Time for Some Frog Anatomy

 

As in other higher vertebrates, the frog body may be divided into a head, a short neck, and a trunk (see Vertebrates). The flat head contains the brain, mouth, eyes, ears, and nose. A short, almost rigid neck permits only limited head movement. The stubby trunk forms walls for a single body cavity, the coelom.

The internal organs of humans are housed in one of three distinct hollow cavities—the chest, the abdomen, and the pelvis. The human chest is separated from the abdomen by a powerful muscular partition, the diaphragm. There is no such partition in the frog’s coelom. All the frog’s internal organs—including the heart, the lungs, and all organs of digestion—are held in this single hollow space.

THE SKELETON AND MUSCLES

The frog’s body is supported and protected by a bony framework called the skeleton.

The skull is flat, except for an expanded area that encases the small brain. Only nine vertebrae make up the frog’s backbone, or vertebral column. The human backbone has 24 vertebrae. The frog has no ribs.

The frog does not have a tail. Only a spikelike bone, the urostyle, remains as evidence that primitive frogs probably had tails. The urostyle, or “tail pillar,” is a downward extension of the vertebral column.

The shoulders and front legs of the frog are somewhat similar to humman shoulders and arms. The frog has one “forearm” bone, the radio-ulna. Humans have two forearm bones, the radius and the ulna. Both frog and human have one “upper arm” bone, the humerus.

The hind legs of the frog are highly specialized for leaping. The single “shinbone” is the tibiofibula. Humans have two lower leg bones, the tibia and the fibula. In humans and in the frog, the femur is the single upper leg (thigh) bone. A third division of the frog’s leg consists of two elongated anklebones, or tarsals. These are the astragalus and the calcaneus. The astragalus corresponds to the human talus. The calcaneus in the human skeleton is the heel bone.

As in other vertebrates, the frog skeleton is moved by muscles. Skeleton-moving muscles are made of skeletal, or “striated,” muscle. Internal organs contain smooth muscle tissue.

THE CIRCULATORY SYSTEM

The frog heart is the only organ contained within the coelom which has its own protective covering. This is the pericardium. There are two upper chambers of the heart, the right atrium and the left atrium. The frog heart, however, has only one lower chamber, a single ventricle. In humans, the lower heart chamber is divided into two compartments, the right ventricle and the left ventricle.

Oxygen-laden blood and oxygen-poor blood containing waste gases are present together in the frog ventricle at all times. The oxygen-laden and oxygen-poor bloods, however, do not mix. Such mixing is prevented by a unique arrangement of the frog’s heart. Instead of “perching” on top of the ventricle, the right atrium dips downward into the ventricle. This causes oxygen-poor blood entering the right atrium to pass all the way down to the bottom of the ventricle.

Meanwhile, oxygen-laden blood is received by the left atrium and enters the same single ventricle. The pool of oxygen-poor blood at the bottom of the ventricle holds up the oxygen-laden blood and prevents it from sinking to the bottom. When the oxygen-poor blood flows from the ventricle into vessels leading to the lungs, the oxygen-laden blood tries to “follow” it. The lung vessels, however, are filled with oxygen-poor blood, blocking the oxygen-laden blood and forcing oxygen-laden blood to detour into the arteries. These carry the oxygen-laden blood to the tissues.

Frog blood has both a solid and a liquid portion. The liquid plasma carries solid elements such as red blood cells and white blood cells.