ANATOMY OF A FROG

EXTERNAL ANATOMY

A. HEAD

Mouth- large opening at front of head end of frog. It has both upper and lower jaws.

Nares- (nostrils) two small openings just above the mouth.

Eyes- two large eyes protrude from the head just back of the nostrils. Note the upper and lower eyelids on each eye and if possible observe the nictitating membrane that can extend from the lower eyelid to cover and protect the eye.

Tympanum- eardrum is a round membrane just back of and lower than the eye on each side. Note the frog has not external ear structure.

B. TRUNK

Cloacal Opening- the hole at the tail end of the frog.

C. APPENDAGES

Forelegs- short extensions from the front end of the trunk for support. Each foreleg consists of the upper arm, forearm, and hand.

Elbow joint- formed where the upper arm meets the lower forearm.

Digits- four fingers and a medial nonfunctional (vestigial) thumb. Note the male frog, especially during mating season, has an enlarged pad on the most medial digit of each hand. This helps to distinguish the male from the female.

Hindlegs- longer extensions from the tail end of the trunk for locomotion. Each hingleg consists of a thigh, leg and foot.

Digits-five webbed toes and a vestigial sixth toe on the inside (toward the middle) of the foot.

D. SKIN

Observe the colour and distribution of pigment in the skin. The pigment granules are dispersed through the outer layer (epidermis) and in pigment cells in the under layer (dermis). Note loose attachment of the skin.

E. MOUTH

Maxillary teeth- found on the margin of the upper jaw

Vomerine teeth- found in the front of the mouth near the centre.

Internal nares- internal opening for breathing.

Eustachian tubes- opening found near the hinge of the jaw on each side. It brings in sound, and equalizes pressure.

Pharynx- throat area in the back of the mouth cavity, begins at the hinge of the jaw and extends back toward the chest.

Esophagus- the opening of this tube is seen in the back of the pharynx.

Glottis- the opening for the windpipe in the middle of the pharynx.

Tongue- found on the lower mouth cavity. Note the free notched back end and the attached front end.

Throat muscles- observe the muscular nature of the floor of the mouth.

INTERNAL ANATOMY

A. CIRCULATION

Heart – a three chambered muscular organ in the centre of the chest. Observe the major artery (conus arteriosus) from the heart and its major branches near the heart. Observe the major veins (precaval and postcaval) to the heart and their major branches near the heart.

B. RESPIRATION

Lungs- the two lungs lie to the right and left of the heart in the chest area. Observe the bronchus that connects each lung to the very short laryngotrachea (windpipe). Note how this connects to the glottis (opening of the pharynx cavity). The lung is a sac like structure and contains small chambers called alveoli.

C. DIGESTION

Esophagus- a tube that begins at the pharynx and ends at the stomach. It is found behind the windpipe and heart.

Stomach- this is a long white organ on the left side of the abdomen. The stomach has a muscular valve (pyloric sphincter) at its lower end.

Liver- this brownish organ of three lobes is the largest of the internal organs. It covers the area around and below the heart and over the lungs. The bile from the liver aids in digestion.

Spleen- a dark reddish round body found near the back body wall of the abdomen. It stores good blood cells and gets rid of bad ones. Wait to identify until after the other abdominal organs are removed.

Small Intestine- this begins at the pyloric sphincter and extends as a coiled tube of small diameter. It ends near the lower end of the abdomen at the large intestine. Note the supporting membrane (mesentery) which holds the intestine in place.

Large intestine- (colon) this is a short tube of larger diameter that opens into the cloaca at the anus.

Cloaca- this is a chamber for receiving urine, digestive waste and reproductive cells (eggs or sperm). It opens to the outside through the cloacal opening.

Gall bladder- a small, round, dark (often greenish) sack.

Pancreas- this is a flattened organ between the small intestine and the stomach. It has a duct to the bile duct to allow pancreatic juice to flow into the small intestine.

D. EXCRETION

Kidneys – are two flattened, elongated organs lying on the dorsal surface of the abdomen and covered with a membrane (peritoneum). Each kidney connects through ducts to the cloaca. The long narrow structure on each kidney is an adrenal gland.

Urinary bladder – a small sack connected by a short tube to the cloaca.

E. REPRODUCTION

Ovaries- the ovaries are many lobed organs on the female frog which in the spring of the year contain thousands of ripe eggs. When the eggs are ripe the ovaries may fill much of the space in the abdomen.

Oviducts- thin white coiled tubes along each side of the female frog.

Testes- each testis is a small oval organ of the male frog attached by mesentery to the front surface of the kidney. The testes produce the sperm cells.

Fat bodies- fat bodies are yellowish tissue with long finger like lobes in front of the testes or ovaries. They function in insulation and storage of energy.

F. MUSCLES AND SKELETON

Muscles- muscles are formed into separate bundles. As a muscle is removed and pulled apart, you should note the long narrow fibres that make up the muscle bundles.

Tendons- the Achilles tendon is observed as a whitish cord at the back of the ankle joint and extending to the calf muscle (gastrocnemius) of the leg. Note that the tendons connect muscles to the bones.