Frog dissection provides a hands-on learning experience that reveals the intricate organization of biological systems. This practical activity offers insights into anatomy and physiology, allowing for a direct observation of how different structures contribute to an organism’s overall function. Engaging with a physical specimen helps bridge the gap between theoretical knowledge gained from textbooks and the tangible reality of biological structures. It also provides an opportunity to develop precise observation and manipulation skills.
Setting Up for Dissection
Before beginning the dissection, gather all necessary tools and prepare the workspace. A typical dissection kit includes a dissection tray, pins to secure the specimen, a scalpel for precise incisions, forceps for gripping and lifting tissues, and dissecting scissors for cutting through various layers. Essential safety equipment consists of protective gloves and safety glasses to prevent contact with preservative fluids and accidental splashes. Good ventilation is also important in the dissection area.
Arrange the tools neatly on the dissection tray or a clean surface adjacent to it. Remove the preserved frog from its packaging, rinse with tap water to remove excess preservative, and pat dry if needed. Position the frog on the dissection tray with its ventral side facing upwards, ready for the initial observation.
External Observation
Examine the frog’s external features before making any incisions. Observe its natural coloration, typically olive green to spotted for camouflage, by positioning the dorsal (back) surface upwards. The ventral (belly) side is usually lighter, often pale yellow, aiding countershading. Note the four limbs: forelimbs have four digits, hind limbs have five webbed digits adapted for movement and swimming.
Locate the two external nares (nostrils) at the tip of the head, used for respiration. Behind each bulging eye, identify the round, flat tympanic membranes (eardrums). A clear third eyelid, the nictitating membrane, is often visible, protecting and moistening the eye. Observe the cloaca, a common exit point for digestive, urinary, and reproductive systems, located between the hind legs.
Internal Exploration Steps
Pin the frog to the dissection tray by laying it on its back, spreading its limbs, and securing them through the hands and feet. Pinch the skin midway between the hind legs with forceps and make a small cut with scissors or a scalpel. Extend this cut along the ventral midline towards the lower jaw, cutting only through the skin. Make horizontal cuts above the hind legs and below the forelegs to create skin flaps, then pin them back to expose the muscle layer.
Lift the muscle midway between the hind legs and extend the cut along the midline towards the chin with scissors. At the chest, turn scissors sideways to cut through the sternum and ribs, avoiding damage to underlying organs. Make horizontal cuts through the muscle layer near the limbs, then pin back the flaps to expose the body cavity. A spiderweb-like membrane, the peritoneum, may cover organs and can be removed with forceps for a clearer view.
Identify the large, brownish liver, typically three-lobed and often the most prominent organ. Lift the liver to locate the small, greenish gallbladder underneath, which stores bile. The heart, a triangular, three-chambered organ, resides near the top of the liver and may be enclosed in the pericardium. Locate the lungs, two small, spongy sacs, beneath and behind the heart and liver.
Follow the digestive tract from the esophagus (mouth to stomach). The curved stomach connects to the small intestine, a coiled tube held by the mesentery. The small intestine is where most nutrient absorption occurs. The small intestine leads into the shorter, wider large intestine, terminating at the cloaca. The spleen, a small, reddish organ, is typically attached to the mesentery near the small intestine and stores blood.
Urogenital System
Observe the urogenital system, combining reproductive and excretory functions. The kidneys are flattened, bean-shaped, dark organs located near the spine, filtering blood wastes. Yellowish, finger-like fat bodies are often attached to the kidneys or within the abdominal cavity.
In males, small, pale testes are at the top of the kidneys. In females, large, dark ovaries filled with eggs may occupy much of the body cavity, and coiled oviducts are visible. Ureters transport urine from kidneys to the urinary bladder, all leading to the cloaca.
Nervous System (Brain)
While typically not fully dissected, the brain is located within the skull. It consists of regions like the olfactory lobes, cerebrum, optic lobes, cerebellum, and medulla oblongata, which connect to the spinal cord.
Concluding the Dissection
Proper cleanup and disposal are necessary after internal exploration. Remove the dissected specimen from the tray. Place biological remains in a designated biohazard bag or double-bag them in non-transparent plastic bags. While preserved specimens are generally not hazardous, follow local guidelines for biological waste disposal, often involving securely covered trash containers.
Clean all dissection tools (scalpels, scissors, forceps, pins) with a 10% bleach solution or bleach-based cleaner, then soap and water. Rinse and dry tools completely before storing to prevent corrosion. Clean the dissection tray with disinfectant and wash it. Dispose of used gloves and any contaminated paper towels in the appropriate waste receptacles. Maintaining hygiene is important for a safe and responsible conclusion.