I have already spoken of the influence of the muscles upon the circulation, which among other things is concerned with the distribution to the whole organism of the oxygen with which the blood has become loaded in the lungs.

I shall return below to the "internal" parenchymatous respiration, by means of which the oxygen in the tissues is used for oxidation and the production of energy.

Each respiration or breath consists of inspiration or breathing in and expiration or breathing out. By inspiration, as is well known, we enlarge the thorax in all three directions, and draw into the air passages the atmospheric air, which contains 20.93 per cent. oxygen, 0.03 to 0.05 per cent. C02, and 79.04 per cent. nitrogen. not reckoning that in this latter figure is included a certain amount of argon and other gases (0.9 per cent.). The expired air changes its composition very considerably, but contains on an average 16.7 per cent. oxygen, 3.6 per cent. C02, and 79.7 per cent. nitrogen. The C02 in the lungs has thus become increased about 100 times, and oxygen has lost quite a fifth of its volume. Moreover, the expired air is much richer in watery vapour than the inspired, and we give off by the lungs under usual conditions 1/3 litre of water in twenty-four hours.

During physical exercise our need of oxygen is increased, owing to its increased use by the working muscles, which use more than the other tissues; and the muscle work for respiration itself is increased, because many muscles are working, and because the work of the respiratory muscles is thereby increased.

During rest we take very small breaths and inspire and expire 280 to 500 c.cm. of air. In lying or sitting position the latter figure is decidedly above the average. During rest we perform only twelve to fifteen respirations a minute,* and the whole ventilation of the lung may sink below 5 litres of air in a minute. As soon as we begin to move about we increase both the number of respirations and their depth; in forced work, especially in mountain climbing, we breathe something over twenty times. in hard "panting" even as many as thirty times, a minute, and we inspire and expire as much as 3,000 c.cm. with every respiration, so that the ventilation of the lungs is multiplied many times.

In calm and shallow respiration we only use for inspiration our external intercostal muscles and the diaphragm, possibly also to a certain extent the scaleni. In this men use the diaphragm more (abdominal breathing), women use the intercostal muscles more (costo-abdominal type). In forced inspiration we further contract sterno-cleido-mastoid, serratus posticus superior, the rhomboids, trapezius, levator anguli scapulae, pectoralis minor, and serratus magnus.

* L. Hermann considers the average frequency for adults, not considering rest or movement, to be eighteen to twenty times a minute.

In quiet and shallow respiration, expiration, as opposed to inspiration, takes place largely as a passive movement, with or without very slight help from the action of the internal intercostals.

The previously raised ribs sink of their own weight, the elastic attempt of the expanded lungs (and pleurae) to contract also draws the ribs inward and downward and the diaphragm upward, and the elasticity of the thoracic wall and of the rotated costal cartilages also helps to make them return to the mid-position between deep inspiration and deep expiration. But as soon as we move there arises muscle action in expiration, and then besides the internal intercostal muscles, triangularis sterni, serratus posticus inferior, quadratus lumborum, sacro-costalis, latissimus dorsi, as well as the abdominal muscles, i.e., rectus abdominis, the external and internal oblique muscles and transversalis are also brought more or less strongly into the work of respiration.

The vagus nerve is known to regulate the frequency of respiration and to contain both accelerating and retarding fibres. As already said, we have ceased to believe that there is an accelerating impulse as a reflex from the tissues thirsting for nourishment, but instead we consider that there is a direct stimulation of the respiratory centre by the products of muscular activity. These products consist not only of C02, since the increased ventilation of the lung during strong physical exercise diminishes the quantity of this gas in the blood (Zuntz and Geppert), but to a considerable extent also of lactic acid.

From the above it is clear that the work of respiration changes enormously, and that above all it accommodates itself to active exercise.

When Wislicenus (weighing 76 kilos) made his famous ascent of the Faulhorn, 1,596 metres high, the amount of work in circulation and respiration performed during the long seven and a half hours journey was 30,000 kgm., of which certainly considerably more than two-thirds was done in respiration.

Zuntz has given an average for the amount of respiratory work done in the twenty-four hours under ordinary conditions as about 25,000 kgm.

I would remind you that the air in the lungs can be divided into - (1) Residual air = the air which we retain in the lungs after the strongest possible expiration. Opinions on the volume of this vary at least from 800 to 1,500 c.cm. This is divided into "collapse air," which leaves the thorax on its being opened, and the air which then remains, i.e., "minimal air."

(2) Supplemental air (reserve air) = that portion of the air in the lungs which after ordinary expiration remains in the lungs, but can be expired by means of the strongest possible expiration. This is on an average 1,000 c.cm. (Loewy and Zuntz).

(3) Tidal air = the air taken in and expired during ordinary quiet respiration (during rest). About 500 c.cm.

(4) Complemental air = the air which can be inspired after an ordinary inspiration by means of forced inspiration. Amounts to about 1,600 c.cm.

Supplemental air, tidal air, and complemental air together constitute the so-called "vital capacity," or, in other words, the greatest possible amount of air which we can expire (3,000 or 5,000 c.cm.).