Rat Anatomy: Interactive 3D Model of the Laboratory Rat

The reference animal here is the laboratory rat, the brown or Norway rat (Rattus norvegicus) bred for research, modelled as an albino strain. It was the first animal domesticated mainly for science and is the most used laboratory animal after the mouse.^[8] Rats have every organ system that people have, but several organs work differently. Biologists use four direction words for a four-legged animal: cranial (towards the head), caudal (towards the tail), dorsal (the back) and ventral (the belly).

Rat body by the numbers

The outside of a rat shows a pointed snout with long whiskers, large ever-growing front teeth, eyes and ears on the sides of the head, a furred body, four paws and a long tail with hairless skin covered in scales.^{[1, 19]}

Whiskers (vibrissae) are long touch hairs in rows on each side of the snout. A rat sweeps them back and forth, called whisking, to feel its surroundings; tracking in one study measured an average of about 8.5 sweeps a second, slower than a mouse's 11.3.^[16]

Eyes and ears. Rat hearing is acute and reaches into ultrasound, but vision is poor.^[8] Sources disagree on colour: a university handout says rats cannot see colour and are blind to red light,^[8] while tests of rat cones found two types, one sensitive to ultraviolet and one to green, and evidence of two-colour (dichromatic) vision.^[17]

The tail: a radiator for body heat

The rat's tail is its main organ for losing heat.^{[8, 19]} Heat lost without evaporating water leaves mainly through the tail, whose blood vessels widen when the brain senses warmth.^[18] Nerves that narrow these vessels control the flow, so a cold rat keeps warm blood away from the tail.^[18] The tail makes up about 9% of a rat's body surface but can dissipate an estimated 17% of its body heat, far more than a mouse's tail.^[20] Hairless skin elsewhere, such as the soles of the paws, helps too.^[19]

A rat has 16 teeth: 2 × (I 1/1, C 0/0, P 0/0, M 3/3). It has no canines or premolars, and a long toothless gap, the diastema, separates the incisors from the molars.^{[11, 12]} The incisors are open-rooted and keep growing and erupting throughout life, while the molars have true roots and stop growing.^{[11, 12]} Only the front of each incisor is covered with hard enamel over softer dentine, which makes the incisors self-sharpening chisels.^{[11, 14, 1]}

In young rats a lower incisor that does not meet its partner erupts about 1.0 mm a day, and a normally worn one about 0.6 mm a day; the shorter the tooth is cut, the faster it grows.^[13] Rats need to gnaw to keep them short: overgrown incisors are common in pet rats and can grow into the nasal cavity.^{[4, 9]}

Why are rat teeth orange? The enamel of rodent incisors contains iron, which was long thought to give the colour. A 2024 study of seven rodents, including laboratory rats, found that the iron-rich layer looks like ordinary enamel; the orange-brown colour comes mainly from a thin surface layer and the zone under it, and the thicker that layer, the darker the tooth.^{[14, 15]}

The rat's backbone has 7 cervical, 13 thoracic, 6 lumbar, 4 sacral and about 28 caudal (tail) vertebrae. There are 13 pairs of ribs: the first 7 pairs reach the sternum, and of the 6 pairs of false ribs the last 3 float free.^[1] The rat has a collarbone, and the clavicle is the only bony link between the shoulder blade and the trunk.^[1] Turn on Skeletal and switch off the other systems in the viewer to see every bone.

A muscular diaphragm divides the chest, which holds the heart, lungs and thymus, from the abdomen, which holds the liver, stomach, spleen, pancreas, intestines, kidneys, bladder and reproductive organs.^[1]

The rat stomach is a half-moon-shaped sac with two regions: a non-glandular forestomach, about three fifths of its area, and a glandular stomach, divided by a raised limiting ridge. The human stomach is glandular only.^{[22, 1]}

Why can't rats vomit? Vomiting is widespread among mammals, from cats and dogs to humans, but rats and other rodents do not do it. A comparative study found that rodents have a less muscular diaphragm and a less funnel-shaped stomach than vomiting species, yet the main reason is in the brain: rodents lack the brainstem circuit that drives the coordinated mouth, throat and breathing movements of a vomit.^[21] Because rodents do not vomit, lab rats are not usually fasted before an anaesthetic.^[8]

Liver without a gallbladder. The rat liver is cleft into distinct lobes. Hunt's manual names four main lobes (median, left lateral, right lateral and caudate), while a 2016 surgical study counts six by splitting the median and adding a quadrate lobe; authors simply name the lobes differently.^{[1, 23]} It weighs about 5–6% of the body.^[23] There is no gallbladder: the hepatic ducts join into one common hepatic duct that carries bile straight to the duodenum.^{[1, 23]}

Intestines and caecum. In one surgical study the small intestine was about 1.49 m long and the large intestine 0.26 m; the caecum measured 45–70 mm and was as large as the stomach, and the colon was simple, without the pouches of the human colon.^[22] Hunt gave smaller figures, about 2.5 cm for the caecum and 20 cm for the whole large intestine.^[1] Rats are hindgut fermenters: bacteria in the large intestine ferment plant material, and rats eat some of their droppings to recover the amino acids and vitamins made there.^[10]

The heart has four chambers, two atria and two ventricles,^[1] and beats 250–400 times a minute at rest, against 450–750 in a mouse.^[5] The left lung is a single lobe and the right lung has four: cranial, middle, caudal and accessory.^{[1, 25]}

Rats keep a pad of brown fat between the shoulder blades (the interscapular depot). Brown fat makes heat directly, a process called non-shivering thermogenesis, using a protein unique to it, uncoupling protein 1 (UCP1).^[26]

The rat's cerebrum is smooth: it lacks the grooves (sulci) and ridges (gyri) seen on the brains of many larger mammals. Large olfactory bulbs at its front end serve the sense of smell.^[1]

Behind each eyeball lies the Harderian gland.^{[4, 9]} It occurs in most land vertebrates, and in rodents it makes lipids, porphyrins and indoles.^[27] Its reddish-brown secretion lubricates the eye; around the eyes and nose it looks like blood but contains none, which is why it is called red tears.^[4] The pigments are porphyrins that fluoresce pink under ultraviolet light, and heavy staining (chromodacryorrhea) usually signals stress or illness.^{[6, 9]}

Male and female rats differ in size, in the distance between the anus and the genital opening, in nipples and in their reproductive organs. Use the Female/Male switch in the viewer to compare them, and see the urogenital diagram in the dissection guide for every gland.

Male. The inguinal canal stays open in rats, so the cremaster muscle can withdraw the testes into the abdomen; preserved lab rats often show them partly retracted. The large, warty vesicular glands were once called seminal vesicles, although they hold no sperm.^[1]

Female. The uterus has two long horns with two separate cervical canals opening into a single vagina (uterus duplex bicollis).^{[30, 1]} Pregnancy lasts 21–23 days^{[3, 8]} and a litter has 8–18 pups.^[3] Mammary tissue is spread over a wide area of the belly, and mammary tumours are the most common cancer in pet rats.^[9]

Rats and mice share the same body plan, but a rat is more than ten times heavier and differs in a few organs. For the mouse in 3D, see mouse anatomy.

The two human small-intestine figures disagree: OpenStax gives the length in a living person, and the larger figure is the one quoted in the rat surgical study.

This is the text version of the 3D rat: all 114 labeled structures, grouped by body system, with what each does. Structures marked "males only" or "females only" appear when you switch sex in the viewer. For a step-by-step bench dissection with a worksheet and answer key, use the rat dissection guide.