Вариант 5

The cells of the heart allow potassium ion to enter but not sodium ion. These ions are electrically charged particles, and by creating, a difference in the concentration of such particles within and without the cell an electric potential is set up across the cell membrane. The rise and fall of this electric potential, as ions move across the membrane, sets off a series of contractions (although the de-tails by which this is managed are not yet well understood) and the rhythmicity with which the ions move is reflected in the rhythmicity of the contractions. This means that the working of the heart depends upon the concentration of various ions in the blood, and that these must be controlled within narrow limits. This indeed the body manages; and man himself can imitate this outside the body by using, an appropriate fluid.'

A heart taken out of the body can be kept alive and beating if it is perfused with a solution containing the proper concentrations of various ions. (By "perfusion" is meant the forcing of liquid through the blood vessels that normally feed an organ.) The first fluid found to be effective for such a purpose was devised by an English physician, Sidney Ringer, and is still known as "Ringer's solution."

Вариант 6

Deglutition, or the act of swallowing, is really a very complicated movement brought about by the co-ordination of a great many different muscles. One of the reasons for its being complicated is that it is necessary to ensure that the food, when it has reached the back of the tongue, shall enter the oesophagus and not pass by mistake into the larynx or to the back of the nose. To shut off the nasal cavities the soft palate is drawn upwards so as to close the nasopharynx. To prevent the food entering the larynx there is a special structure at the back of the tongue, known as the epiglottis. This is a flap of cartilage covered with mucous membranes which projects backwards from the base of the tongue so as to overhang the entrance to the glottis. At the moment of swallowing the whole of the Adam's apple containing the glottis is drawn upwards under the epiglottis. Once the food has traversed this critical stage in its journey, it is grasped by the oesophagus and carried by a wave of muscular movement towards the stomach. In human beings gravity assists its descent, but in an animal, such as a horse, the contractions passing along the oesophagus are sufficient to carry food and drink in a direction opposite to that of gravity.

Вариант 7

The cell represents the body in miniature. It may be looked upon as a working model in which we can study on a smaller scale the activities that take place in the body as a whole. The cells out of which the body is built are to small that they cannot be teen with the naked eye. On examination under the microscope, a typical animal cell is seen to consist of а mass of protoplasm, or bioplasm surrounded by a containing membrane. It appers to be a colourless, semi-fluld, jellylike substance. Actually, protoplasm is not homogeneous (uniform), but consists of two parts, namely, a fine network, or reticulum, and a more fluid portion held in its meshes. A more useful examination of a cell reveals the fact that it is not made up of undifferentiated jelly, but that it роssesses a definite structure. This structure is best rovealed either by staining the cell with some aniline

dye.

The cell is seen to contain a vast number of solid particles which are aggregates of living matter.

These are suspended in the more fluid portion of the cell and its internal surface is enormously increased by their presence. The nucleus exercises control over all the cell's activities, and if it were to be removed the cell would immediately die.