Internal structure of mammals

Digestive system

The digestive system of mammals, like other vertebrates, is represented by the digestive tract and glands.

Digestive tract: mouth – pharynx – esophagus – stomach – small intestine – large intestine – rectum – anus.

The digestive glands secrete substances (enzymes) necessary for the digestion of food:

• salivary glands (4 pairs) – secrete saliva;
• liver – secretes bile;
• pancreas – secretes pancreatic juice.

The mammalian digestive system has a number of features.

For mammals, cheeks and lips are characteristic, which delimit the space in front of the teeth – the pre-oral cavity.

Food captured by the soft lips is bitten off and chewed with the teeth in the mouth. A muscular tongue promotes grasping food, determining its taste, and turning it over in the mouth. In the oral cavity, food is moistened with saliva flowing through the ducts from the salivary glands. This makes it easier to swallow and move through the esophagus. Under the influence of saliva, complex organic substances (starch, sugar) contained in food are converted into less complex ones.

In the oral cavity there are teeth that do not grow to the jaws, like in other vertebrates, but are located in the cells of the jaws. The teeth are divided into incisors, canines, small molars and large molars.

Food enters the stomach through the esophagus. Its walls contain numerous glands that secrete digestive juice.

The structure of the stomach depends on the type of food. The stomach in most mammals is unicameral. A multicameral stomach is characteristic of ruminant mammals (deer, cows, goats, sheep) – it is divided into a rumen, a net, a oma and an abomasum.

The food in the rumen is fermented and then fed into the net. From the net, it burps into the mouth, where it is chewed. Then the food goes into the book and the abomasum. In these departments, its final digestion takes place.

From the stomach, food enters the initial part of the small intestine – the duodenum. It also receives juice from the pancreas and bile from the liver, which facilitate the process of digesting food.

From the duodenum, food moves further along the small intestine, where nutrients are absorbed. The remnants of undigested food enter the large intestine and then are expelled out through the anus.

In many animals feeding on coarse plant food (for example, in rabbits, beavers), a long cecum departs at the transition of the small intestine to the large intestine (in some animals with a vermiform appendix). In it, under the influence of bacteria, there is a change in difficult-to-digest food substances (fiber).

Respiratory system

The respiratory organs of mammals consist of the airways and lungs.

Airways:

• nasal cavity;
• larynx;
• trachea;
• bronchi.

In the larynx of mammals there are vocal cords, with the help of which animals moo, meow, bark, roar, howl, bleat. By emitting various sounds, animals notify their relatives about the danger, about their location, about their relationship to each other.

The trachea and bronchi are well developed.

In the lungs, the bronchi turn into thin bronchioles, ending in thin-walled vesicles, densely braided capillaries – alveoli (due to them, the surface of the lungs is 50-100 times larger than the entire surface of the mammalian skin).

Gas exchange occurs in the alveoli of the lungs.
Inhalation and exhalation are carried out with the participation of the intercostal muscles and the diaphragm.

The respiratory center is located in the medulla oblongata. After its excitation, the following processes occur sequentially:

• contraction of the intercostal muscles and diaphragm;
• an increase in lung volume;
• enrichment of blood with oxygen in the alveoli of the lungs and release it from excess carbon dioxide;
• relaxation of the intercostal muscles;
• reduction of lungs in volume and removal of air from them.

The respiratory system is also involved in thermoregulation. Species in which the sweat glands are poorly developed evaporate water from the surface of the tongue. So in hot weather, the amount of air exhaled in 1 minute in dogs increases by about 30 times. As a result, the amount of evaporated water also increases.

Circulatory system

The circulatory system is closed and consists of a four-chambered heart and blood vessels. Two circles of blood circulation.
The mammalian heart has a complete septum and consists of four chambers: two atria and two ventricles.

In the heart, the blood does not mix, it is completely divided into venous (on the right side of the heart) and arterial (on the left side of the heart), and the organs are washed with pure arterial blood.

There is only the left aortic arch extending from the left ventricle, the walls of which are thicker than those of the right.

The blood moves in two circles of blood circulation: a large circle – from the left ventricle of the heart throughout the body to the right atrium; small (pulmonary) circle – from the right ventricle of the heart through the lungs to the left atrium.

Venous blood is collected from the internal organs into the portal vein of the liver, and then into the posterior (inferior) vena cava. From the head, venous blood returns to the heart through the superior vena cava.

Metabolism

Mammals have a high metabolic rate. Mammals, like birds, are warm-blooded animals.
Warm-blooded animals are animals with a constant body temperature that does not depend on the ambient temperature.

Excretory system

The excretory system is represented by paired pelvic kidneys (bean-shaped). The urine formed in them enters the bladder through the ureters, and from it through the urethra – outward.

Nervous system

Unlike other vertebrates in mammals, the forebrain (large hemispheres) reaches a special development. Its cortex, formed by several layers of nerve cell bodies, covers the entire forebrain. In most mammalian species, it forms cerebral folds and convolutions with deep grooves.

A highly developed brain provides a high level of nervous activity and complex adaptive behavior in mammals.

For example, an animal responds with a reaction of alertness or listening to any new stimuli (turns its head, eyes and ears towards a new object or stimulus) – this is an orienting reflex. The center of orienting reflexes is located in the midbrain.

The behavior of mammals is determined not only by complex instincts, but also by higher nervous activity (HNI) associated with the rapid formation of conditioned reflexes. Conditioned reflexes are formed in the cerebral cortex.

Sense organs

All the senses are well developed in mammals. The organs of sight, hearing, smell, taste and touch help animals find food, navigate in space, and recognize individuals of their own species or enemy at a distance.

Most terrestrial mammals have a keen sense of smell (for many of them, sense of smell is important when searching for food).

The hearing organs are well developed.

In mammals, the external auditory canal and the auricle (outer ear) appear. Mammals pick up sounds with their auricles (many animals have movable ears and turn them in the direction the sound comes from).

Behind the eardrum, in the middle ear, there are three ossicles – the malleus, incus, and stapes.

The organs of vision (eyes) have two leathery eyelids with eyelashes (a transparent nictitating membrane, as it was in reptiles and birds, is underdeveloped).

Visual acuity and the degree of development of eyes in mammals are associated with the conditions of existence. Those who are nocturnal, as well as inhabitants of open landscapes (horses, zebras, antelopes) have especially large eyes.

Only monkeys are capable of distinguishing many colors.

The organs of touch are represented by vibrissae – tactile hair.