A long time ago, more than 300 million years ago, the waters parted and the first vertebrate capable of living on land emerged. True, it and its descendants did not completely melt with water, they moved as needed from one habitat to another, therefore much later, when experimenters appeared on land, they gave these creatures the name "amphibians", that is, amphibians. By that time, amphibians were crushed so much that it became possible to observe them without risking their lives. And there is something to watch. Even the most common grass frog easily, and most importantly, quickly demonstrates an amazing achievement of evolution - access to land. An aquatic herbivore emerges from the egg and becomes predatory and land-based in 66 days.
The tadpole is actually a fish; it has a lateral line (typically a fishy sense organ), a horny beak, with which it scrapes greens from snags and stones, gills and a tail that replaces fins. In front of the amazed audience, this creature grows paws. (Just imagine a fish with knees!) Lungs appear, and in order to use them, it is necessary to redo the circulatory system designed for the gills. The narrow mouth widens, the eyes bulge to look out of the water. The vegetarian's long intestine is being rebuilt to accept animal food. At the same time, gills and other swimming devices disappear. There was a fish - it became a four-legged.
Of course, the transformation of a caterpillar into a butterfly is also stunning, but it happens secretly from us, in a cocoon, and here everything is in plain sight. Something is absorbed, something is rebuilt, and the animal swims to itself, as if nothing had happened. Unless he fasts for several days. Not surprisingly, embryologists and evolutionists, one might say, do not take their eyes off the frogs. But, unfortunately, the matter was not limited to glances alone.
From the second half of the 18th century, frogs began to be slaughtered: "everyone is terribly interested in what is hidden inside." It is difficult to find a creature more convenient for comprehending the fundamentals of physiology. They are easy to catch (they began to breed frogs specially later, but this is also not difficult); the size is the most suitable - it fits in the fist, and the insides are clearly visible. And inside the frog has a heart, skeletal muscles and nervous system, which work according to the same principles as in other vertebrates. Before cutting, the mammal must be shaved, and the frog is naked, and this is also a great convenience. All its organs, muscles and nerves are very easy to dissect, and the equipment for this requires the simplest.
The famous dispute between two Italian scientists: the doctor and anatomist Luigi Galvani and the physicist Alessandro Volta took place over the severed hind legs of the frog. Galvani found that a freshly prepared frog's leg, suspended on a copper hook from an iron balcony railing, contracted whenever it touched the iron. This experiment entered the history of physiology under the name "balcony". Galvani was convinced that the reason for the contraction was the “animal electricity” generated in the frog's leg. Galvani's experiments began to be repeated by Volta and quickly found out that a frog's foot was not needed for an electric discharge between two metals - an electrolyte solution was enough, that is, there was no electricity in the frog. And Galvani stood his ground and proved that metals are not needed to twitch the paws, you can simply throw the prepared sciatic nerve onto the damaged area of the leg muscle. So L. Galvani laid the foundation for a new direction in physiology - the doctrine of electrical processes in the body, and A. Volta invented a device that was later called a galvanic cell.
In the 19th century, frogs' plastering took on such proportions that it became a sign of the times and ended up on the pages of many literary works. Two medical students, future husbands of Vera Pavlovna, do their thesis on frogs, and Dr. Bazarov does not part with them even on vacation. “I saw (with regret I must say) that the ladies, just like men, for example, go for walks every day with empty pill boxes and catch lizards, beetles, spiders and frogs, and when they return home, they stick in the unfortunate pins or cut them into pieces without the slightest remorse ... And when you wonder what this disgusting cruelty means, you are told that a young gentleman or a young lady has an inclination for the natural sciences, "grumbles the butler Betteredge from Wilkie Collins' novel Lunar a rock".
While writers were composing, real scientists made epoch-making discoveries on frogs. Thus, our famous physiologist I.M. Sechenov described the phenomenon of central inhibition. It consists in the fact that the higher parts of the central nervous system can inhibit or facilitate the reflex activity of the spinal cord. For example, a frog reflexively pulls its paw away from sulfuric acid, but if a crystal of table salt is placed on the visual hillocks of the brain, as Sechenov did, the frog does not pull back its paw. Students at the physiological workshop are still repeating Sechenov's experiments, as well as Galvani's experiments, as well as the experiments of the Austrian physiologist Otto Levy, which laid the foundation for the mediator theory.
Mediators are substances that are released from nerve endings when the nerves are excited. With their help, the nervous system controls the body. Levy proved the existence of neurotransmitters by performing a complex experiment with two isolated frog hearts. Then the scientist isolated one of the mediators, acetylcholine, and in 1936 he received the Nobel Prize for his work.
To list all the experiments that have been and are being performed on frogs means rewriting most of the physiological practice. The massive physiology building is supported by a myriad of frog-headed caryatids. Over the two and a half centuries of development of the physiology of frog blood, the sea has spilled. Sometimes students save a puppy or kitten they like from the vivarium, but no one will save the frog. Due to the fact that they are so wet, cold and not attached to people, almost no one thinks of frogs as creatures worthy of compassion or at least respect. And the frog, by the way, is a very complex and interesting organized animal. She has unique eyes that not only perceive visual information, but also partially process it, that is, they perform the function of the brain. Signals from the eye come mainly not to the visual part of the brain, but to the reflex center. Having received the information, the brain no longer thinks about what to do, but immediately activates the muscles of the body in accordance with the decision of the retina. Even such a skilled hunter as a cat cannot do this. By the way, frogs can live no less than cats, up to eighteen years old. Although a rare frog succeeds.