The structure and functions of the internal organs of amphibians briefly. Development of a lesson “Structure and operation of internal organ systems of amphibians” (7th grade). Characteristics of the external structure of the frog

Goal: to reveal the structural features and vital functions of internal organ systems in connection with the life of amphibians on land and in water.

During the classes

Work in the lesson takes place by first dividing students into 3 groups.

Motivational conversation.

What monuments do you know? What feelings arise when you pass by the monument? To whom are monuments usually erected?

There are frog monuments in Paris and Tokyo. (Presentation). Why were frogs awarded monuments?

Memory:

Warm-up: Fill in the missing words in the text.

Amphibians are:............ animals whose life is connected both with:............ and with:......... .................... On her head there are noticeable 2 bulging eyes, protected by:................... ......... The frog breathes:...... air, which enters its body through:......... .......... The skin of a frog, like that of all amphibians:.................................... ....., always moist, thanks to the liquid mucous secretions of the skin:....................... Amphibians have......... ........................ body temperature. The respiratory organs are:......................... and:................. ............. One of the adaptations for swimming are:................................. ....... between the toes.

Group assignments (oral response).

General characteristics of amphibians, their habitat

The external structure of the frog, a combination of terrestrial and aquatic features.

Skeleton and musculature of frogs.

Learning new material.

Outwardly a little disgusting, some are disgusted. There is a misconception that they cause warts on the skin. Their skin secretes mucus. Previously, in the old days, they were placed in a jar with milk, and the milk did not sour for a long time. Frogs are the first slobberers. To be able to swallow dry prey on land, saliva was required. But this saliva did not contain enzymes. The frog's eyes are designed so that it is able to see moving insects. Having smelled food, they begin to search for it with renewed vigor. And if the prey is in front of their nose, then the frogs throw out their sticky tongue inside out. The tongue jumps out of the mouth at lightning speed. Frogs' eyes can disappear from their faces. They use their eyes to push food into the esophagus. But this is not at all why frogs were awarded monuments. And why, you will only find out by studying new material.

Independent study of the material (questions in groups) In 5 minutes we will listen to your messages.

Digestive and excretory system (compared to fish)

Respiratory and nervous system, comparative characteristics with the respiratory and nervous system of fish.

Circulatory system and metabolism (compared to fish).

When presenting messages, class students fill out the table:

After presenting the messages, the material is summarized and the most important is highlighted (Presentation):

1. Food in the oral cavity is moistened with saliva - this is an important device for swallowing food on land.
2. The intestine is differentiated, it includes the duodenum, small intestine, large intestine, and rectum.
3. The frog's respiratory organs are the lungs and skin. It breathes with its lungs on land, and with its skin both in water and on land. Gas exchange only through wet skin.
4. Amphibian larvae breathe through gills
5. The surface of the lungs is small.
6. The heart has 2 atria and 1 ventricle. It is not divided by a septum and therefore the blood in the ventricle is mixed.
7. The excretory system, like the respiratory system, is also closely connected with the circulatory system. The excretory system is represented by a pair of kidneys, ureters and a bladder.
8. The nervous system consists of the brain, spinal cord and nerves. The brain has 5 sections: medulla oblongata, middle, cerebellum, intermediate, anterior. The cerebellum is especially poorly developed. The forebrain is large.
9. The organs of hearing, vision, and smell are well developed.
10. Amphibians are cold-blooded animals. Their body temperature depends on the environment.

Maybe you can guess why the monument was erected? If not, you will learn about it at the end of the lesson.

Effective consolidation.

Distribute organs by system:

1. Muscles
2. Cloaca
3. Heart
4. Lungs
5. Arteries and veins
6. Nerves
7. Spinal cord
8. Forelimb belt
9. Stomach
10. Intestines
11. Kidneys
12. Bladder
13. Hind limb belt
14. Scull
15. Brain
16. duodenum

Biological tasks:

1. Frogs move by jumping, why can't newts move by jumping?
2. Frogs are good swimmers, what features allow them to be such?
3. In frogs, 49% of oxygen comes through the lungs, how does 51% come?
4. The lungs of toads are better developed than the lungs of frogs, why?
5. Frogs can open and close their eyes. Why is this possible?
6. Frogs have a poorly developed cerebellum, what is the reason for this?

6. Unraveling the intrigue.

The monument to frogs was erected by physicians and physiologists. Thousands of experiments have been carried out on frogs. The vast majority of knowledge in physiology was obtained from studies on frogs, very unpretentious and patient creatures.

Summing up, grading.

Homework: paragraph 37. oral answers to questions after the paragraph.

"Structure and activity of the internal organs of amphibians." 7th grade. Animals. Lesson 41: “Structure and activity of the internal organs of amphibians.” Completed by: Poltavtseva O.A. – biology teacher, Proletarskaya secondary school 4 named after. Nisanova Kh.D. Purpose of the lesson: Continue studying the class Amphibians; Identify adaptations to terrestrial and aquatic habitats; Continue to develop the ability to work with a textbook, diagram, drawing.

1) Checking homework: working with the drawing “external structure of a frog”, working with terms, checking the homework table “Skeleton and Musculature”. 2) Study of a new topic: digestive system, respiratory system, circulatory system, excretory system, nervous system, metabolism. 3) Conclusions: make sure that Amphibians got their name deservedly. 4) Consolidation of new material. 5) Homework.

Checking homework. 1) Name the parts of the frog's body. 2) List the external organs of the frog located on the head. 3) Name the parts of the forelimb of a frog. 4) Name the parts of the frog's hind limb. Why are the hind limbs longer than the front ones?

Diagram of the internal structure of amphibians. The internal structure is associated with the aquatic-terrestrial habitat. Amphibians have a more complex internal structure compared to fish. The complication concerns the respiratory and circulatory systems due to the appearance of lungs and two circulatory systems. The nervous system and sensory organs have a more complex structure than that of fish.

Respiratory system of amphibians. The structure of the lungs. The mechanism of respiration of amphibians. Lungs - are small elongated sacs with thin elastic walls. Breathing occurs due to the lowering and raising of the floor of the mouth. The lungs of amphibians are primitive; therefore, the skin is important in gas exchange.

Let us consider the systems of internal organs of amphibians also using the example of the lake frog (Fig. 98). The digestive system of amphibians is more complex than the digestive system of fish. It begins with the oral cavity, at the bottom

To which the tongue is attached at the front end. (Consider how the way a frog's tongue is attached differs from the way a human's tongue is attached.) The main function of the tongue is to catch insects, not to push food through. The ducts of the salivary glands open into the oral cavity, producing saliva intended for moistening food. (Remember which other animals have salivary glands and what their purpose is.) The oral cavity opens into the esophagus, which opens into the stomach, where food digestion begins. The duodenum lies under the stomach and receives the bile duct of the liver and the pancreatic duct. The intestine forms several loops, passes into the rectum and ends in the cloaca. Water is absorbed in the intestines.

Oddly enough, some amphibians are helped to swallow food... by their eyes. Perhaps you have watched a frog roll its eyes in pain as it eats a fly. This is by no means a sign of sympathy for the victim, but the process of swallowing food.

The excretory system of amphibians is similar to the excretory system of fish. The bare skin of the lake frog does not prevent water from penetrating into the body, so excess water constantly accumulates in it. This excess water is removed by large body buds (fish also have similar ones). From the kidneys, urine is discharged through the ureters into the cloaca.

Amphibians living in the desert know how to save water. That is why urine is not produced in the South American quakia - phylomedusa. Harmful substances in it accumulate in the bladder in the form of solid crystals and dissolve when the animal enters the water.

Respiratory system. The frog's respiratory organs - the lungs - look like bags divided into cells. The lungs are penetrated by a branched network of blood vessels.

The inhalation mechanism is as follows: when the bottom of the oropharyngeal cavity drops, a rarefied space is created in it, and air enters through the open nostrils. Next, the nostrils are closed with special valves, and the bottom of the cavity rises, pushing air into the lungs.

Due to the insufficient development of the lungs in amphibians, skin respiration becomes of great importance. The amount of oxygen entering the body through the skin, and carbon dioxide also released through the skin, is half the total volume of gases necessary for the animal to breathe.

A pond frog, which was placed in water and thus deprived of the ability to breathe with its lungs, can live for three weeks, a grass frog - for a month. An ordinary newt once stayed underwater for seven months! He felt great and would have sat there for who knows how long, but the scientists were tired of watching him. A whole family of amphibians - bezlegenevi salamanders - do without lungs at all, and breathe only through the skin and using the surface of the oral cavity.

The circulatory system of amphibians is adapted to the pulmonary type of breathing. (Remember the structure of the perch’s heart.) This means that the lake frog has a three-chambered heart, consisting of two atria and a ventricle, and two circles of blood circulation allow partial separation of arterial and venous blood and better saturation of body tissues with oxygen. Mixed blood collects in the right atrium: venous, rich in carbon dioxide, from the internal organs, and arterial, rich in oxygen, from the skin. Only arterial blood enters the left atrium and is enriched with oxygen in the lungs. Both atria contract simultaneously, and blood from them enters the ventricle. The blood in the ventricle is mixed. Thanks to the presence of the heart valve, blood is distributed: arterial blood goes to the brain, venous blood goes to the lungs and skin, where it is saturated with oxygen and turns into arterial blood, and mixed blood goes to all organs. Due to the fact that in amphibians arterial and venous blood mix and oxygen saturation of tissues does not occur quickly enough, metabolism is slowed down. (Remember what metabolism is.) Therefore, the body temperature of amphibians, like fish, does not differ from the ambient temperature.

The nervous system of the lake frog is more complex than the nervous system of fish. The frog's more developed forebrain is divided into two large hemispheres, thanks to which the frog is a smarter creature than, for example, a perch.

Sense organs. Complications of the nervous system are directly related to the development of the sensory organs.

The frog's visual organs - the eyes - are well adapted to terrestrial conditions. She is able to see moving objects at a considerable distance, and also follow prey without turning her head and body.

The frog's hearing organs consist not only of the internal (like in fish), but also of the middle ear. The outer opening of the middle ear cavity is covered with the eardrum - a special elastic membrane. The middle ear includes the auditory ossicle - the stapes, which transmits signals from the eardrum to the inner ear.

The olfactory organs are represented by paired external and internal nostrils and are quite well developed. With their help, amphibians find their way to their native reservoir, covering a distance of more than a kilometer.

The organ of taste is the taste buds, which are located on the tongue. You can observe how the frog first grabs an inedible object with its tongue, and then gets rid of it by pushing it out of its mouth with its front paw.

Unlike fish, amphibians have a more complex structure of the nervous, digestive and circulatory systems, lungs, and better developed sense organs. However, the structure of the excretory system is similar to the excretory system of fish.

Terms and concepts: duodenum, bile duct, pancreas, rectum, oropharyngeal cavity, cutaneous respiration, cerebral hemispheres, middle ear, stirrup.

Check yourself. 1 . What is the structure of the lake frog's digestive system? 2. How does a frog breathe?

3. Why does a frog have pulmonary respiration and not suffocate under water?

4 . How does the circulatory system of a frog differ from the circulatory system of a fish, what is this difference? 5 . What blood goes to the left atrium and what blood goes to the right? 6. How is the perfection of the nervous system and sensory organs of amphibians manifested in comparison with the nervous system and sensory organs of fish?

How do you think? Why is the presence of two circulation circles and a three-chambered heart a progressive sign of the structure of the animal?

Lesson 10. INTERNAL STRUCTURE OF AMPHIBIDES USING THE EXAMPLE OF THE FROG OF THE GENUS RANA

Equipment and materials

1. Freshly killed frogs (one for two students).

2. Ready-made preparations: 1) dissected frog; 2) digestive system; 3) injected circulatory system; 4) excretory organs; 5) reproductive organs; 6) brain.

3. Tables: 1) appearance of the frog; 2) general location of internal organs; 3) digestive system; 4) respiratory organs; 5) circulatory system; 6) excretory organs; 7) reproductive organs of the male and female; 8) brain.

4. Dissecting instruments: scalpel; scissors; tweezers; dissecting needle; stationery pins (one set for two students).

5. Baths (one for two students).

6. Glass tubes with a drawn-out spout, connected to a rubber bulb (2 - 4 per group).

Introductory Notes

Amphibians, or amphibians, are the first relatively small group of primitive terrestrial vertebrates. However, they still maintain a close connection with the aquatic environment. This is most fully manifested during the period of embryonic and initial postembryonic development. The laying of caviar (eggs) and its development in the vast majority of amphibians occurs in water. The larvae that emerge from the eggs - tadpoles - also live in the aquatic environment. They have the characteristics of typical aquatic animals: gill breathing, a two-chambered heart, one circle of blood circulation, lateral line organs, etc. After metamorphosis, amphibians acquire the characteristics of terrestrial vertebrates.

Adult amphibians are characterized by pulmonary breathing. The circulatory system changes accordingly: the heart becomes three-chambered; pulmonary circulation occurs; the branchial arteries are replaced by homologous carotid arteries, systemic aortic arches and pulmonary arteries. The posterior vena cava, characteristic of terrestrial vertebrates, appears. The sense organs are noticeably improved: the shape of the cornea of ​​the eye becomes convex, the lens becomes lenticular, movable eyelids and the middle ear cavity with the eardrum and auditory bone - the stirrup - appear. The digestive tract is much more differentiated than in fish. Ground limbs of the five-fingered type appear. The limb girdles become more complex. A strong articulation of the hind limb girdle with the axial skeleton, etc. is achieved.

However, despite these transformations, amphibians are still poorly adapted to living on land. This is reflected in poor development of the lungs, and therefore bare skin plays an important role in the breathing process. The skin, which is easily permeable to gases and water, does not protect the body from drying out, which necessitates the need to constantly replenish water losses. A number of aquatic species retain external gills for life, so many experts consider amphibians to be a transitional group between fish and true terrestrial vertebrates. The three-chambered heart does not provide complete separation of blood, and more or less mixed blood is distributed throughout the body. The limbs are still poorly developed and cannot hold the body in an elevated position above the ground. The genitourinary system of almost all amphibians is not fundamentally different from that of fish. Amphibians, like fish, are characterized by poikilothermy (inconstancy of body temperature).

Consider the features of the internal structure of the frog.

Digestive system: oropharyngeal cavity; teeth; esophagus; stomach; duodenum; small and rectal intestine; liver; gallbladder; pancreas.

Respiratory system: laryngeal fissure; larynx; bronchi; lungs.

Circulatory system: three-chambered heart (two atria and a ventricle); abdominal aorta; two systemic aortic arches; anterior vena cava, posterior vena cava, two circles of blood circulation. Using the preparation and drawing, trace the blood circulation pattern.

Excretory organs: kidneys; ureters; bladder.

Reproductive organs: testes; vas deferens; seminal vesicles; ovaries; oviducts; fat bodies.

Central nervous system: brain (large hemispheres of the forebrain with the olfactory lobe, diencephalon, optic lobes of the midbrain, cerebellum, medulla oblongata); spinal cord.

Sketch:

1) general location of internal organs; 2) brain (top view); 3) diagram of the circulatory system (homework).

Internal structure

Opening

For dissection, freshly killed frogs of the largest possible size are most convenient. Animals are killed

Rice. 39. Dissected frog:
1 - heart; 2 - lung; 3 - liver; 4 - gallbladder; 5 - stomach, 6 - pancreas; 7 - duodenum; 8 - small intestine; 9 - rectum; 10 - spleen; 11 - cloaca; 12 - bladder; 13 - kidney; 14 - ureter; 15 - right ovary (left ovary removed); 16 - fat body; 17 - right oviduct; 18 - uterine section of the oviduct; 19 - dorsal aorta; 20 - posterior vena cava; 21 - carotid artery; 22 - left aortic arch; 23 - pulmonary artery

20 - 30 minutes before the start of class. For this purpose, frogs are placed in a tightly closed vessel containing cotton wool, abundantly moistened with chloroform or ether.

Place the frog in the bath with its belly up and, stretching its limbs, attach them with pins. After pulling back the skin at the back of the abdomen with tweezers, use scissors to make a small transverse incision in front of the base of the limbs. Then insert the scissors into the hole formed and from there make a longitudinal incision in the skin along the midline of the body up to the chin. In order not to damage the underlying organs, when cutting, it is necessary to pull the scissors upward. At the level of the forelimbs, cut the skin perpendicular to the longitudinal section to the base of the forelimbs. Turn the resulting flaps of skin to the sides and secure them with pins. After this, look at the exposed muscles and some blood vessels.

In the middle part of the body, above the abdominal cavity, lies the rectus abdominis muscle, divided by transverse tendon septa into separate segments. In the area of ​​the forelimbs there is a paired pectoral muscle, which extends from the middle of the body (from the sternum) in three bundles to the forelimbs. In front of the pectoral muscle between the branches of the lower jaw is the submandibular muscle, which plays an important role in the breathing mechanism. Noteworthy is the dark blood vessel - the abdominal vein, which stretches along the midline of the rectus abdominis muscle. In addition, a large number of vessels are found located on the inner surface of the skin. These are branches of the skin arteries and veins.

Continuing with the dissection, cut through the wall of the body cavity. The longitudinal incision should not be made along the midline, but to the side of the abdominal vein in order to avoid bleeding. When cutting the bones of the forelimb girdle, care must be taken to avoid damaging the underlying heart. After this, turn to the sides and secure the muscle flaps with pins, reattach the forelimbs (their tension has weakened after cutting the shoulder girdle) and carefully rinse the preparation with water. It is not recommended to remove any of the internal organs. You can only carefully straighten the intestines and lay them out next to the animal (Fig. 39).

General location of internal organs

At the top of the body cavity lies the three-chambered heart. In a recently killed frog it continues to pulsate. The dark-colored atria and the lighter

ventricle (note the asynchronous contraction of these chambers).

On the sides of the heart lie dark gray thin-walled lungs. As a rule, they collapse when opened and are therefore difficult to see. In order to see them better, insert the thin end of a glass tube into the laryngeal slit and, using a rubber bulb, carefully fill the lungs with air. Note the thin-walledness of the lung sacs, the weak cellularity of their surface and the network of blood vessels in their walls.

Below the heart is a large three-lobed liver. A round greenish-brown gall bladder is visible between the lobes of the liver. Under the liver on the left side of the body is the stomach, which passes into the duodenum. In the loop between the duodenum and stomach, a small orange-yellow pancreas is attached to the mesentery. The duodenum passes into the small intestine, which is curled into a ball. The large intestine is poorly visible, but the rectum, on the contrary, is very clearly defined. On the mesentery, approximately at the level of the anterior edge of the rectum, lies a burgundy round body - the spleen. Above the rectum, at the point where it exits into the cloaca, there is a transparent, two-lobed bladder (often upon opening it is damaged, collapses and is difficult to see).

The kidneys are located on the dorsal side of the abdominal cavity and are covered by the intestines, and in female frogs, by the genitals. By lifting the intestines (and ovaries in females) with tweezers, you can see the kidneys and the fat bodies lying in front of them, which are represented by multi-lobed flat formations. If the male is opened, a pair of oval testes is found under the intestines. In a sexually mature female, the entire posterior part of the body cavity is occupied by ovaries filled with eggs (spawn) and long oviducts rolled into a complex ball. It should be emphasized that the reproductive system of females is usually so developed that it even covers the intestines. Therefore, to examine the latter, it is necessary to move the ovaries and oviducts to the sides.

Organ systems

Digestive system

Compared to the digestive system of bony fish, the digestive system of amphibians is more complex and differentiated. The digestive tube begins with an oral slit leading into the oropharyngeal cavity (the latter was studied during an external examination of the frog). The tongue is placed in this cavity. The ducts of the salivary glands, which first appeared in

amphibians. However, in frogs these glands serve only to wet the food bolus and are not yet involved in the chemical processing of food. The oropharyngeal cavity passes into a short but wide esophagus (Fig. 40), and the latter into a relatively voluminous stomach, which has a slightly curved shape.

The pyloric part of the stomach, bending strongly, passes into the duodenum, which is the beginning of the small intestine. As already indicated, the pancreas lies in the loop between the stomach and duodenum. The small intestine forms many bends and loops and smoothly passes into the large intestine, which ends in a clearly visible rectum. The rectum opens into the cloaca. The entire intestine is suspended from the walls of the cavity on special folds of the peritoneum - the mesentery. The digestive glands - the liver with the gallbladder and the pancreas - are well developed. The liver ducts, together with the gallbladder duct, open into the duodenum. The pancreatic ducts flow into the gallbladder duct, so this gland does not have independent communication with the intestines.

Rice. 40. Digestive tract of a frog:
1 - esophagus; 2 - stomach; 3 - duodenum; 4 - small intestine; 5 - rectum; 6 - cloaca; 7 - the place where the rectum flows into the cloaca; 8 - bladder

Respiratory system

The respiratory organs of amphibians are of a completely different type than those of fish. They are represented by light ones - two thin-walled oval-shaped bags with narrow lower ends. The inner surface of the lungs is slightly honeycombed. When the lungs are filled with air (see page 87), a network of blood vessels is clearly visible on their walls. However, due to the imperfection of the lungs (small oxidation surface), the skin plays an important role in breathing. For example, in green frogs, over 50% of the oxygen necessary for blood oxidation passes through the skin. In connection with pulmonary breathing, internal nostrils, or choanae, appear, connecting the nasal cavity with the oropharyngeal cavity. Airways due to

due to the absence of the cervical spine, they are very short. They are represented by the nasal and oropharyngeal cavities, as well as the larynx. The larynx opens directly into the lungs with two openings.

The breathing mechanism of a frog is of the pressure type. The role of the pump is performed by the oropharyngeal cavity. When its bottom is lowered, the volume of the cavity increases and air through the external nostrils (the valves of which are open at this time) and then through the choanae is sucked into the cavity. In this case, the laryngeal fissure is closed. Then the laryngeal slit opens, the valves of the nostrils close and air from the lungs, as a result of contraction of the abdominal muscles, is also pushed into the oral cavity. After this, the mixed air from the oropharyngeal cavity, when its bottom is raised, is pushed into the lungs (the nostril valves continue to be closed). Exhalation occurs when the valves of the nostrils open due to the contraction of the elastic walls of the lungs.

Circulatory system

The circulatory system of amphibians, in connection with pulmonary respiration, has undergone significant transformations and is significantly different from that of fish. In connection with the appearance of the lungs, a second circle of blood circulation and a three-chambered heart arose. The branchial arteries were replaced by the carotid arteries, systemic aortic arches and pulmonary arteries. In higher (tailless) amphibians, the posterior cardinal veins disappeared and the posterior vena cava, characteristic of terrestrial vertebrates, appeared, and the abdominal vein appeared. In connection with cutaneous respiration, cutaneous blood vessels have reached great development, which is a specific feature of amphibians.

The frog's heart is three-chambered (Fig. 41), it consists of the right and left atria and ventricle. Both thin-walled atria communicate with the ventricle through one common opening. The right atrium is more

Rice. 41. Scheme of an opened frog heart from the ventral side:
1 - right atrium; 2 - left atrium; 3 - ventricle; 4 - valves; covering a common hole; leading from both atria into the ventricle; 5 - arterial cone; 6 - common arterial trunk; 7 - pulmonary cutaneous artery; 8 - aortic arch; 9 - common carotid artery; 10 - carotid gland; 11 - spiral valve of the arterial cone

voluminous - blood from the whole body collects through the veins into it, while the left one receives blood only from the lungs.

The ventricle is thick-walled, its inner surface is covered with numerous projections, between which there are pocket-like depressions. In addition to the indicated main parts of the heart, there is a venous sinus (sinus), which communicates with the right atrium, and an arterial cone extending from the right side of the ventricle.

Three pairs of arterial vessels (arterial arches), homologous to the gill arteries of fish, extend from the conus arteriosus. Each vessel arising from the conus arteriosus begins with an independent opening. All three vessels (arches) of the left and, respectively, the right side first go through a common arterial trunk, surrounded by a common membrane, and then branch (see Fig. 41).

The vessels of the first pair (counting from the head), homologous to the first pair of gill arteries of fish, are called carotid arteries. The carotid arteries carry blood to the head. These vessels depart from the common arterial trunk in the form of the common carotid arteries, each of which almost immediately splits into the external and internal carotid arteries (Fig. 42). At the site of their separation lies the carotid gland, which apparently regulates blood pressure in the carotid arteries.

Rice. 42. Diagram of the arterial system of a frog:
1 - ventricle; 2 - right atrium; 3 - left atrium; 4 - arterial cone; 5 - common carotid artery; 6 - systemic aortic arches; 7 - subclavian artery; 8 - dorsal aorta; 9 - iliac artery; 10 - femoral artery; 11 - sciatic artery; 12 - enteromesenteric artery; 13 - pulmonary artery; 14 - cutaneous arteries; 15 - carotid gland; 16 - external carotid artery; 17 - internal carotid artery. Arteries with venous blood are painted black, arteries with arterial and mixed blood are shaded

Through the vessels of the second pair (homologous to the second pair of gill arteries of fish) - the systemic arches of the aorta - blood is directed to the back of the body. The systemic arches go around the heart on the right and left sides, respectively, and merge under the spine into a common trunk - the dorsal aorta. The subclavian arteries depart from the systemic arches, carrying blood to the forelimbs.

Through the vessels of the third pair, homologous to the fourth pair of gill arteries of fish (vessels homologous to the third pair of gill arteries are absent in the frog), the pulmonary arteries, blood is sent to the lungs. From each pulmonary artery there arises a large cutaneous artery, through which blood is directed to the skin for oxidation (see Fig. 42). From the dorsal aorta, blood is carried through a series of arteries to the internal organs and hind limbs.

Venous blood from the anterior end of the body is collected through two pairs of jugular veins (Fig. 43). The latter, merging with the cutaneous veins, which have already absorbed the subclavian veins, form two anterior vena cava. These veins carry mixed blood into the venous sinus, since oxygenated arterial blood moves from the skin through the cutaneous veins. Blood from the hind limbs and back of the body moves through the iliac veins to the kidneys, where it passes through the portal system. The vessels leaving the kidneys merge to form

Rice. 43. Scheme of the venous system of a frog:
1 - venous sinus (shown as if visible through the contours of the heart); 2 - external jugular vein; 3 - internal jugular vein; 4 - great cutaneous vein; 5 - subclavian vein; 6 - anterior vena cava; 7 - posterior vena cava; 8 - femoral vein; 9 - sciatic vein; 10 - iliac vein; 11 - renal portal system; 12 - subintestinal vein; 13 - portal system of the liver; 14 - hepatic veins; 15 - abdominal vein; 16 - pulmonary vein Veins with arterial blood are shaded

powerful posterior vena cava. The lower (posterior) section of this vein is homologous to the posterior cardinal veins of fish, while its upper (anterior) section is a neoplasm. Through the posterior vena cava, blood is directed to the venous sinus, from which it then enters the right atrium.

From the intestines, blood is collected by the subintestinal vein, which flows into the liver, where the portal system functions. Blood also passes through the portal system of the liver from the abdominal vein, which carries it from the hind limbs. From the liver, blood flows through the hepatic veins into the posterior vena cava.

From the lungs, blood moves through the pulmonary veins to the left atrium.

The blood circulation in the heart of a frog can be schematically represented as follows. Mixed blood enters the right atrium (venous blood comes from all parts of the body, arterial blood comes from the skin), and arterial blood (from the lungs) enters the left atrium. When the atria contract, blood flows through the common opening into the ventricle. This is where further mixing of the blood occurs. However, venous blood predominates in the right part of the ventricle, and arterial blood predominates in the left. The opening leading from the ventricle to the conus arteriosus is located on the right side of the ventricle. Therefore, when the ventricle contracts, the first portion of blood, containing more venous blood, enters the opening of the nearest pulmonary arch, the next portion, with a predominance of arterial blood, enters the systemic arches of the aorta, and the portion with the least content of venous blood enters the carotid arteries.

Excretory organs

The excretory organs (Fig. 44 and 45) are represented in amphibians, as well as in fish, by the trunk kidneys (mesonephros). They are elongated, compact, reddish-brown bodies that lie on the sides of the spine. From each kidney a thin Wolffian canal stretches to the cloaca. In female Wolffrogs, the canal serves only as an excretory duct, or ureter, while in males

Rice. 44. Urogenital organs of a male frog:
1 - testis; 2 - fat body; 3 - kidney; 4 - ureter; 5 - seminal vesicle; 6 - cloaca; 7 - bladder; 8 - posterior vena cava; 9 - seminiferous tubules; 10 - adrenal gland

Rice. 45. Urogenital organs of a female frog:
1 - oviduct funnel; 2 - oviduct; 3 - uterine section of the oviduct; 4 - cloaca; 5 - bladder; b - right ovary; 7 - kidney; 8 - fat body

it simultaneously performs the function of the reproductive duct, or vas deferens (about this, see page 93). In the cloaca, the Wolffian canals open with independent openings. It also opens separately into the cloaca and bladder. Urine enters first into the cloaca, and from it into the bladder. After filling the latter, through the same hole, urine is discharged again into the cloaca, and then out.

Reproductive organs

The reproductive organs of amphibians are represented by paired gonads. In males, these are oval-shaped testes, attached by the mesentery to the anterior part of the kidneys (see Fig. 44). Thin seminiferous tubules stretch from the testes to the kidneys. Sexual products from the testis are sent through these tubules to the bodies of the kidneys, then into the already known Wolffian canals and through them to the cloaca. Before flowing into the cloaca, the Wolffian canals form small expansions - seminal vesicles, which serve to temporarily reserve sperm.

The ovaries of females (see Fig. 45) are thin-walled sacs, in adults filled with pigmented eggs. In the lateral parts of the body cavity there are highly convoluted light oviducts, or Müllerian canals. These genital canals are not directly connected to the ovaries; they open through small funnels near the lungs into the body cavity. Before flowing into the cloaca, each oviduct expands into the so-called “uterus”. Mature eggs fall out through breaks in the walls of the ovary into the body cavity, then are captured by the funnels of the oviducts and move along them to the cloaca. Passing through the oviducts, the eggs are covered with a gelatinous membrane. In the “uterus” the formation of lumps ready for laying eggs occurs. Thus, in females the excretory and reproductive ducts are completely separated.

In front of the kidneys in both sexes lie yellow multilobed fatty bodies (in males they are more developed), the function of which is to supply the gonads with nutrients during the breeding season.

central nervous system

Compared to the brain of fish, the brain of amphibians has a number of progressive features. This mainly concerns the forebrain, which in amphibians is relatively larger than in fish, its hemispheres are completely separated, and the nervous substance also lines the sides and roof in addition to the bottom of the lateral ventricles, i.e., amphibians have a real brain vault - the archipallium. Among bony fishes, a true brain vault is characteristic only of lungfishes.

To study the structure of the brain, remove the skin from the animal's head. Next, make a small transverse incision into the skin and muscle just behind the head. Having bent the frog's body along the cut, insert the tip of the scissors into the opened occipital region and carefully cut the skull from the side to the eye. Do the same on the other side. Carefully lift the cut roof of the skull up with tweezers, bend it forward and cut it off. If after this part of the brain remains covered by bones, they should be broken off with tweezers.

The frog's brain consists of five sections (Fig. 46). In front is the forebrain, consisting of two elongated hemispheres separated by a deep fissure. In front of the hemispheres

Rice. 46. ​​Frog brain from above (A) and below (B):
1 - cerebral hemispheres of the forebrain; 2 - olfactory lobe; 3 - olfactory nerve; 4 - diencephalon; 5 - visual chiasm; 6 - funnel; 7 - pituitary gland; 8 - optic lobes of the midbrain; 9 - cerebellum; 10 - medulla oblongata; 11 - spinal cord

the common olfactory lobe arises from which two olfactory nerves originate. Behind the forebrain is the diencephalon. On its roof is the epiphysis (endocrine gland). The midbrain is presented in the form of two rounded optic lobes. Behind the optic lobes lies the underdeveloped cerebellum. Immediately behind it is the medulla oblongata with the rhomboid fossa (fourth ventricle). The medulla oblongata gradually passes into the spinal cord.

To view the brain from the bottom, cut off the nerves extending from the brain and carefully lift it up by the medulla oblongata. On the lower side of the brain there is an optic chiasm, or chiasm, an infundibulum extending from the bottom of the diencephalon, and the pituitary gland (lower cerebral gland). 10 pairs of head nerves arise from the amphibian brain, the eleventh pair is not developed, and the twelfth departs outside the skull.