A log floats down the river -
Oh, how furious it is!
To those who fell into the river,
The nose will be bitten off...
(Crocodile.)
Crocodiles
In terms of their general body structure, crocodiles resemble enlarged lizards.
Types of crocodiles: 1 - gharial; 2 - Nile crocodile; 3 - Chinese alligator
However, this is purely external resemblance. Crocodiles differ from lizards not only in size, but also in significant features of their anatomical structure. They are allocated to a special squad.
The huge mouth of crocodiles is armed with sharp teeth, which do not grow to the jaw bones, like all lower vertebrates, but sit in special recesses, cells, and in this respect resemble the teeth of mammals. The brain of crocodiles is well developed and its structure is similar to the more highly organized brain of birds. The lungs of crocodiles have a large volume and complex structure. Thanks to this, animals can stay under water for a long time. The skin of crocodiles, unlike the skin of frogs, is covered with a horny cover that does not allow oxygen to pass through.
The crocodile's heart is not three-chambered, like all other reptiles, but four-chambered. Not only the atrium, but also the ventricle are divided by a longitudinal septum into right and left parts. Pure arterial blood coming from the pulmonary vessels to the left side of the heart does not mix here with venous blood passing through the right atrium and right ventricle. Consequently, in this respect, crocodiles differ from amphibians and other reptiles and are closer to higher vertebrates - birds and mammals, which also have a four-chambered heart.
But still, the circulatory system of crocodiles differs from the circulatory system of higher - warm-blooded - animals: in the latter, only pure arterial blood from the left ventricle of the heart enters the arteries, and in crocodiles venous blood also enters the main arterial trunk, and therefore, the arteries carry mixed blood throughout the body blood. In this respect, crocodiles, despite their four-chambered heart, differ little from other reptiles. And only the head (brain!) receives pure arterial blood from crocodiles through the carotid arteries.
As a result, crocodiles, like all other reptiles, generally remain cold-blooded animals, and their life activity is highly dependent on the surrounding temperature conditions.
So, the higher organization of crocodiles compared to other reptiles is expressed in the structure of the teeth, heart, lungs and brain. These features bring them closer to animals of higher groups - mammals and birds.
Crocodiles are large and strong animals, active predators. The length of some species can reach 6 m. Crocodiles live in countries with a warm, tropical climate. Their life is closely connected with bodies of water - on land they usually only bask and lay eggs, and catch prey mainly in the water. Crocodiles are excellent swimmers and divers. Their long, muscular tail is laterally compressed and serves as a good propulsion device, and the toes on the hind legs are partially connected by a swimming membrane. The body of crocodiles is dressed in a shell of horny scutes and scales, which are arranged in longitudinal and transverse rows. On the back, these scutes ossify, making the shell more durable.
Having plunged into the water, the crocodile exposes only the upper part of its head, where its slightly raised nostrils and eyes are located. Let us remember that the frog also raises its flattened head out of the water in the same way; this similarity is explained by the adaptation of both animals to similar living conditions. The main prey of crocodiles are fish and frogs. But they can also attack land animals that come to water and swim across a body of water. Large species of crocodiles are also dangerous to humans.
Crocodile skin has long been used to make suitcases, briefcases and other products. Crocodile meat is also edible.
Among the most dangerous predators in the world, one of the first places are crocodiles (Latin name - Crocodilia) - the only surviving heirs of dinosaurs, which belong to the order of aquatic vertebrates. The average length of an adult is from 2 to 5.5 meters, and the weight of a crocodile can reach 550-600 kilograms.
External structure of a crocodile
The structural features of crocodiles, both internal and external, help them survive in incredible conditions. It is interesting that, despite the long process of evolution, these reptiles have retained almost all the features of their ancestors, in particular the body of a crocodile , adapted to aquatic habitat:
Few people know that the body of a crocodile can have different colors, although, as a rule, the color of a crocodile is greenish-brown. The upper part of the skin consists of rows of extremely strong and tightly connected horny plates that grow with the individual itself, so that they do not shed. The color that crocodile skin acquires can change depending on external factors, and more precisely the ambient temperature. These animals are cold-blooded, so the normal body temperature of a crocodile varies from 30 to 35 degrees.
Crocodile teeth
Representatives of this species are often confused with alligators, although in reality they have a number of differences, the main of which is the location and structure of the dentition. For example, if the jaws of a crocodile are closed, you can see the 4th tooth from the bottom, whereas in an alligator they are all closed. The total number of teeth in a crocodile is from 64 to 70, depending on the variety, and they have the same conical shape and a hollow inner surface where new incisors develop. On average, each crocodile's fang is replaced once every two years, and over the course of a lifetime there can be up to 45-50 such updates. In turn, the crocodile’s tongue is completely fused to the lower jaw, so some generally think that reptiles do not have this organ. 
Despite the fact that the crocodile's mouth looks very scary, in fact its teeth are not designed for chewing food, so it swallows its prey in large pieces. The digestive system of a crocodile has a number of specific features, for example, the stomach has very thick walls, and to improve digestion it contains stones (gastroliths). Their additional function is to change the center of gravity to improve swimming performance.
Features of the internal structure of crocodiles
In general, the internal structure of a crocodile is similar to that of other reptiles, but there are also some unusual features. For example, the skeleton of a crocodile is very similar to the structure characteristic of dinosaurs: two temporal arches, a diapsid skull, etc. The most vertebrae are found in the tail (up to 37), while in the cervical region and trunk there are only 9 and 17, respectively. For additional protection, there are ribs in the abdominal part that are not connected to the spine.
The crocodile's respiratory system is designed in such a way that the animal feels comfortable both on land and under water. The crocodile's respiratory organs are represented by the choanae (nostrils), the nasopharyngeal passage with a secondary bony palate, the palatine curtain, the trachea and the lungs with the diaphragm. The very powerful and complex lungs of a crocodile are capable of holding a large volume of air, while the animal can, if necessary, adjust its center of gravity. To prevent the crocodile's breathing from interfering with its ability to move quickly, there are special muscles in the diaphragm area. 
The crocodile’s circulatory system is unique in its own way, which is much more advanced than that of other reptiles. Thus, the crocodile’s heart is four-chambered (2 atria and 2 ventricles), and a special mechanism for mixing blood from arteries and veins makes it possible to regulate the blood supply process. If you need to speed up the digestion process, the structure of the crocodile’s heart allows you to change arterial blood to venous blood, which is more saturated with carbon dioxide and promotes the production of additional gastric juice. It should also be noted that crocodile blood has a high content of antibiotics, and hemoglobin is saturated with oxygen and works independently of red blood cells.
By the way, these predators do not have a bladder, and to search for a mate during the breeding season, there are special glands on the lower half of the jaw, which emit a musky odor.
Their nervous system is very developed, in particular, the crocodile’s brain (or rather, the cerebral hemispheres) is covered with a cortex, and among the sensory organs, hearing and vision are especially developed. We can say with confidence that the crocodile’s memory is very good, since it manages to remember the paths along which other animals go to the watering hole.
I'll tell you a story that happened a few years ago. Now I am writing a school textbook on zoology according to a program in which I myself participated. When this version of the program was first conceived, I convinced a ministerial official [Not the Russian ministry, don’t worry!] that before systematically studying individual groups, it was necessary to consider a fairly large topic that would talk about animals in general.
“Okay, where should I start?” - the official asked me. I said that the way animals live is determined primarily by what they eat and how they move. This means we need to start with a variety of ways of eating. “What are you talking about!” my interlocutor exclaimed. “How can I take such a program to the minister? He will immediately ask why we instill in children that the most important thing is food!”
I tried to argue. In general, the division of living organisms into kingdoms (animals, plants, fungi and others) is associated primarily with the method of nutrition, which, in turn, determines the features of their structure. The peculiarities of multicellular animals are a consequence of the fact that they need external sources of organic substances and at the same time do not absorb them through the surface of the body, but eat them in pieces. Animals are creatures that eat other organisms or their parts! Alas, my interlocutor was adamant. The minister will be primarily interested in the educational aspect of the program.
Thinking about how to organize the introductory part differently, I then made an unforgivable mistake. My next idea was the proposal to start studying the zoology course with the variety of life cycles. When my interlocutor realized that I was going to consider reproduction, not food, as the “main thing in life”, he seemed to think that I was making fun of him... In the end I wrote something that, as I hoped, no one won't be shocking. Then the methodologists worked their magic on this program, corrected everything in it that they did not understand, and replaced the formulations with those that were in use in historical eras when these same methodologists studied at pedagogical institutes. Then the ill-fated program was corrected by officials, then it was rethought in the spirit of new guidelines, then... - in general, I’m writing a textbook on my “own” program and I don’t get tired of swearing.
And I remembered this sad story because I was once again convinced: the most important thing for animals is the notorious “food”. When we compare different groups of our relatives with each other, we often do not realize what characteristics led them to success or failure. Do you know, for example, what has become one of the main trump cards of mammals? A successful schoolchild will name feeding offspring with milk, warm-bloodedness, high development of the nervous system, or some other property that became possible due to a sufficient amount of energy obtained from food. And one of the main trump cards of mammals is the structure of the jaws and teeth!
Try moving your lower jaw: up and down, left and right, back and forth. Its “suspension” allows movement in all three planes! In addition, on the jaws of mammals there are teeth, the structure of which is determined by the task assigned to them - to pierce, crush, grind, cut, crush, bite, tear, hold, gnaw, crush, pry, grind, scrape, etc. Our jaws are an evolutionary biomechanical masterpiece. Apart from mammals, almost no land vertebrates are capable of biting off pieces of food! A few exceptions include the archaic hatteria, which can saw off the head of a petrel chick with its jaws, and turtles, which abandoned teeth in favor of a horny scissor-shaped beak. Both birds of prey and crocodiles do not bite off pieces of food, but simply tear them off - resting with their claws (the former) or spinning with their whole body (the latter).
By the way, about crocodiles - this column is dedicated primarily to them. Thanks to sophisticated experiments, biologists from the University of Utah have learned something new about the functioning of the heart of these reptiles. But first, a few more words about school biology.
Some features of the presentation of biological material have been preserved from the times when the school was supposed to form a materialistic worldview, promoting evolution. Generally speaking, the fact of evolution has no special relation to the “materialism-idealism” dilemma (while we verbally reject the mossy dichotomy, for some reason we still attach excessive importance to this dubious dichotomy). Alas, when instead of modern ideas about evolution some stale dogmas are taught, this only causes damage to the natural scientific worldview. Such dogmas include the linear view of evolution. Think about it, the history of vertebrates is a “bush” of many branches, each of which followed its own path, adapted to its own way of life. And the school teacher, jumping from branch to branch of this bush, builds a progressive sequence of “typical representatives”: lancelet-perch-frog-lizard-dove-dog. But the frog never tried to become a lizard, it lives its own life, and without taking into account this life (and the background history of frogs) it is impossible to understand it!
What will the school teacher tell you about crocodiles? He uses them to illustrate the statement that the most progressive animals are those with a four-chambered heart and “warm-blooded” (homeothermic). And - look, children! - a crocodile has a four-chambered heart, almost like those of mammals and birds, but there is only one extra hole left. We see with our own eyes how the crocodile wanted to become a man, but did not get there, stopping halfway.
So, the crocodile has a four-chambered heart. From its right half, blood goes to the lungs, from the left - to the systemic circulation (to the organs that consume the oxygen received in the lungs). But between the bases of the vessels extending from the heart there is a gap - the foramen of Panizzia. During the normal operation of the heart, part of the arterial blood passes through this hole from the left half of the heart to the right half and enters the left aortic arch (look at the picture so as not to get confused in the right-left relationship!). Vessels leading to the stomach depart from the left aortic arch. The right aortic arch departs from the left ventricle, supplying the head and forelimbs. And then the aortic arches merge into the dorsal aorta, which provides blood supply to the rest of the body. Why is it so difficult?
First, let’s figure out why we need two circles of blood circulation at all. Fish make do with one thing: heart - gills - consumer organs - heart. Here the answer is clear. The lungs will not be able to withstand the pressure required to pump blood through the entire body. This is why the right (pulmonary) half of the heart is weaker than the left; That is why it seems to us that the heart is located in the left side of the chest cavity. But why does part of the blood flowing through the systemic circulation (from the left half of the heart) pass through the right, “pulmonary” part of the heart and the left aortic arch in crocodiles? In humans, incomplete separation of blood flows can be caused by heart disease. Why do crocodiles need such a “vice”? The fact is that the heart of a crocodile is not an unfinished human heart, it is “conceived” more complex and can function in two different modes! When the crocodile is active, both aortic arches carry arterial blood. But if the foramen of Panizzia is closed (and crocodiles “know how” to do this), venous blood will flow into the left aortic arch.
Traditionally, such a device is explained by the fact that it supposedly allows a crocodile hidden at the bottom to turn off the pulmonary circulation. In this case, venous blood is sent not to the lungs (which cannot be ventilated anyway), but immediately to the large circle - along the right aortic arch. Somewhat “better” blood will go to the head and to the front legs than to other organs. But if the lungs are disabled, is there much use in circulating the blood?
American biologists have figured out how to test the long-standing assumption that crocodiles transfer blood from one circulation to another not in order to hide, but for the sake of better digestion of food (carbon dioxide is the substrate for the production of acid by the stomach glands). Researchers have verified that in healthy young alligators, in the process of digesting food, venous, carbon dioxide-rich blood flows through the left aortic arch (the one that supplies blood to the digestive system). Then they began to interfere with the heart function of experimental crocodiles using surgical methods. In some of them, the transfer of venous blood to the left aortic arch was forcibly blocked; others underwent surgery simulating such an intervention. The effect was assessed by measuring the activity of gastric secretions and by X-ray observation of the digestion of bovine vertebrae swallowed by crocodiles. In addition, semiconductor sensors were placed in the unfortunate alligators, allowing them to measure their body temperature. As a result of these manipulations, it was possible to convincingly confirm the hypothesis put forward - the transfer of venous blood into the systemic circulation increases the production of acid in the stomach and accelerates the digestion of food.
Crocodiles are capable of feeding on fairly large prey, swallowing the prey whole or in large pieces (remember what we said about the structure of the jaws?). The body temperature of these predators is unstable, and if they do not have time to digest their prey quickly enough, they will simply be poisoned by it. The complex structure of the circulatory system and its ability to work in two different modes is a way to activate digestion. And the digestive system of crocodiles justifies its purpose: a series of X-ray photographs show how solid bovine vertebrae “melt” in acid in the stomachs of predators!
So, now we know what is important in the life of crocodiles. What integral beings they are!
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Fish
The heart of fish has 4 cavities connected in series: sinus venosus, atrium, ventricle and conus arteriosus/bulb.
- The venous sinus (sinus venosus) is a simple extension of a vein that receives blood.
- In sharks, ganoids and lungfishes, the conus arteriosus contains muscle tissue, several valves and is capable of contraction.
- In bony fishes, the conus arteriosus is reduced (has no muscle tissue and valves), therefore it is called the “arterial bulb”.
The blood in the heart of fish is venous, from the bulb/cone it flows to the gills, there it becomes arterial, flows to the organs of the body, becomes venous, returns to the venous sinus.
Lungfish
In lungfishes, a “pulmonary circulation” appears: from the last (fourth) gill artery, blood flows through the pulmonary artery (PA) into the respiratory sac, where it is additionally enriched with oxygen and returns through the pulmonary vein (PV) to the heart, in left part of the atrium. Venous blood from the body flows, as it should, into the venous sinus. To limit the mixing of arterial blood from the “pulmonary circle” with venous blood from the body, there is an incomplete septum in the atrium and partially in the ventricle.
Thus, arterial blood in the ventricle appears before venous, therefore it enters the anterior branchial arteries, from which a direct road leads to the head. The smart fish brain receives blood that has passed through the gas exchange organs three times in a row! Bathing in oxygen, the rogue.
Amphibians
The circulatory system of tadpoles is similar to that of bony fish.
In an adult amphibian, the atrium is divided by a septum into left and right, resulting in a total of 5 chambers:
- venous sinus (sinus venosus), in which, like in lungfishes, blood flows from the body
- the left atrium (left atrium), into which, like in lungfishes, blood flows from the lung
- right atrium
- ventricle
- arterial cone (conus arteriosus).
1) The left atrium of amphibians receives arterial blood from the lungs, and the right atrium receives venous blood from organs and arterial blood from the skin, so in the right atrium of frogs the blood is mixed.
2) As can be seen in the figure, the mouth of the arterial cone is shifted towards the right atrium, so blood from the right atrium enters there first, and from the left - last.
3) Inside the conus arteriosus there is a spiral valve that distributes three portions of blood:
- the first portion of blood (from the right atrium, the most venous of all) goes to the pulmonary cutaneous artery (pulmocutaneous artery), to be oxygenated
- the second portion of blood (a mixture of mixed blood from the right atrium and arterial blood from the left atrium) goes to the body organs through the systemic artery
- the third portion of blood (from the left atrium, the most arterial of all) goes to the carotid artery to the brain.
4) In lower amphibians (tailed and legless) amphibians
- the septum between the atria is incomplete, so mixing of arterial and mixed blood occurs more strongly;
- the skin is supplied with blood not from the cutaneous pulmonary arteries (where the most venous blood is possible), but from the dorsal aorta (where the blood is average) - this is not very beneficial.
5) When a frog sits under water, venous blood flows from the lungs into the left atrium, which, in theory, should go to the head. There is an optimistic version that the heart begins to work in a different mode (the ratio of the pulsation phases of the ventricle and the arterial cone changes), complete mixing of the blood occurs, due to which not completely venous blood from the lungs enters the head, but mixed blood consisting of venous blood of the left atrium and mixed blood of the right. There is another (pessimistic) version, according to which the brain of an underwater frog receives the most venous blood and becomes dull.
Reptiles
In reptiles, the pulmonary artery (“to the lung”) and two aortic arches emerge from a ventricle partially divided by a septum. The division of blood between these three vessels occurs in the same way as in lungfish and frogs:
- The most arterial blood (from the lungs) enters the right aortic arch. To make it easier for children to learn, the right aortic arch begins from the very left part of the ventricle, and it is called the “right arch” because it goes around the heart on right, it is included in the spinal artery (you can see what it looks like in the next and subsequent figures). The carotid arteries depart from the right arch - the most arterial blood enters the head;
- mixed blood enters the left aortic arch, which goes around the heart on the left and connects with the right aortic arch - the spinal artery is obtained, carrying blood to the organs;
- The most venous blood (from the body organs) enters the pulmonary arteries.
Crocodiles
Crocodiles have a four-chambered heart, but they still mix blood through a special foramen of Panizza between the left and right aortic arches.
It is believed, however, that mixing does not normally occur: due to the fact that there is higher pressure in the left ventricle, blood from there flows not only into the right aortic arch (Right aorta), but also - through the foramen of Panicia - into the left aortic arch (Left aorta), thus the crocodile’s organs receive almost entirely arterial blood.
When a crocodile dives, the blood flow through its lungs decreases, the pressure in the right ventricle increases, and the flow of blood through the foramen of panicia stops: the left aortic arch of an underwater crocodile flows blood from the right ventricle. I don’t know what the point is in this: all the blood in the circulatory system at this moment is venous, why should it be redistributed where? In any case, blood enters the head of the underwater crocodile from the right aortic arch - when the lungs are not working, it is completely venous. (Something tells me that the pessimistic version is also true for underwater frogs.)
Birds and mammals
The circulatory systems of animals and birds in school textbooks are presented very close to the truth (all other vertebrates, as we have seen, are not so lucky with this). The only little thing that you are not supposed to talk about in school is that in mammals (B) only the left aortic arch is preserved, and in birds (B) only the right one is preserved (under the letter A is the circulatory system of reptiles, in which both arches are developed) - There is nothing else interesting in the circulatory system of either chickens or people. Except for the fruits...
Fruit
Arterial blood received by the fetus from the mother comes from the placenta through the umbilical vein. Part of this blood enters the portal system of the liver, part bypasses the liver, both of these portions ultimately flow into the inferior vena cava (interior vena cava), where they mix with venous blood flowing from the fetal organs. Entering the right atrium (RA), this blood is once again diluted with venous blood from the superior vena cava (superior vena cava), thus resulting in hopelessly mixed blood in the right atrium. At the same time, some venous blood from the non-functioning lungs enters the left atrium of the fetus - just like a crocodile sitting under water. What shall we do, colleagues?
The good old incomplete septum, which the authors of school textbooks on zoology laugh at so loudly, comes to the rescue - in the human fetus, right in the septum between the left and right atria, there is an oval hole (Foramen ovale), through which mixed blood from the right atrium enters the left atrium. In addition, there is a ductus arteriosus (Dictus arteriosus), through which mixed blood from the right ventricle enters the aortic arch. Thus, mixed blood flows through the fetal aorta to all its organs. And to the brain too! And you and I pestered frogs and crocodiles!! And themselves.
Tests
1. Cartilaginous fish lack:
a) swim bladder;
b) spiral valve;
c) conus arteriosus;
d) chord.
2. The circulatory system in mammals contains:
a) two aortic arches, which then merge into the dorsal aorta;
b) only the right aortic arch
c) only the left aortic arch
d) only the abdominal aorta, and there are no aortic arches.
3. The circulatory system of birds contains:
A) two aortic arches, which then merge into the dorsal aorta;
B) only the right aortic arch;
B) only the left aortic arch;
D) only the abdominal aorta, and there are no aortic arches.
4. The arterial cone is present in
A) cyclostomes;
B) cartilaginous fish;
B) cartilaginous fish;
D) bony ganoid fish;
D) bony fish.
5. Classes of vertebrates in which blood moves directly from the respiratory organs to the tissues of the body, without first passing through the heart (select all correct options):
A) Bony fish;
B) adult amphibians;
B) Reptiles;
D) Birds;
D) Mammals.
6. The heart of a turtle in its structure:
A) three-chamber with an incomplete septum in the ventricle;
B) three-chamber;
B) four-chamber;
D) four-chamber with a hole in the septum between the ventricles.
7. Number of blood circulation in frogs:
A) one in tadpoles, two in adult frogs;
B) one in adult frogs, tadpoles have no blood circulation;
C) two in tadpoles, three in adult frogs;
D) two in tadpoles and adult frogs.
8. In order for a carbon dioxide molecule that has passed into the blood from the tissues of your left foot to be released into the environment through the nose, it must pass through all of the following structures of your body except:
A) right atrium;
B) pulmonary vein;
B) alveoli of the lungs;
D) pulmonary artery.
9. There are two circles of blood circulation (choose all the correct options):
A) cartilaginous fish;
B) ray-finned fish;
B) lungfishes;
D) amphibians;
D) reptiles.
10. A four-chambered heart has:
A) lizards;
B) turtles;
B) crocodiles;
D) birds;
D) mammals.
11. Here is a schematic drawing of a mammalian heart. Oxygenated blood enters the heart through the following vessels: 
A) 1;
B) 2;
AT 3;
D) 10.
12. The figure shows arterial arches:
A) lungfish;
B) tailless amphibian;
B) tailed amphibian;
D) reptile.
I'll tell you a story that happened a few years ago. Now I am writing a school textbook on zoology according to a program in which I myself participated. When this version of the program was just being conceived, I convinced the ministerial official that before systematically studying individual groups, it was necessary to consider a fairly large topic that would talk about animals in general.
“Okay, where should I start?” - the official asked me. I said that the way animals live is determined primarily by what they eat and how they move. This means we need to start with a variety of ways of eating. “What are you talking about!” my interlocutor exclaimed. “How can I take such a program to the minister? He will immediately ask why we instill in children that the most important thing is food!”
I tried to argue. In general, the division of living organisms into kingdoms (animals, plants, fungi and others) is associated primarily with the method of nutrition, which, in turn, determines the features of their structure. The peculiarities of multicellular animals are a consequence of the fact that they need external sources of organic substances and at the same time do not absorb them through the surface of the body, but eat them in pieces. Animals are creatures that eat other organisms or their parts! Alas, my interlocutor was adamant. The minister will be primarily interested in the educational aspect of the program.
Thinking about how to organize the introductory part differently, I then made an unforgivable mistake. My next idea was the proposal to start studying the zoology course with the variety of life cycles. When my interlocutor realized that I was going to consider reproduction, not food, as the “main thing in life”, he seemed to think that I was making fun of him... In the end I wrote something that, as I hoped, no one won't be shocking. Then the methodologists worked their magic on this program, corrected everything in it that they did not understand, and replaced the formulations with those that were in use in historical eras when these same methodologists studied at pedagogical institutes. Then the ill-fated program was corrected by officials, then it was rethought in the spirit of new guidelines, then... - in general, I’m writing a textbook on my “own” program and I don’t get tired of swearing.
And I remembered this sad story because I was once again convinced: the most important thing for animals is the notorious “food”. When we compare different groups of our relatives with each other, we often do not realize what characteristics led them to success or failure. Do you know, for example, what has become one of the main trump cards of mammals? A successful schoolchild will name feeding offspring with milk, warm-bloodedness, high development of the nervous system, or some other property that became possible due to a sufficient amount of energy obtained from food. And one of the main trump cards of mammals is the structure of the jaws and teeth!
Try moving your lower jaw: up and down, left and right, back and forth. Its “suspension” allows movement in all three planes! In addition, on the jaws of mammals there are teeth, the structure of which is determined by the task assigned to them - to pierce, crush, grind, cut, crush, bite, tear, hold, gnaw, crush, pry, grind, scrape, etc. Our jaws are an evolutionary biomechanical masterpiece. Apart from mammals, almost no land vertebrates are capable of biting off pieces of food! A few exceptions include the archaic hatteria, which can saw off the head of a petrel chick with its jaws, and turtles, which abandoned teeth in favor of a horny scissor-shaped beak. Both birds of prey and crocodiles do not bite off pieces of food, but simply tear them off - resting with their claws (the former) or spinning with their whole body (the latter).
By the way, about crocodiles - this column is dedicated primarily to them. Thanks to sophisticated experiments, biologists from the University of Utah were able to learn something new about the functioning of the heart of these reptiles. But first, a few more words about school biology.
Some features of the presentation of biological material have been preserved from the times when the school was supposed to form a materialistic worldview, promoting evolution. Generally speaking, the fact of evolution has no special relation to the “materialism-idealism” dilemma (while we verbally reject the mossy dichotomy, for some reason we still attach excessive importance to this dubious dichotomy). Alas, when instead of modern ideas about evolution some stale dogmas are taught, this only causes damage to the natural scientific worldview. Such dogmas include the linear view of evolution. Think about it, the history of vertebrates is a “bush” of many branches, each of which followed its own path, adapted to its own way of life. And the school teacher, jumping from branch to branch of this bush, builds a progressive sequence of “typical representatives”: lancelet-perch-frog-lizard-dove-dog. But the frog never tried to become a lizard, it lives its own life, and without taking into account this life (and the background history of frogs) it is impossible to understand it!
What will the school teacher tell you about crocodiles? He uses them to illustrate the statement that the most progressive animals are those with a four-chambered heart and “warm-blooded” (homeothermic). And - look, children! - a crocodile has a four-chambered heart, almost like those of mammals and birds, but there is only one extra hole left. We see with our own eyes how the crocodile wanted to become a man, but did not get there, stopping halfway.
So, the crocodile has a four-chambered heart. From its right half, blood goes to the lungs, from the left - to the systemic circulation (to the organs that consume the oxygen received in the lungs). But between the bases of the vessels extending from the heart there is a gap - the foramen of Panizzia. During the normal operation of the heart, part of the arterial blood passes through this hole from the left half of the heart to the right half and enters the left aortic arch (look at the picture so as not to get confused in the right-left relationship!). Vessels leading to the stomach depart from the left aortic arch. The right aortic arch departs from the left ventricle, supplying the head and forelimbs. And then the aortic arches merge into the dorsal aorta, which provides blood supply to the rest of the body. Why is it so difficult?
First, let’s figure out why we need two circles of blood circulation at all. Fish make do with one thing: heart - gills - consumer organs - heart. Here the answer is clear. The lungs will not be able to withstand the pressure required to pump blood through the entire body. This is why the right (pulmonary) half of the heart is weaker than the left; That is why it seems to us that the heart is located in the left side of the chest cavity. But why does part of the blood flowing through the systemic circulation (from the left half of the heart) pass through the right, “pulmonary” part of the heart and the left aortic arch in crocodiles? In humans, incomplete separation of blood flows can be caused by heart disease. Why do crocodiles need such a “vice”? The fact is that the heart of a crocodile is not an unfinished human heart, it is “conceived” more complex and can function in two different modes! When the crocodile is active, both aortic arches carry arterial blood. But if the foramen of Panizzia is closed (and crocodiles “know how” to do this), venous blood will flow into the left aortic arch.
Traditionally, such a device is explained by the fact that it supposedly allows a crocodile hidden at the bottom to turn off the pulmonary circulation. In this case, venous blood is sent not to the lungs (which cannot be ventilated anyway), but immediately to the large circle - along the right aortic arch. Somewhat “better” blood will go to the head and to the front legs than to other organs. But if the lungs are disabled, is there much use in circulating the blood?
American biologists have figured out how to test the long-standing assumption that crocodiles transfer blood from one circulation to another not in order to hide, but for the sake of better digestion of food (carbon dioxide is the substrate for the production of acid by the stomach glands). Researchers have verified that in healthy young alligators, in the process of digesting food, venous, carbon dioxide-rich blood flows through the left aortic arch (the one that supplies blood to the digestive system). Then they began to interfere with the heart function of experimental crocodiles using surgical methods. In some of them, the transfer of venous blood to the left aortic arch was forcibly blocked; others underwent surgery simulating such an intervention. The effect was assessed by measuring the activity of gastric secretions and by X-ray observation of the digestion of bovine vertebrae swallowed by crocodiles. In addition, semiconductor sensors were placed in the unfortunate alligators, allowing them to measure their body temperature. As a result of these manipulations, it was possible to convincingly confirm the hypothesis put forward - the transfer of venous blood into the systemic circulation increases the production of acid in the stomach and accelerates the digestion of food.
Crocodiles are capable of feeding on fairly large prey, swallowing the prey whole or in large pieces (remember what we said about the structure of the jaws?). The body temperature of these predators is unstable, and if they do not have time to digest their prey quickly enough, they will simply be poisoned by it. The complex structure of the circulatory system and its ability to work in two different modes is a way to activate digestion. And the digestive system of crocodiles justifies its purpose: a series of X-ray photographs show how solid bovine vertebrae “melt” in acid in the stomachs of predators!
So, now we know what is important in the life of crocodiles. What integral beings they are!
D. Shabanov. Heart of a crocodile // Computerra, M., 2008. – No. 10 (726). - pp. 36–37
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