Rat Dissection


Rat External Anatomy

Step 1: Obtain a rat. Rats are ordered from biological supply companies. They come stored in a preserving fluid, either in bags or in a bucket. You will use latex (or similar) gloves when handling your rat and safety goggles are required to protect your eye from chemical splattering or debris.

rats in bucket

These rats are stored in "carosafe" a chemical that keeps them preserved. Over time, rats stored in buckets become bloated with the chemical, use caution when opening the body cavity of these rats as the chemical is prone to spray and splatter. Once an incision is made, the rats can be drained of fluid.

Step 2:  Place your rat in a dissecting tray and examine the external features. The rat (and all vertebrates) has anatomical regions to help locate structures.

rat regions

Step 3: Locate the vibrissae (whiskers) and the pinna (ears). Rats are gnawing mammals and two large incisors should be visible. Eyes are usually pink or black with a black pupil and covered by a nictitating membrane.

 

 

rat head

The sex of the rat can be determined by looking for external testes, found on males and the presence (or absence) of teats, which are only found on female rats.

rat teats

Female rat, urogenital opening visible between legs.

male rat

Male Rat


The Muscular and Skeletal System of the Rat

You will carefully remove the skin of the rat to expose the muscles below. This task is best accomplished with scissors and forceps where the skin is gently lifted and snipped away from the muscles. You can start at the incision point where the latex was injected and continue toward the tail. Use the lines on the diagram to cut a similar pattern, avoiding the genital area. Gently peel the skin from the muscles, using scissors and a probe to tease away muscles that stick to the skin.

rat incision

rat skinning

skinning the rat

rat muscles labeled

rat forearm muscles

abdomen


Exposing the Bones of the Leg

Carefully tease away the biceps femoris and gastrocnemius to expose the 3 leg bones:

Tibia, Fibula, and Femur.

The fibula is a very thin bone that may have been broken in the process of exposing the bones.

Note that the joint of the hip is called a ball and socket joint. Examine how the bones fit into the pelvis

rat leg


The Abdominal Organs


1. Locate the liver, which is a dark colored organ suspended just under the diaphragm. It has four lobes:

median or cystic lobe - located at the top, there is an obvious central cleft
left lateral lobe - large and partially covered by the stomach
right lateral lobe - partially divided into an anterior and posterior lobule, hidden from view by the median lobe
caudate lobe - small and folds around the esophagus and the stomach, seen most easily when stomach is raised

liver

caudate lobe

2. Find the stomach, its a curved organ lying just under the liver. At the top of the stomach you can see the esophagus where it pierces the diaphragm and joins the stomach. Lifting the stomach up may reveal a bumpy glandular organ: the pancreas.

rat digestive

3. The spleen is about the same color as the liver and is attached to the greater curvature of the stomach.

4. The small intestine is a slender coiled tube that receives partially digested food from the stomach (via the pyloric sphincter). It consists of three sections: duodenum, jejunum and  ileum, (Listed in order from the stomach to the large intestine.)  The duodenum is recognizable as the first stretch of the intestine leading from the stomach, it is mostly straight.  The jejunum and ileum are both curly parts of the intestine, with the ileum being the last section before the small intestine becomes the large intestine. 

5. Locate the colon, which is the large greenish tube that extends from the small intestine and leads to the anus. The colon is also known as the large intestine and it consists of four sections:

cecum - large flattened sac in the lower third of the abdominal cavity, it is a dead-end pouch and is similar to the appendix in humans.
ascending colon – food travels upward.
transverse colon – a short section that is parallel to the diaphragm
descending colon – the section of the large intestine that travels back down toward the rectum.
rectum - the short, terminal section of the colon that leads to the anus. The rectum temporarily stores feces before they are expelled from the body.

cecum

 


Organs of the Head and Neck

1. Locate the salivary glands, which on the sides of the neck, between muscles and the external layer of skin.  You should not have to cut very deep.  Peel the skin back to expose the salivary glands.  They are soft spongy tissue that secrete saliva and amylase (an enzyme that helps break down food).  The texture of these glands resembles chewing gum.


There are three salivary glands - the sublingual (yellow pin), submaxillary (green pin), and parotid (red pin). The parotid is easiest to find, it lies just beneath the ear and extends to the neck.  See if you can find the others also.

2. Find the lymph glands which lie anterior to the salivary glands. Lymph glands are dark and circular and are pressed against the jaw muscles

3. Tease away the muscles of the neck to reveal the trachea. The trachea is identifiable by its ringed cartilage which provides support. The esophagus lies behind the trachea, but can be difficult to locate in this area.

4. Locate the larynx, which is just anterior to the trachea. The larynx is the voicebox, and allows rats to make squeaking noises.

rat salivary glands

rat trachea

In this photo, the esophagus is not visible, it lies behind the trachea. The trachea is easily distinguished from the esophagus because it has rings on its surface.

 


Thoracic Organs

Procedure: Cut through the abdominal wall of the rat following the incision marks in the picture. Be careful not to cut to deeply and keep the tip of your scissors pointed upwards. Do not damage the underlying structures. Once you have opened the body cavity, you will need to rinse it in the sink.

1. Locate the diaphragm and the heart is centrally located in the thoracic cavity. The thymus gland, may be visible at the upper part of the heart.

. The lungs are spongy organs that lie on either side of the heart and should take up most of the thoracic cavity. They lie closer to the back of the rat, you will need to push the ribs to the side to find them.

3. A sheet of muscle can be found just under the heart (and above the liver) - this is the diaphragm. This muscle is only found in mammals. (The diaphragm may have been cut when you opened the thoracic cavity.)

 

rat thoracic

On this image, the heart and thymus are obscured by fatty tissue at the top of the heart. The pericardium is also not easily seen, as it is a clear membranous sack that surrounds the heart.

 


The Urogenital System

Excretory Organs

1. The primary organs of the excretory system are the kidneys. These organs are large bean shaped structures located toward the back of the abdominal cavity on either side of the spine. Renal arteries and veins supply the kidneys with blood.


2. Locate the delicate ureters that attach to the kidney and lead to the bladder. Wiggle the kidneys to help locate these tiny tubes.

Procedure: Remove a single kidney (without damaging the other organs) and dissect it by cutting it longitudinally. Locate the cortex (the outer area) and the medulla (the inner area).  


3. The urethra carries urine from the bladder to the urethral orifice (this orifice is found in different areas depending on whether you have a male or female rat). 

4. The small yellowish glands embedded in the fat atop the kidneys are the adrenal glands.

 


The Reproductive Organs of the Male Rat

1. The major reproductive organs of the male rat are the testes (singular: testis) which are located in the scrotal sac. Cut through the sac carefully to reveal the testis. On the surface of the testis is a coiled tube called the epididymus, which collects and stores sperm. The tubular vas deferens moves sperm to the urethra, which carries it though the penis and out the body.  
 

2. The lumpy brown glands located to the left and right of the urinary bladder is the seminal vesicles. The gland below the bladder is the prostate gland and it is partially wrapped around the penis. The seminal vesicles and the prostate gland secrete materials that form the seminal fluid (semen).   Procedure: Pin the organs of the urogenital system.

The Reproductive Organs of the Female Rat

1. The short gray tube lying dorsal to the urinary bladder is the vagina. The vagina divides into two uterine horns that extend toward the kidneys. This duplex uterus is common in some animals and will accommodate multiple embryos (a litter). In contrast, a simple uterus, like the kind found in humans has a single chamber for the development of a single embryo.
2. At the tips of the uterine horns are small lumpy glands called ovaries, which are connected to the uterine horns via oviducts

 

Labeled Internal Organs