The mode of breathing in all mammals is the same. The trunk of the animal is hollow, and the space within is divided into two compartments by a thin muscular sheet called the diaphragm. The cavity nearer the fore limbs contains the heart and paired lungs. This cavity is bounded at the back by the vertebral column, at the front by the breast bone and at the sides by the ribs between which are stretched thin sheets of muscle called the intercostal muscles. At the front is a narrow entrance bounded by the first pair of ribs. Through this passes the trachea connecting the larynx with the lungs. The trachea is kept open by rings of cartilage in its walls. At its base it branches to form two main bronchial tubes one of which enters the right lung and the other the left. Each lung is a lobed structure and is enclosed in a double-walled covering formed by the inner and outer pleural membranes, which are continuous at the place where the bronchial tube enters the lung. The inner pleural membrane forms a covering to the lung while the outer one is attached to the wall of the thoracic cavity and to the diaphragm. Between chioles terminating in thin walled air sacs and their projections. It is these which give the lung its spongy structure. Accompanying the bronchial tubes are branches of the pulmonary artery and vein and these are connected by capillary networks covering the air sacs and alveoli. On examining a freshly killed rabbit, the lungs will be found to fill all the space available. If the chest wall is now punctured, the lungs collapse. Each rib is hinged where it joins the back bone, and its other end is connected with the breast bone by a short length of gristle. When a breath has been exhaled the ribs make an angle of less than 90 degrees with the back bone. When the intercostal muscles connecting each rib with the one above and the one below contract, all the ribs are brought into a position more perpendicular to the back bone, thus increasing the cavity of the thorax. The diaphragm when at rest is concave upwards, and possesses muscles Called the pleural cavity. which when contracted flatten it out, thereby further increasing the volume of the thoracic cavity. When all these muscles relax, the thoracic region to some extent collapses, decreasing the volume of its cavity, aided by other muscles, particularly those running round the belly which by their contraction push the organs lying in the abdominal ity up against the under side of the diaphragm.

To take in air the intercostal muscles contract, ribs towards the head, while at the same time the diaphragm muscles contract, flattening the diaphragm. The pleural cavity is airtight, so that this increase in size decreases the pressure in it. The air spaces inside the lungs are connected with the outside air, and at rest contain air at atmospheric pressure. The walls of the air spaces are extremely thin and elastic, and therefore expand as the pressure around them decreases, and so a ’sucking ’action results, air passing into the lungs by way of nostrils and mouth, windpipe, bronchi and bronchioles sucoessively. The internal area of each lung is tremendously increased by the possession of the tiny air spaces, called alveoli, each of which is plentifully supplied with capillaries. In an adult man the combined area of the alveoli is over 6000 sq. ft., I.e. enough to cover an area over 30 ft. square. The oxygen in the air in the alveoli continually diffuses through the walls into the blood in the capillaries, combining with the haemoglobin of the red blood corpuscles to form a bright red compound, oxyhemoglobin.

The formation of this compound enables much more oxygen to be taken up than would be the case if it merely dissolved. At the same time carbon dioxide diffuses into the alveoli from the blood, being brought there partly in solution and partly as sodium bicarbonate.

The air in the alveoli thus becomes progressively poorer in oxygen and richer in carbon dioxide and this leads to more oxygen diffusing into the alveolar air from the inhaled air while carbon dioxide diffuses in the reverse direction.

Gaseous exchange has now been carried out. The muscles next. relax, while the abdominal muscles can be contracted to push tJ organs lying in it against the diaphragm and so make the thoracic cav still smaller. This increases the pressure in the pleural cavities, lungs shrink, being elastic, and the ’used ’air is forced out. must be remembered that the lungs of any animal are never completely emptied, only a small fraction being exhaled with each breath. The deepest breathing only expels three-quarters of the air in the lungs. The air which passes in and out at each breath is called the tidal air.

Experiment 17—To demonstrate the Importance of the Pleural Cavity in Respiration and the Action of the Diaphragm

The diaphragm is represented by a football bladder, cut open and tied firmly over the mouth of the bell jar with the inflating tube A in the centre. The lung is represented by a thin rubber balloon fitted inside the bell jar to the end of the tube passing through the cork. The space inside the bell jar between the balloon and the ’diaphragm ’represents the pleural cavif

The pressure in the ’pleural cavity ’is decreased and the ’diaphragm. made to assume its relaxed position by removing some air through the tube A by using a suction pump and then closing the clip. By pulling the tube A down, the ’diaphragm ’assumes its contracted position, the cavity of the ’thorax ’is increased and the artificial ’lung ’is seen to increase in size.

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