Rat Dissection Protocol

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

External Features

As you study the external anatomy of your specimen, identify the following features and compare with Figure 2 and Figure 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 non-pigmented 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 mid-ventral 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.

Skeletal System

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.

Axial Skeleton

Study Figure 4 and Figure 5 to compare the rat and human skeletons.

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.

Appendicular Skeleton

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.

Digestive System

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.

Opening the Body Cavity:
To continue your study, you must expose the viscera.

  • 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.


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.

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. By carefully removing this lobe you may easily observe the entire stomach and esophagus. 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.

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.

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.

Caecum - At the junction of the small and large intestine you will find a blind sac, the caecum or caecum. 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.

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.

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.

Review Questions

Skeletal System

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?

Digestive System

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 using (Figure 8 and Figure 9)

Respiratory, Circulatory, and Urogenital Systems

Respiratory System

Food materials are broken down into simpler subunits by the digestive system, and these simpler substances are delivered to the cells by the circulatory system. Within body cells, simple molecules may be broken down to release energy. Some of this energy is used to form ATP, which can be used to supply energy for cellular activities. The process of breaking down food molecules to form ATP is called cellular respiration. Part of this energy transaction is to pass hydrogen ions along the electron transport chain. At the end of the chain, the hydrogen is joined to its final acceptor, oxygen, forming water (H2O). In the absence of oxygen, the hydrogen transfers are prevented and no ATP can be formed. This is the reason that oxygen must be delivered to the cells. In many invertebrates, the size of the body is small enough for oxygen to reach the cells by diffusion. In larger animals such as rats and humans, however, a respiratory system has evolved, creating an internal surface area large enough to allow oxygen to diffuse into and be carried to the cells by the circulatory system. The respiratory and circulatory systems must function together.

It is now time to cut open the thoracic cavity on your specimen:

  • You will now make the lateral cuts at the top of the chest.
  • Even though you already have the cuts at the bottom of the rib cage, the body wall may not peel back because it is attached at the bottom of the rib cage to the diaphragm.
  • You will have to carefully separate the body wall from the diaphragm.
  • It will also be necessary to break the ribs close to where they join at the sternum.
  • Cut the rib bones with the points of the scissors always angled slightly upwards, so that you do not damage any internal organs.
  • Now that the left and right rib cages are no longer joined, carefully open them to expose the thoracic cavity.

Trachea - To reach the lungs, air travels through the nose or mouth to the pharynx and then to the trachea. Find the trachea in the neck region. You will notice that it has C-shaped rings of cartilage to prevent it from collapsing as air rushes through it. Note that in the anterior region the cartilaginous bands are replaced by a single, larger housing of cartilage. This initial portion of the trachea is the larynx or voice box, within which lie the vocal cords. The vocal cords are folds of epithelium that vibrate, producing sounds as air passes over them. The small brownish glandular mass found on either side of the anterior end of the trachea is the thyroid gland. It is not part of the respiratory system, but you may have wondered what it was. In the thoracic cavity, the trachea branches into the right and left bronchi. The forking of the trachea occurs immediately dorsal to the aorta and cannot be seen at this time. Each bronchus leads to a lung where it branches further into bronchioles.

Lungs - Identify the lungs and note that there are four lobes on the right lung and only one on the left lung. Within the lungs, the bronchioles carry the air to their endings, tiny air sacs. Inside, these air sacs are further partitioned into chambers called alveoli. This greatly increases the surface area available for gas exchange. In the human, the internal surface area of the lung is equal to about half the area of a tennis court. In fact, our lungs have a greater surface area than our skin. The alveoli are only one cell layer thick and have capillaries immediately outside of them. The gas exchange occurs across these moist surfaces by simple diffusion. The oxygen, being in higher concentration in the inspired air, diffuses across the alveolar and capillary walls and is picked up by the red blood cells of the blood. The concentration of carbon dioxide is higher in the blood that has carried it from the cells, where it was produced by the oxidation of foodstuffs. The carbon dioxide will diffuse into the alveoli and leave the body during exhalation.

Diaphragm - Notice that the lungs are located within closed cavities, the thoracic or pleural cavities, which are lined by membranes, called the pleura. The parietal pleura line the wall of the cavity (body wall, diaphragm, and median septum) and visceral pleura line the lungs. Find the diaphragm, a muscular wall separating the thoracic and abdominal cavities.

The fact that the lungs lie within closed cavities is critical to the mechanism of breathing. During inspiration, the size of the chest cavity is increased, creating a negative pressure or vacuum that draws air into the lungs. This action is accomplished by the contraction and flattening of the dome-shaped diaphragm and the contraction of the muscles between the ribs. The contraction of the rib muscles raises the ribs, thus increasing the size of the thoracic cavity. Air fills the lungs. During expiration, the diaphragm and rib muscles relax and decrease the size of the chest cavity, forcing the air out. The maintenance of a closed chest cavity is, therefore, essential to the breathing mechanism. This is similar to the way a bicycle pump works. When you pull out the handle, a piston is drawn back inside the cylinder, increasing its volume and creating a negative pressure that draws air into the pump. Pushing in the handle moves the piston so that the internal volume is lessened and air is forced out of the pump. Obviously, a hole in the side of the pump would prevent it from working.

Circulatory System

The Heart
The rat heart is small so that the details of its structure are difficult to observe.

The heart is basically two pumps: one circulates blood to the lungs for oxygenation and the other circulates blood to the body cells. Each side of the heart has an atrium (the anterior chamber that receives blood from a vein) and a ventricle (a thicker walled chamber whose contractions drive the blood into an artery) (Figure 7).

Orient the heart so that the thicker-walled left ventricle is on your right. This is the position of the heart when it is exposed from the ventral side of the animal, as your rat’s heart is now.

Right side - The right side of the heart circulates blood to the lungs. The ''right atrium'' receives blood returning from the body via two large veins called the ''superior and inferior vena cava''. When the right atrium contracts, blood is forced into the ''right ventricle''. The contraction of this chamber pushes the blood into the ''pulmonary artery'' and on to the lungs, where it picks up oxygen.

Left side - The oxygen-rich blood is returned from the lung to the ''left atrium'' of the heart through the ''pulmonary vein''. Contraction of the left atrium moves the blood into the ''left ventricle''. The powerful contraction of the muscles of the left ventricle drives the blood out of the heart through the ''aorta'' to the rest of the body.

Valves - The effectiveness of these contractions is increased by the presence of valves that prevent the backflow of blood. Locate the ''semilunar valves'' between the aorta and the left ventricle. A similar set of valves is found at the junction of the pulmonary artery and the right ventricle. These valves prevent blood from flowing back into the ventricles from the arteries. Between each atrium and its corresponding ventricle is a set of ''atrioventricular valves''. When the ventricles contract, blood is forced against these valves, forcing them shut and preventing the flow of blood back into the atria. Do you see the heartstrings? Technically called the ''chordae tendineae'', these prevent the valves from flapping back into the atria, which would permit the backflow of blood.

Blood Vessels

The circulatory system performs the essential duties of transporting oxygen and nutrients to metabolizing body tissue and carries off carbon dioxide and other metabolic waste for eventual removal from the body. The rat has a closed circulatory system, which means that the blood remains within a system of vessels through which it is pumped by the heart. A vessel that carries blood away from the heart to a capillary bed is an ''artery''. A ''vein'' carries blood in the reverse direction, from the capillaries back to the heart. A ''portal vein'' carries blood from one capillary bed to another. Note that normally an artery carries oxygenated blood, and a vein carried deoxygenated blood. However, the pulmonary artery and vein are backward in this regard because they carry the blood between the heart and lungs.


Begin your dissection by locating the rat’s heart. The vessels of your specimen have been injected with latex to make them easier to locate. Those with blue latex are veins. The arteries contain red latex.

Vena cava - The vena cava returns blood from the body to the right atrium.
Pulmonary vein- This vein enters the left atrium with blood from the lungs.
Internal and external jugular veins - locate these veins in the neck region. These carry blood from the head region back toward the heart.


Aorta - Emerging from the left ventricle, near the midline of the heart, is the '''aortic arch'''. It bends around dorsally, giving rise to three branches very near its origin.

The first is the ''innominate artery''. This vessel divides again very shortly to form the ''right subclavian'' and ''right common carotid''.
The next branch of the aorta is the ''left carotid".
The third branch is the ''left subclavian''.

The aorta continues dorsally, giving rise to the other arteries of the body.

Urogenital System

The urogenital system comprises the excretory and reproductive systems. The relationship between the systems is closer in males than in females. As you begin your study of this system compare your findings with Figure 10 and Figure 11.

The Excretory System

The excretory system functions in removing the nitrogenous waste products of cellular metabolism, as well as in removing a number of other materials that may be present in the blood in excess of the body’s needs. However, it conserves materials that are not in excess. In this way it plays a vital role both in excretion and in maintenance of a fairly constant internal environment.

Kidneys - Find the pair of bean-shaped kidneys lying against the back muscles on the dorsal side of the animal. These are often embedded in fat. The blood is brought into the kidney for processing and taken out of the kidney via the vessels that can be seen near the medial indentation. Remove some of the fat from around the kidneys and locate the adrenal glands. These are endocrine glands, not involved in excretion and found just anterior to the kidney. The hormone they are most famous for is adrenaline (epinephrine). This substance has a variety of effects on a body, preparing it for emergency situations. The cortex of the adrenal glands produces other hormones that can be categorized into three groups: (1) regulators of carbohydrate and protein metabolism, (2) regulators of salt and water balance, and (3) sex hormones.

Ureter - The material removed from the blood by the kidney is gathered internally in collecting ducts that empty into the larger ureter through which the urine leaves the kidney. Locate this duct as it passes from the indentation on the medial side of each kidney close to the blood vessels. The path of the ureter is toward the posterior end of the animal.

Urinary bladder- This sac stores the urine prior to its passage outside the body. The organ is usually contracted in a preserved specimen and appears as a small pear-shaped muscular sac.

Urethra - This is the tube the urine flows through as it exits the body. It can be seen if the mesenteries holding the bladder are dissected away and the bladder is displaced dorsally. In the male, the urethra extends through the penis. It will carry sperm from the testes and secretions of reproductive accessory glands as well as the urine. In the female, the urethra opens to the exterior separately from the reproductive system.

The Reproductive System

The Male

Scrotum - You have already identified the scrotum, the sac housing the testes. In the nonreproductive season, the testes may be withdrawn into the abdominal cavity.

Testes - Cut open the scrotum to reveal the testes. Slit one testis open and notice that it contains a mass of tubules. These are the ''seminiferous tubules'', where the sperm are produced. In addition to sperm, the testis produces male sex hormones.

Epididymis - The sperm pass from the seminiferous tubules into this highly coiled tubule. The epididymis covers both ends and the lateral surface of the testis.

Vas deferens - This tubule carries the sperm from the epididymis to the urethra. It passes anteriorly and joins the urethra very close to the spot where the latter leaves the urinary bladder.

Urethra - You have previously identified this tubule.

Accessory glands - The secretions of all the accessory glands form the seminal fluid that carries the sperm during ejaculation, activates the sperm, provides some nutrients for them, and contains substances that help to neutralize the acidity of the vagina. The products of the coagulating gland probably contribute to the vaginal plug, which is formed in the female rat after copulation. There are several accessory glands, we will only concern ourselves with "seminal vesicles" and the "prostate gland".

a. The vesicular glands and the coagulating glands comprise the "seminal vesicles". They are shaped like wings and are located anterior to the urinary bladder.

b. The ''prostate gland'' is large and has lobes. Look for it where the urethra and vas deferens join. It usually surrounds the bladder.

Penis - You have already identified this structure. Its erectile tissue is composed of three cylindrical masses of sponge-like vascular tissue. During sexual excitement, the arteries leading to this tissue become dilated and the veins draining it become constricted. This causes the spongy tissue to become engorged with blood, making the penis hard and erect. This rigidity facilitates insertion into the female’s vagina. In addition, the rat has a rod of connective tissue called a baculum or “penis bone” enhancing this rigidity.

The Female

Uterus - Locate this structure in the region posterior to the kidneys. In the rat, the uterus is actually divided into two complete uteri, which open separately into the vagina. The two uterus system is know as the unterine horn. Open the uteri and examine any embryos you find.

Ovaries - These are located near the anterior end of the uterus. They are often embedded in fat, which must be carefully dissected away. The ovaries produce the ova, which, if fertilized, will develop into embryos.

Oviducts - Each oviduct is a highly coiled tubule found on the surface of an ovary. The ova pass from the ovary into the oviduct, which carries them to the uterus. The ovary is actually surrounded by the funnel-shaped opening of the oviduct, so that the eggs are released directly into this tubule.

 Figure 1. Orientation terms as depicted on a rat (lateral view).

 Figure 2. Lateral view of a rat, indicating major external features.

 Figure 3. Ventral views of a male and female rat, noting external reproductive features.

 Figure 4.

 Figure 5.

 Figure 6.

 Figure 7. Placement of incision lines to enter the rat. The anterior-most lateral cut will be made next

 Figure 8.

 Figure 9.

 Figure 10. Photograph of the female urogenital system in the rat.

 Figure 11. The female urogenital system of the rat.