Birds

BIRDS (Aves), a highly organized and specialized class of vertebrates, the representatives of which use the aerial environment for locomotion with perfection. Birds are characterized by integumentary appendages in the form of feathers, which are horny formations. Contour feathers (large) and down feathers (small and soft) are distinguished. Feathers and the air located between them are poor conductors of heat, which contributes to the preservation of the bird's own body temperature; birds, along with mammals, are classified as warm-blooded. Old feathers are replaced by new ones (molting). The skin of birds is dry, as it is devoid of glands, except for the uropygial gland lying above the cloacal opening. This gland produces a fatty secretion for lubricating the feathers, which is of great importance for birds swimming on water (the feathers do not become wet with water, which allows birds to passively stay on its surface). A fat layer lies under the skin; when birds are fattened, fat is deposited in various parts of the body. The head of birds is lightened in weight due to the delicacy of the skull bones. The neck is generally elongated; the torso is compact, and the tail is reduced. There are two pairs of limbs—the anterior pair is transformed into organs of flight, called wings; the hind limbs are the only support when moving on the ground. The skeleton of birds is bony, characterized by the thinness of the bones and a tendency for them to fuse entirely or partially into complex bones or to merge into a single whole without any trace. The skeleton of birds is pneumatic, i.e., filled with air due to the penetration of air sacs into the bone cavities. Teeth in currently living birds have disappeared without any trace, even in embryonic life. A beak—a horny sheath covering the upper and lower jaws—serves for grasping or tearing prey. The digestive organs are characterized by a crop, a muscular stomach, a glandular stomach, a great length of the intestine, and the presence of a cloaca, into which the bursa of Fabricius opens in young birds. The heart is four-chambered with a right aortic arch. The lungs form thin-walled sac-like outgrowths—air sacs. In addition to the upper larynx, birds have a lower, or syrinx, at the place where the trachea branches into the bronchi. In females of most birds, only the left half of the reproductive organs (ovary and functioning oviduct opening into the cloaca) has been preserved. In males, the reproductive apparatus is paired. The organs of vision are well developed. In addition to the inner and middle ear, there is a short external auditory canal. In the brain, the cerebral hemispheres and the cerebellum are very strongly developed. Birds reproduce by laying eggs, which are very rich in yolk and covered with a thick protein membrane and a strong calcareous shell. The development of the embryo occurs only when the eggs are warmed (incubation or warming in an incubator). Birds are remarkable for their instincts (nest building, care for offspring, migrations, flights, etc.). Phylogenetically, birds descended from reptiles. Some birds feed on plant food, others on various invertebrates and vertebrates, using them alive or as carrion. The number of bird species is about 20,000. Birds are of great importance to humans. Feathers, down, and skins are used in everyday life and in industry. Bird meat is used for food and for dietary nutrition. Many birds exterminate various harmful animals (mice, rats, gophers, snakes, etc.), while other birds, on the contrary, harm humans themselves (destruction and spoilage of fruits, destruction of beneficial insects, etc.). Chicken eggs have long been a classic object for embryological work. Chickens are used for genetic work and are used (like pigeons) as laboratory animals in experimental zoology and medicine. Birds are rich in various parasites, which in most cases have no relation to humans. However, some species of external bird parasites attack humans and vice versa—for example, the mites Argas persicus, Dermanyssus gallinae, the bedbug, etc. Birds are divided into the subclass Ratitae—ratites (African ostriches and rheas, cassowaries, kiwis) and Carinatae—carinates. Orders of carinates: Gaviiformes, penguins, Procellariiformes, Ciconiiformes, Pelecaniformes, herons, storks, flamingos, Anseriformes, Falconiformes, Galliformes, Gruiformes, Charadriiformes (gulls, pigeons, etc.), Cuculiformes, parrots, Coraciiformes, Passeriformes.

Figure 1. Ptosis of the left [eyelid].

Figure 2. Bilateral [ptosis].

Ptosis. [Paralysis] of the oculomotor muscles (one or several). With complete ptosis, the upper eyelid completely covers the eyeball; when ptosis is not pronounced, the upper segment of the pupil is covered by the lower edge of the upper eyelid; between these two positions of the upper eyelid, various degrees of its drooping are observed. The patient cannot voluntarily lift the eyelid despite all efforts; he tries to help lift the eyelid by contracting the forehead muscles, thanks to which, with ptosis, one can note the presence of numerous folds on the forehead and high standing of the eyebrows, which is especially demonstrative in unilateral ptosis. Ptosis can be acquired or congenital; congenital ptosis depends on a lack of development of the corresponding nuclei of the oculomotor nerve (in the brainstem), while acquired ptosis can be observed with damage to the oculomotor nerve at various levels, starting from its nuclei and up to the peripheral branches. Some authors admit a cortical origin of ptosis with damage to the angular gyrus (Landouzy) or the anterior central gyrus (Dejerine). The etiology and course are the same as with complete paralysis of the oculomotor muscles (see Ophthalmoplegia, Polioencephalitis). Isolated paralysis of Müller's muscle is accompanied not by ptosis, but by narrowing of the palpebral fissure. Treatment is surgical (see Hess operation).

N. Tolkachevskaya