelimbs. The fingers are greatly elongated and joined by a membrane that extends to the sides of the body and legs, as well as between the legs and the tail. Notice that the shoulder girdle is more well developed than the pelvic girdle. The sternum usually has a keel for muscle attachment. Some of the structural modifications accompanying an erect posture can be seen in a comparison of the rat and human skeletons. The major change is in the balancing of the skull atop the backbone, instead of slinging it in front of the backbone. Notice that the human vertebral column has a double curvature instead of a single arch as is seen in the quadruped rat. This keeps the head and shoulders balanced over the hips. The hipbones of the rat are long and narrow. Compare these to the short broad bones of the human hip. This modification found in humans helps to support the internal organs. Look now at the feet of the skeletons. All vertebrates except humans stand either on the toes or the flattened sole. Humans stand with the heels, toes and outer border of the foot pressed to the ground. The arch formed by the rest of the foot provides a means of distributing the weight over a triangular area much larger than the base of the leg bones.
The digestive system is a long tube inside the animal, with the mouth as the opening at the anterior end and the anus as the opening at the posterior end. The process of digestion, the enzymatic breakdown of complex food substances into their simpler components, occurs in the lumen (cavity) of the digestive tube. The small molecules resulting from digestion are then absorbed by the cells lining the gut and transferred to all the other cells of the body via the circulatory system. Within the cells, these molecules may be burned to release energy for cellular activity, built into the structural elements of the cell, or stored for later use. The undigested material passes along the gastrointestinal tract and out of the anus as feces.
1. Mouth - The mouth is the most anterior part of the digestive system. Within the mouth, the food is ground up by chewing and mixed with saliva, which contains carbohydrate-splitting enzymes and lubricating mucus.
2. Salivary glands - Make an incision on one side of the body from the region of the shoulder to the angle of the jaw. Continue cutting along the lower jaw to reveal the salivary glands. There are three pairs of salivary glands. The largest lies just behind the ear and extends to the ventrolateral surface of the neck. The other glands are more ventral and extend anteriorly under the lower jaw (Figure 6). The saliva, as previously mentioned, contains enzymes, which begin the digestion of carbohydrates, and mucus, which moistens food and sticks it together to facilitate swallowing.
3. Floor of the mouth and pharynx - Locate the tongue. It plays a role in the swallowing response. The food moves from the mouth into a chamber shared by the respiratory system called the pharynx and on into the esophagus. The esophagus can be seen under the trachea (a tube recognized by its cartilage rings) in the neck region.
4. Viscera - The other organs of the digestive system are located within the body cavities. All the organs of the body cavity, particularly those of the digestive system, are called the viscera. These organs are supported from the dorsal body wall by mesenteries. The wall of the body cavities and the organs are lined with a thin, moist membrane, the peritoneum.
To continue your study, you must expose the viscera (Figure 7). From the cut at the throat of the rat, cut down the center of the rat until you reach the genitals. Be careful to only cut the body wall. At the posterior end of your cut (by the genitals), cut laterally so that you are creating two flaps of skin that will open from the center of the rat. Feel for the bottom of the rib cage. Make similar lateral cuts just below the rib cage. You should now be able to open the flaps of skin. Use pins to secure them open while you observe the viscera. Next week we will open the rib cage to investigate the respiratory and circulatory systems.
a. Liver - Locate the liver first, because it is an obvious landmark. It is the large, reddish brown mass that lies immediately posterior to the diaphragm (the muscle dividing the thoracic and abdominal cavities). The liver has a great number of functions. However, its role in digestion is to produce bile, a substance that emulsifies fats (breaks them into minute droplets), making them easier to digest. In humans, the bile is stored in the gall bladder before being released into the small intestine. However, the rat lacks a gall bladder. Therefore, the bile is released through a duct directly into the small intestine, where it acts.
b. Stomach - The food passes from the esophagus into the stomach. Locate this bean-shaped sac, which is partially covered by the left lateral lobe of the liver. One function of the stomach is to act as a storage organ so that fewer and larger meals can be consumed. Within the stomach, food is coated with mucus and digestion of proteins begins. The cells lining the stomach secrete the protein-splitting enzyme pepsin. This enzyme is active only in an acidic environment. The cells of the stomach lining also produce hydrochloric acid, which activates the pepsin. The muscular walls of the stomach churn the food, mixing it with enzymes and helping to fragment it. During this time, circular muscles, called sphincters, located at each end of the stomach, prevent the food from escaping.
c. Small intestine - Examine the stomach and locate the place where it joins with the small intestine. Without tearing the mesentery that binds the coils together, trace the small intestine to its junction with the large intestine. Most of the digestion and the absorption of the products of digestion take place in the small intestine. Glands in the wall of the small intestine secrete enzymes for the breakdown of both proteins and carbohydrates. Secretions of the pancreas enter the small intestine and contain enzymes for the breakdown of fats, carbohydrates, and proteins. We have already noted that the digestion of fats is aided by bile. Although bile is not an enzyme, it helps digestion by emulsifying the fats. The alkaline environment of the small intestine inactivates the pepsin from the stomach. Enzymes from the small intestine continue the digestion of protein. Epithelial cells lining the small intestine absorb the digested substances and pass them on to the blood capillaries or the lymphatic system for distribution.
d. Pancreas - The pancreas is an irregular mass of brownish glandular tissue in the mesentery dorsal to the stomach. We have already discussed the digestive function of this diffuse gland. It also produces a hormone, insulin, which passes directly into the circulatory system and is not involved with digestion.
e. Caecum - At the junction of the small and large intestine you will find a blind sac, the caecum or ceacum. The caecum is a place where ingested cellulose is diverted from the main track and is digested by microbial fermentation. Rats and lagomorphs (rabbits, hares) will produce a special feces formed from the caecum product. They will then ingest this feces again, to digest it a second time. This behavior is called coprophagy. Not all mammals have a caecum. Humans do have a short caecum terminating in the appendix. The appendix may serve a function in the immune system, making antibodies. The human caecum provides space for digestion, but does not have the microbes for cellulose fermentation.
f. Large intestine or colon - Running from the caecum, the colon ascends, crosses the abdominal cavity, and descends again. The colon connects posteriorly with the poorly differentiated rectum of the rat. The rectum connects the colon and the anus. The primary function of the large intestine is to absorb most of the water of the digestive secretions, conserving it for use within the body.
The spleen is not part of the digestive system. However, it is an obvious structure, and this is a convenient time to locate it. It is a dense, red, elongate structure located on the left side of the rat’s body. It is part of the circulatory system and plays a role in the production and destruction of blood cells.
Figure 8. Photographs of rat abdomen. When the intestines are moved aside, the stomach and pancreas may be located.
Cross Section of the Intestine
We will use prepared slides of a cross section of the small intestine of a salamander (Necturus) for this study. Identify the structures working from the cavity or lumen to the outside. Compare with Figure 10.
1. Mucosa - The mucosa is a single layer of columnar epithelial cells. Notice that this layer is thrown into folds, increasing the surface area of the small intestine and facilitating absorption. You may see clear “bubbles” at the distal ends of some cells. These cells are called goblet cells. The clear area is an accumulated mass of their secretions.
2. Submucosa - The submucosa is a layer of connective tissue beneath the mucosa. Notice that it extends up into the folds of the mucosa and that it contains many blood vessels. The blood vessels may appear as empty spaces because the blood has drained out of them. These vessels will carry the digested food material to the cells in the rest of the body.
3. Circular muscle - A circular layer of smooth muscle cells encircles the gut. In this preparation, the cells have been cut lengthwise. Notice the shape of each cell: long and slender, tapering at each end. The oval nucleus is in the center of each cell.
4. Longitudinal muscle - These smooth muscle cells are oriented perpendicularly to the circular layer. They extend the length of the intestine. Because of their orientation, they will be cut transversely.
5. Peritoneum - This is a thin layer of connective tissue covered by a single layer of squamous epithelial cells. Only the scattered oval nuclei of this layer will be visible.
6. Mesentery - The mesentery is a continuation of the peritoneum that suspends the intestine from the dorsal body wall. Blood vessels may be visible within it.
Figure 9. Diagram of the rat digestive system.
Rat I Worksheet
Describe at least 3 differences between the rat and the human skeleton that evolved with an upright posture.
Describe the differences in the wing structure of the bat and the bird.
Describe the differences in the articulation between the skull and the vertebral column of a rat, bird, and human.
Describe the differences in the trunk region of a cat and a bird. What modifications for flight can be seen in the bird?
What modifications for way of life can be seen in the in the skeleton of the snake?
What modifications for way of life can be seen in the in the skeleton of the fish?
Trace the path of food through the digestive system, naming each structure.
Describe the observed anatomy of each of the digestive structures you observed and relate the anatomy of the structure (its form) to its function .