Rat Dissection: Step-by-Step Labeled Guide

A rat dissection is the standard introduction to mammal anatomy because the rat has every organ system of a human in a body small enough for one lab period. You will open the body in this order: external anatomy and sexing, skin, abdominal cavity, digestive organs, thoracic cavity and heart, kidneys, reproductive organs and, optionally, the brain. Expect to spend two to three lab periods if you do all of it.

Directions are given for the rat, not for you. Cranial means towards the head, caudal towards the tail, dorsal the back and ventral the belly. When the rat lies on its back, its right side is on your left. Most lab diagrams, including the ones below, show this ventral view.

Keep a few headline facts in mind, because they come up in almost every rat dissection quiz: rats have no gallbladder; their stomach has two regions separated by a limiting ridge; the caecum is huge; the left lung has one lobe and the right lung four; the right kidney sits further forward than the left; and females have separate urethral and vaginal openings.

You need a preserved rat, a dissecting tray, a few sharp tools and protective equipment. A double-injected rat (red latex in the arteries, blue in the veins) makes the blood vessels much easier to follow.

Materials

• Preserved rat (Rattus norvegicus), often double-injected: arteries red latex, veins blue latex
• Dissecting tray with wax, or a cutting board, and 6–8 dissecting pins or string to tie the legs
• Sharp scissors (one blunt-tipped blade is ideal), fine forceps, a blunt probe and a scalpel
• Bone cutters or strong scissors for the ribs and pelvis; a single-edge razor blade for the brain
• Hand lens or dissecting microscope (teeth, pancreas, oviducts)
• Nitrile gloves, safety glasses, apron or lab coat
• Paper towels, wash bottle, a ruler, and a labelled bag or container to store the specimen

Safety

• Rinse the rat under running water before you start: preserved specimens are stored in fixative. Work in a ventilated room.
• Wear gloves and eye protection. Cut away from your fingers and your partner, and keep scalpels in the tray when not in use.
• Lift the skin and body wall with forceps before cutting, so the scissors never plunge into the organs underneath.
• No eating or drinking in the lab. Wash your hands when you finish.
• Put tissue, sharps and gloves in the containers your teacher provides.

This is the view after you open the thoracic and abdominal cavities and pin back the body wall: the trachea and thymus in the neck and upper chest, the heart between the lung lobes, the dome of the diaphragm, the dark liver lobes, the two-part stomach and spleen on the rat's left, the coiled small intestine with the pancreas in its mesentery, the large caecum, and the bladder at the back.

Compare the diagram with your specimen before you move anything. In a real rat the liver covers more of the stomach than shown, and the intestine fills the abdomen more tightly. Lift the liver lobes forward to see the stomach, and fan the intestine to one side to see the pancreas and mesentery.

Before cutting, identify the external features. The head carries the long vibrissae (whiskers), the nostrils, large eyes (pink-red in albino rats) and thin ear flaps (pinnae). Open the mouth to see the two upper and two lower incisors with orange enamel on their front faces, and the toothless gap behind them, the diastema. Rats have only 16 teeth: one incisor and three molars in each half of each jaw, with no canines or premolars.

Each forepaw has four clawed fingers and a tiny thumb with a flat nail; each hind foot has five clawed toes and the rat walks on the whole sole. The tail is covered in rings of scales with short hairs between them; in the Norway rat it is shorter than the head and body. Hunt's manual asks students to count the foot pads and to note that scales on a mammal's tail are unusual.

The order below follows Hunt's classic rat manual and modern school labs. Each numbered step matches a step in the 3D viewer above, so you can switch between the two.

1. 1. External anatomy

   1. Lay the rat on its side. Identify the head, neck, trunk and tail.
   2. Find the vibrissae (whiskers), nostrils, eyes and pinnae (ear flaps).
   3. Open the mouth with forceps. Find the two upper and two lower incisors and the toothless gap (diastema) behind them.
   4. Count the digits: four clawed fingers plus a tiny thumb on each forepaw, five toes on each hind foot. Note the scaly rings on the tail.

   Identify: Vibrissae, Nose and nostrils, Eye, Pinna, Upper incisors, Lower incisors, Forepaw, Hindpaw, Tail, Skin and fur.

2. 2. Sexing the rat

   1. Turn the rat on its back and lift the tail. Find the anus under the base of the tail.
   2. Male: a large scrotum lies below the anus and a single urogenital opening (the prepuce) lies in front of it. The anogenital distance is long.
   3. Female: three openings in a row — anus, vaginal opening and urethral papilla — and a short anogenital distance. Look for six pairs of nipples in two rows along the belly (wet the fur to see them).
   4. Use the Female/Male switch on the model to compare.

   Identify: Anus, Scrotum, Prepuce and urogenital opening, Vaginal opening, Urethral papilla, Nipples (teats).

3. 3. Pin the rat and remove the ventral skin

   1. Pin or tie the rat on its back with the legs spread.
   2. Lift the skin at the midline just in front of the genital opening and make a small snip. Cut forward along the midline to the chin, keeping the scissors pointing up.
   3. Make side cuts towards each leg, then peel the skin back. Separate it from the body wall with your fingers or a probe; it comes away easily.
   4. In the neck find the salivary glands and the jugular veins. In a female, the thin mammary tissue stays on the skin. Look for small lymph nodes in the armpits and groin.

   Caution: Cut only the skin. The abdominal muscles beneath it are thin and the organs lie just under them.

   Identify: Salivary glands, Jugular veins, Mammary glands, Lymph nodes, Masseter.

4. 4. Open the abdominal cavity

   1. Lift the abdominal muscles with forceps and cut along the midline (linea alba) from the pelvis to the bottom of the rib cage.
   2. Make cuts sideways just behind the ribs and in front of the hind legs, and pin the flaps back.
   3. Before moving anything, sketch what you see: the dark red liver at the front, the stomach on the left, coils of small intestine and the large caecum.

   Caution: Do not cut the diaphragm yet; the thoracic cavity is opened later.

   Identify: Liver: median lobe, Forestomach (non-glandular stomach), Glandular stomach, Jejunum, Caecum, Diaphragm, Urinary bladder.

5. 5. The digestive tract

   1. Lift the liver forward. Find the oesophagus entering the stomach through the diaphragm.
   2. Identify the two regions of the stomach: the thin, translucent forestomach on the left and the thick, opaque glandular stomach on the right. Optionally open the stomach along its greater curvature to see the limiting ridge between them.
   3. Follow the gut from the pylorus: the U-shaped duodenum, the long jejunum and the darker ileum, then the caecum, the colon and the rectum.
   4. Spread the coils of intestine to see the thin, transparent mesentery and its blood vessels.

   Identify: Oesophagus, Forestomach (non-glandular stomach), Glandular stomach, Limiting ridge, Duodenum, Jejunum, Ileum, Caecum, Colon, Rectum, Mesentery.

6. 6. Liver, bile duct, pancreas and spleen

   1. Identify the four main liver lobes: median (with a deep notch), left lateral, right lateral and the small caudate lobe around the stomach.
   2. Look for a gallbladder: there is none. Find the thin bile duct running from the liver to the duodenum instead, with the larger hepatic portal vein beside it carrying blood from the gut to the liver.
   3. Find the pancreas: lift the duodenum and hold the mesentery up to the light. The pancreas is the pinkish, branching tissue spread in the duodenal loop and towards the spleen. White, smooth tissue is fat.
   4. Find the long, dark red spleen along the left side of the stomach.

   Identify: Liver: median lobe, Liver: left lateral lobe, Liver: right lateral lobe, Liver: caudate lobe, Bile duct, Hepatic portal vein, Pancreas, Spleen, Duodenum.

7. 7. Open the thoracic cavity

   1. Cut through the ribs on each side of the sternum, from the diaphragm to the front of the chest. Cut the diaphragm away from the body wall and lift the sternum forward.
   2. Find the pale thymus over the front of the heart, the heart in its pericardial sac, and the trachea in the neck. Just behind the larynx, look for the small, dark red lobes of the thyroid gland on each side of the trachea.
   3. Identify the lungs: one lobe on the left; cranial, middle, caudal and accessory lobes on the right. Lift the heart to find the accessory lobe behind it.

   Caution: Rib ends are sharp. Cut with the tips of the scissors pointing up.

   Identify: Thymus, Heart (ventricles), Trachea, Larynx, Thyroid gland, Left lung (one lobe), Right cranial lobe, Right middle lobe, Right caudal lobe, Accessory lobe, Diaphragm.

8. 8. Heart and great vessels

   1. Remove the thymus and the pericardium. Identify the ventricles (apex pointing back and left) and the two atria at the base.
   2. Find the aorta arching to the left, the pulmonary trunk, the two cranial venae cavae (one each side) and the caudal vena cava passing through the diaphragm.
   3. Follow the common carotid arteries up the neck beside the trachea.

   Identify: Heart (ventricles), Right atrium, Left atrium, Aorta, Pulmonary trunk, Cranial venae cavae (left and right), Caudal vena cava, Common carotid arteries.

9. 9. Kidneys and urinary system

   1. Move the intestines to one side (or cut them free at the oesophagus and rectum and remove them).
   2. Find the kidneys against the dorsal body wall. The right kidney lies further forward than the left.
   3. Find the small adrenal gland at the front of each kidney, the renal artery and vein at each hilum, the aorta and caudal vena cava between the kidneys, and the ureters running back to the bladder.
   4. Optional: remove one kidney and slice it lengthwise to see the cortex, medulla and renal pelvis.

   Identify: Kidneys, Adrenal glands, Renal arteries and veins, Ureters, Urinary bladder, Aorta, Caudal vena cava.

10. 10. Male reproductive system

    1. Set the model to Male. Cut open the scrotum to expose the testes and the coiled epididymis on each.
    2. Trace each vas deferens up into the abdomen, over the ureter, to the urethra behind the bladder.
    3. Identify the large, curved seminal vesicles with the thin coagulating glands along their inner curve, and the prostate around the bladder neck.
    4. Find the preputial glands under the skin beside the penis. Cut the pubic symphysis to follow the urethra into the penis and find the bulbourethral glands.

    Identify: Testes, Epididymis, Vas deferens, Seminal vesicles, Coagulating glands, Prostate, Bulbourethral glands, Preputial glands, Penis, Os penis, Urethra (male).

11. 11. Female reproductive system (and the pregnant rat)

    1. Set the model to Female. Find the small ovaries behind the kidneys, each in a fatty capsule, with tiny coiled oviducts.
    2. Follow the two long uterine horns back to where they meet at the cervix above the bladder. The vagina runs from the cervix to its opening.
    3. Note that the urethra runs separately to the urethral papilla.
    4. If your rat is pregnant, each uterine horn looks like a string of beads. Count the embryos in each horn and measure one.

    Identify: Ovaries, Oviducts, Uterine horns, Cervix, Vagina, Urethra (female), Clitoral glands, Mammary glands.

12. 12. Brain (optional)

    1. Remove the skin of the head. With bone cutters, carefully chip away the top of the skull from behind the eyes to the back of the head.
    2. Lift the brain out by cutting the cranial nerves underneath. Identify the olfactory bulbs, the smooth cerebral hemispheres, the cerebellum and the brainstem.
    3. Cut one hemisphere lengthwise with a razor blade to look for the hippocampus inside.

    Caution: Preserved brain tissue is soft. Remove bone in small pieces.

    Identify: Olfactory bulbs, Cerebrum, Hippocampus, Cerebellum, Brainstem (medulla oblongata), Spinal cord, Skull (cranium).

13. 13. The skeleton (review)

    1. Use the model to review the skeleton: skull and mandible, 7 cervical, 13 thoracic, 6 lumbar and 4 sacral vertebrae, the caudal vertebrae, 13 pairs of ribs, sternum, girdles and limbs.
    2. Compare it with your own skeleton: the rat has a clavicle like you, but a long tail, a plantigrade foot and a fused tibia and fibula.

    Identify: Skull (cranium), Mandible, Cervical vertebrae (7), Thoracic vertebrae (13), Lumbar vertebrae (6), Sacrum (4 sacral vertebrae), Caudal vertebrae, Ribs (13 pairs), Sternum, Scapula, Clavicle, Humerus, Radius and ulna, Pelvis (os coxae), Femur, Tibia and fibula.

Food passes through the mouth, pharynx and oesophagus, into the forestomach (storage) and glandular stomach (acid and pepsin), then through the pylorus into the duodenum, jejunum and ileum. At the ileocaecal junction it enters the large intestine: the caecum (fermentation), the ascending, transverse and descending colon (water absorption and pellet formation) and the rectum. The accessory organs are the salivary glands, the liver (bile, with no gallbladder to store it) and the diffuse pancreas.

A good way to finish the digestive step is to cut the gut free at the oesophagus and rectum, lay it out straight on wet paper towel and measure each part. The small intestine is several times longer than the body; the caecum is the widest part of the tract.

A rat is a small, complete mammal. Every organ system in it has a direct human counterpart: a diaphragm separating thorax and abdomen, a four-chambered heart, lungs with lobes, a liver, kidneys with a cortex and medulla, a placenta-forming uterus, mammary glands and a brain with a cerebral cortex. That is why the rat is the classic mammal of school and first-year university labs, usually after the earthworm and frog. Hunt's 1924 laboratory manual made the same argument a century ago: the Norway rat and its albino variety are easy to obtain, of a convenient size and anatomically typical of mammals. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

The rat also differs from humans in instructive ways, and good lab questions use them. It has no gallbladder, a stomach with a non-glandular forestomach, a huge caecum, ever-growing incisors, a duplex uterus, a penile bone, and separate urethral and vaginal openings in the female. Spotting these differences, and explaining them, is the point of the dissection, not just naming organs.

Most preserved rats used in schools are laboratory-type albino Norway rats (Rattus norvegicus) bred for the purpose and sold by biological supply companies. A Carolina Biological Supply paper on how the company procures animals says its specimens come from cultures (captive breeding), from managed habitats, and from individuals or organisations, that endangered species are never used, and that non-aquatic vertebrates such as rats are euthanised with pure carbon monoxide before being preserved. It also notes that movement of a body during perfusion with fixative is a post-mortem response, not a sign that the animal was alive. (Flagg (Carolina Biological Supply), Procuring and preserving animals for biological instruction, ABLE Tested Studies vol. 15, PDF.)

After euthanasia, supply-house rats are injected with a preservative (traditionally formalin-based, often replaced today by lower-odour fixatives) and many are double-injected with coloured latex: red in the arteries, blue in the veins. That is why the vessels in your rat may be bright red and blue and firm to the touch. Some teachers instead use frozen, unpreserved rats sold as food for pet reptiles. Fresh or thawed tissue is softer and its colours are natural, but it must be handled like raw meat and disposed of the same day.

If you object to dissecting an animal, ask your teacher about an alternative assignment. The 3D model on this page follows the same thirteen steps as the bench dissection and works with the same worksheet questions.

Lift the tail and look at the perineum (the area around the anus). In rats and other rodents the anogenital distance, the gap between the anus and the genital opening, is normally about twice as long in males as in females. The difference is produced by androgens (male hormones) acting on the fetus. Male rats also lack nipples: the nipple buds regress before birth under androgen control, so visible nipples are a quick sign of a female. (Schwartz et al. (2021), Areola/nipple retention as a biomarker for anti-androgenic effects in rat toxicity studies, Front. Toxicol..)

An adult male is unmistakable: two large testes fill a scrotum below the anus, and the single urogenital opening sits in a hairy prepuce in front of the scrotum. A female has three openings in a line: the anus under the tail, the vaginal opening just below it, and the urethral opening on top of a small cone, the urethral papilla. Hunt points out that this is unusual among mammals, most of which have a single urogenital opening in the female. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Count the nipples on a female. Rats of both sexes have six pairs of mammary glands, three in the cervicothoracic region and three in the inguinoabdominal region, but only females have nipples. Mice have five pairs (three and two). Wetting the fur with a little water makes the nipples easier to see in a preserved specimen. (NTP Nonneoplastic Lesion Atlas: mammary gland.)

The skin of a rat is loose. After the first midline snip, most of it can be separated from the muscles with a finger or a blunt probe. The abdominal muscles underneath are thin, so open them separately and with the scissor tips pointing up. Once the flaps are pinned back, the view is dominated by four things: the dark red liver against the diaphragm at the front, the stomach on the left, the coiled small intestine filling the middle, and the large, dark caecum in the lower left.

The organs are held by thin membranes. The mesentery suspends the intestine and carries its blood vessels; the stomach's mesentery expands into a sac, the greater omentum, which often holds fat and part of the pancreas. In adult laboratory rats the small intestine is roughly 1 to 1.5 m long, most of it jejunum: a surgical-anatomy study gives the duodenum about 10 cm, the jejunum 90 to 135 cm and the ileum only 2.5 to 3.5 cm. (Hunt, measuring the smaller rats of 1924, gave about 70 cm.) Uncoiling it is a good way to show how much surface a small body packs inside. (Vdoviaková et al. (2016), Surgical anatomy of the gastrointestinal tract and its vasculature in the laboratory rat, Gastroenterol Res Pract (PMC).)

The rat stomach has two regions that look different even from outside. The left part, the forestomach (non-glandular stomach), has a thin wall lined with keratinised squamous epithelium like the oesophagus. The right part, the glandular stomach, is thick and opaque, with folds (rugae) and glands that secrete acid and pepsin. Inside, the two are separated by a raised fold called the limiting ridge. (NTP Atlas of the rodent stomach (forestomach, limiting ridge, glandular stomach), PDF.)

Hunt described the forestomach as thin enough, in a full preserved stomach, to see the food inside, and the glandular region as yellowish, opaque and folded. Open the stomach along its greater curvature to see the limiting ridge for yourself. The oesophagus opens into the forestomach, just beside the ridge. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Rats cannot vomit, and the limiting ridge is often given as the reason. The best evidence points elsewhere. Horn and colleagues gave standard emetic drugs to rats, mice and other rodents and none retched or vomited. They also found anatomical features that would make vomiting hard (relatively less diaphragm muscle with a larger central tendon, a stomach shape poorly suited to moving contents towards the oesophagus, a long abdominal oesophagus), but in recordings from the brainstem of rats and mice, the coordinated nerve activity that drives vomiting in shrews never appeared. They concluded that a missing brainstem neural component is the most likely cause. (Horn et al. (2013), Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study, PLoS ONE (PMC).)

The rat liver is deeply divided. Most lab guides use four main lobes: a large median lobe partly split by a notch, a left lateral lobe behind it on the left, a right lateral lobe almost divided into front and back lobules, and a small caudate lobe whose two lobules fit around the lesser curvature of the stomach. The back lobule of the right lobe has a hollow, the renal impression, that holds the front end of the right kidney. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Names and counts vary between books. Vdoviaková and colleagues, describing rat liver morphology for surgeons, distinguish six lobes (left and right medial, left and right lateral, caudate and quadrate) and review authors who use four. If your worksheet says 'four lobes', it is using the traditional grouping; neither is wrong. The same paper is clear on one point: laboratory rats have no gallbladder. (Vdoviaková et al. (2016), Importance of rat liver morphology and vasculature in surgical research, Med Sci Monit (PMC).)

Without a gallbladder to store it, bile flows continuously from the liver into the bile duct, which runs in the ligament between the liver and duodenum, crosses the back of the pancreas and enters the duodenum about 3 cm from the pylorus. Small pancreatic ducts join it on the way, so the bile duct carries pancreatic juice too. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Students miss the rat pancreas because they expect a compact organ like the human pancreas. In the rat it is irregular and divided into many lobes, some joined to each other, spread thinly in the mesentery of the duodenum and in the descending part of the greater omentum. It sends a lobe towards the pylorus, and its left end lies dorsal to the spleen. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

To find it, lift the stomach forward and pull the duodenum gently to the right so its U-shaped loop spreads out. Hold the mesentery up against the light. The pancreas is the soft, pinkish-tan, granular tissue lying in the loop and running along the stomach towards the spleen. Fat looks whiter, smoother and more glossy, and lymph nodes are firm, rounded beads. If your rat is injected, the pancreas may be tinted by latex in its many small vessels.

The caecum is the large blind sac where the ileum meets the colon. In an adult laboratory rat it is about 5 to 7 cm long (Vdoviaková and colleagues), shaped like a comma or a short spiral, and full of dark contents. The ileum is darker than the jejunum because of what it carries. From the caecum the ascending colon runs forward, the transverse colon circles to the left near the stomach, and the descending colon runs back along the midline, beaded with faecal pellets 'shaped like cotton seeds', to the rectum and anus. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

The caecum lies on the left side of the abdomen. That is why research protocols for intraperitoneal injection in rats direct the needle into the right lower quadrant: to avoid penetrating the caecum on the left side. (Virginia Tech SOP ARCD-0320: intraperitoneal injections in the rat, PDF.)

Open the thorax by cutting through the ribs on both sides of the sternum and freeing the diaphragm. The heart sits in the middle in its pericardial sac, with the pale thymus over its base. The left lung has a single lobe and the right lung has four: cranial, middle, caudal and accessory. A 2016 study of the bronchial tree in Wistar rats confirmed exactly this pattern. (Olivares Pérez-Montt & Gil (2016), Lobulación pulmonar de la rata blanca, Int. J. Morphol. 34(3).)

The accessory lobe is a trap. It is a small, pyramid-shaped lobe that sits behind the heart, mostly to the left of the midline, in its own pocket of pleura. Hunt warns that it may be mistaken for part of the left lung unless you trace its bronchus back to the right side. Hunt also describes two cranial (superior) venae cavae entering the right atrium, one on each side: rats keep the left one, which in humans disappears before birth. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

The kidneys lie against the dorsal body wall, behind the peritoneum, with their hilum facing the midline. The right kidney is further forward than the left: its front end fits into the renal impression of the right liver lobe, while the left kidney is displaced so far back that its hilum lies behind the level of the right kidney. Hunt suggests the stomach on the left pushes it back. A small, rounded adrenal (suprarenal) gland caps the front of each kidney. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

To see the inside, remove one kidney and slice it lengthwise along its outer edge with a scalpel or razor. You will see a pale outer cortex, a darker inner medulla and a single cone-shaped papilla projecting into the renal pelvis, where the ureter begins. Human kidneys have many papillae; the rat kidney is unipapillate, which makes it simpler to read. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Only the reproductive steps differ between sexes. Sex the rat externally first (step 2), then use the diagram below while you dissect.

• Male: testes in the scrotum, epididymis, vas deferens looping over the ureter, large question-mark seminal vesicles with coagulating glands, prostate lobes around the bladder neck, bulbourethral glands, preputial glands and a penis with an os penis.
• Female: small ovaries in fatty capsules behind the kidneys, coiled oviducts, two long uterine horns joining at the cervix, a vagina below the rectum, a separate urethra opening on the urethral papilla, clitoral glands and six pairs of mammary glands.

The male accessory glands are the most surprising part of the rat dissection. Behind the bladder lie the seminal vesicles (vesicular glands): a pair of large, warty glands bent like question marks, hard and brittle in a preserved rat and full of coagulated secretion. Despite the old name they contain no sperm. Pressed into their concave side is a thin, flattened gland, the coagulating gland. The prostate has a dorsal part on the urethra and two prominent ventral lobes under the bladder. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Each vas deferens (ductus deferens) comes up from the scrotum through the inguinal canal and loops around the ureter before joining the urethra, a loop produced when the testis descended during development. At the root of the penis the two bulbourethral (Cowper's) glands empty into the urethra. Under the skin beside the penis lie the preputial glands, whose duct contains a soapy secretion. The penis itself bends in a figure-7 and contains one or two small bones, the os penis, at its tip. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

The female tract is a Y. Each tiny ovary sits behind its kidney inside a thin capsule, the ovarian bursa, with a tightly coiled oviduct beside it. From each ovary a long uterine horn runs back to join its partner above the bladder. Hunt showed that the joining is only superficial: a probe passed down each horn emerges through its own opening into the vagina, so the rat uterus is a uterus duplex with two cervical canals. A 2019 study of laboratory rats, mice and guinea pigs confirmed two separate cervical canals in all three (uterus duplex bicollis). The vagina lies directly beneath the rectum. The urethra runs separately, ventral to the vagina, to the urethral papilla. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Lab rats are sometimes pregnant. A pregnant uterus looks like two strings of beads: each swelling is one embryo with its placenta, spaced along both horns. The average gestation is 21 to 23 days and litters are usually 8 to 18 pups, so a late-pregnant rat can hold many fetuses. Count the embryos in each horn, measure the crown–rump length of one, and compare horns. (Merck Veterinary Manual: breeding and reproduction of rats.)

The brain lies in a narrow braincase behind the eyes. Remove the skin of the head, then chip away the top of the skull in small pieces with bone cutters, working forward from the back of the head. Lift the brain out by cutting the cranial nerves underneath. From front to back you will find the two olfactory bulbs (very large compared with a human's, because rats rely on smell), the smooth cerebral hemispheres, the folded cerebellum and the brainstem continuing as the spinal cord. The cerebral surface has no folds: the rat brain is lissencephalic.

Hunt notes that the cribriform plate of the skull, the perforated bone between the nasal cavity and the braincase, holds the olfactory lobes; the olfactory nerves pass through its holes. Damaging this plate is the usual way the bulbs are lost during removal, so leave them for last. (Hunt (1924), A Laboratory Manual of the Anatomy of the Rat, Internet Archive.)

Use this blank diagram for "label the rat dissection" practice or as a pre-lab quiz. Write the name of each numbered structure, then check your answers.

Answer these during or after the dissection. Click a question to reveal the answer.

1. How can you tell a male rat from a female rat without opening it?

   Look at the perineum. A male has a scrotum and a long anogenital distance (about twice the female's), with one urogenital opening. A female has three openings (anus, vaginal opening and urethral papilla), a short anogenital distance and six pairs of nipples.

2. Rats have the dental formula I 1/1, C 0/0, P 0/0, M 3/3. How many teeth does a rat have, and what is the diastema?

   16 teeth: 4 incisors and 12 molars. The diastema is the toothless gap between the incisors and the molars; the cheeks fold into it to seal off the mouth while the rat gnaws.

3. Why do a rat's incisors stay sharp even though they never stop growing?

   Enamel covers only their front surface. The softer dentine behind wears away faster, so the edge stays chisel-sharp. Constant growth replaces what is worn away.

4. Name the two regions of the rat stomach and the structure that separates them.

   The non-glandular forestomach (thin, lined with keratinised squamous epithelium) and the glandular stomach (thick, secretes acid and pepsin). The limiting ridge separates them.

5. Why can't rats vomit?

   Mainly because their brainstem lacks the neural circuit that coordinates vomiting (Horn et al. 2013). Their anatomy also works against it: a relatively weak diaphragm, a stomach shape poorly suited to pushing contents up and a long abdominal oesophagus. The limiting ridge is often blamed, but it is not the main reason.

6. Which organ do humans have that a rat lacks: gallbladder, spleen or caecum? Where does the rat's bile go?

   The gallbladder. Bile flows continuously from the liver through the bile duct into the duodenum.

7. How do you find the pancreas in a rat?

   Lift the stomach and duodenum and hold the mesentery to the light. The pancreas is the soft, pink, branching tissue in the loop of the duodenum, running towards the spleen. It is diffuse, not a compact organ; fat is whiter and smoother.

8. What is the function of the large caecum, and why is it important for herbivores and omnivores like rats?

   It is a fermentation chamber: bacteria digest plant fibre and make vitamins. Rats re-eat some soft faeces (coprophagy) to absorb these products.

9. How many lobes do the left and right lungs of a rat have?

   The left lung has one lobe. The right lung has four: cranial, middle, caudal and accessory.

10. Which kidney is further forward (cranial) in the rat, and why?

    The right kidney. The left one is pushed back, probably by the stomach, which lies on the left (Hunt 1924).

11. Trace the path of sperm from where it is made to where it leaves the body.

    Testis → epididymis → vas deferens (through the inguinal canal, over the ureter) → urethra → penis → out through the prepuce. The seminal vesicles, coagulating glands, prostate and bulbourethral glands add fluid along the way.

12. Describe the shape of the rat uterus. Where do embryos develop?

    It is duplex: two long, separate uterine horns, each with its own opening into the vagina. Embryos implant in a row along both horns.

13. Name two structures found in a male rat that a male human does not have.

    Any two of: os penis (penile bone), coagulating glands, preputial glands. (Humans do have seminal vesicles, but the rat's are relatively huge; rats also keep their inguinal canals open.)

14. Which part of the rat brain is relatively much larger than in a human, and what does that tell you?

    The olfactory bulbs. Rats rely heavily on smell.

Most rat dissection reports end with a comparison to the human body. The table lists the differences you can see in the lab, with the laboratory mouse for comparison.

For the mouse in more detail, including the mouse brain, heart, liver and hindlimb muscles, see our mouse anatomy page.

A 'paper rat dissection' is a worksheet in which students cut out paper organs and glue them onto an outline of a rat in the right order and position. The labeled diagrams on this page, and the blank version for labelling, do the same job. Work from deep to superficial: kidneys, ureters and bladder first; then the reproductive organs; then the intestines, caecum, stomach and spleen; then the liver, heart, lungs and thymus. If an organ cannot be placed, ask which organ lies on top of it in the real animal.

The 3D viewer is a virtual rat dissection. Each step removes the same layers you would remove at the bench and lists the structures to find; clicking any part shows its location and function. It is a practical alternative for students who opt out, for absent students and for revision before a practical exam.

The full list of 114 structures in the model, grouped by system. Click any of them in the viewer to zoom to it.