is relatively most developed in the smallest bronchi; and (3) that the lumina of all the larger air-tubes are kept open by more or less complete rings and plates of cartilage.

Each act of respiration is composed of two phases-inspiration by expansion of the thorax and lungs, expiration by their shrinkage. Normally, inspiration is effected by muscular action, while expiration is literally a shrinkage' to the original volume by virtue of the elasticity of the lungs. It is only when respiration is forced or laboured that expiration is assisted by the action of thoracic muscles. The muscles acting in normal inspiration are the diaphragm and external intercostals with the scaleni, the inter-cartilaginous portion of the internal intercostals, the levatores costarum, and the quadratus lumborum. In laboured inspiration other muscles come into action-the serrati (magnus, superior, and inferior), the sterno-mastoid, and, indeed, any muscle extending between the thorax and upper extremities, e.g. pectorales (major and minor), latissimus dorsi. These lastnamed muscles, which ordinarily act from the thorax as a fixed point of origin, now act upon the thorax, fixed objects being grasped with the hands so that the humerus and scapula afford points of origin instead of points of insertion. At the same time the vertebral column is fixed and extended by the dorsal muscles. In normal expiration, as already stated, the chief factor is the elastic recoil of the lung to its position of rest, with perhaps some slight assistance by the interosseous portions of the internal intercostals and by the triangularis sterni. In laboured expiration the abdominal muscles assist by fixing and compressing the abdomen, forcing the diaphragm upwards, and constricting the lower part of the thorax.

men.

The most important of the muscles above enumerated are those which always act in respiration, i.e. those of normal inspiration, viz. the diaphragm and the external intercostals (in conjunction with the scaleni). The diaphragm is a dome-shaped sheet of muscle forming the partition between thorax and abdoThe external intercostals form two lateral sheets of muscle composed of a series of slips extending from rib to rib. The scaleni fix the first two ribs, thus affording an essential condition of effective action as regards elevation of the ribs by the series of external intercostals; each of these muscles acts from the rib above as its relatively fixed point upon the rib below as a relatively movable lever which it elevates; the quadratus lumborum, acting from the pelvis upon the last rib, affords a fixed point to

the contracting diaphragm. By these means the thorax is enlarged in all its diameters-(1) vertically by the descent of the diaphragm, (2) laterally by the elevation of the ribs, and (3) in an anteroposterior diameter by the elevation of the ribs and of the sternum. And according as one or other of these muscular agencies takes chief part in such inspiratory movement, two types of respiration are distinguished-the costal or thoracic, and the diaphragmatic or abdominal. In the thoracic type, which is characteristic of women, the thoracic muscles play the greater part; in the abdominal type, which is characteristic of men, the diaphragm is comparatively more effectual. But it is a mistake to suppose that these are fundamental

sexual differences; they are probably due to differences of dress.

FIG. 57.

To illustrate the diaphragmatic' and 'costal' types in the male and in the female. The shaded spaces are intended to indicate the range of movement (much exaggerated) of the diaphragm and of the chest in the two cases.

In ordinary easy breathing a moderate amount of air is taken into the chest with each inspiration, and given out with the succeeding expiration. This air is called the tidal air, and amounts to about litre, or 500 c.c., or 30 cubic inches. Beyond this ordinary or tidal amount it is possible by an extraordinary effort of inspiration to introduce into the chest a further 1,500 to 2,000 c.c. of air, which amount is termed complemental air, or by an extraordinary effort of expiration to expel 1,500 c.c. (in excess of the normal tide), which is termed the supplemental air. After the most complete possible expiration, there is left in the lungs quantity of air amounting to another 1,500 c.c., which no effort can expel; this is the residual air. In relation to this matter, two other terms require to be defined—the vital capacity, amounting to between 3,000 and 4,000 c.c., is used to denote the amount of air which can be given out by the deepest possible expiration after the deepest possible inspiration; the capacity of equilibrium or stationary air, amounting to about 3,000 c.c., is the cubic capacity of the chest after normal expiration. These terms will be best brought to mind by referring to the diagram (fig. 58).

We have seen that at each respiration the lungs are not entirely emptied of and refilled with air, but that the greater propor

tion of their contents remains stationary, while a small proportion only, nearest to the outlet (nose and mouth), is actually exchanged. It is by the rapid diffusion of gases taking place between the stationary and the fresh tidal air that the former discharges carbon dioxide and is replenished with oxygen. We have now to consider in detail the manner in which the pulmonary blood and air influence each other. The effect of the blood upon the air is known by comparing expired with atmospheric air; the effect of air upon blood by comparing pulmonary venous with pulmonary arterial blood.

Expired air as compared with atmospheric air contains about 5 per cent. less oxygen and 4 per cent. more CO2. It is warmer,

Amounts of air contained by the lungs in various phases of ordinary
and of forced respiration.

saturated with moisture, fouled by organic emanations, and slightly diminished in volume.

The differences between pulmonary venous and pulmonary arterial blood must obviously correspond with these theoretically at least-seeing that any gained or lost matter in expired air must, regarded from the other side, be lost or gained matter from the pulmonary venous blood. Practically, however, we can only say that the difference between blood in the pulmonary artery and in the pulmonary veins is that the former is blacker, contains more carbon dioxide and less oxygen than the latter; in a word, that the former is venous' in character, the latter arterial.' We cannot state, as directly demonstrated, the minute differences which must exist, either in temperature or in amount of

water, or in amount of organic matter. To return to the differences between expired and atmospheric air, it is to be observed that the minus quantity of oxygen is greater than the plus quantity of carbon dioxide; with this inequality corresponds the fact that the volume of expired air is slightly less than that of the previously inspired air. The fraction denoting the ratio Vol CO, exhaled is spoken of as the respiratory quotient; normally

Vol O, absorbed

this fraction is about or 8.

If for every five volumes of oxygen absorbed, only four volumes of CO, are exhaled, one volume of oxygen remains unaccounted for; it is probable that this excess of oxygen which does not appear in union with carbon, does so in union with hydrogen as water.

A second point, which is of great practical importance, relates to organic matter exhaled from the lungs. It is the chief factor in the fouling of air, but though its effect is thus so pronounced no direct estimate or measurement of its amount can be made. All we may say concerning its nature is, that it is probably proteid in character, condensed respiratory moisture giving a faint xanthoproteic reaction. It may, however, be indirectly estimated by measurement of exhaled CO2, the amount of which indicates to what degree the air is fouled by the more deleterious but impalpable organic emanations of the breath. A rough but excellent guide to the amount of respiratory impurity is the sense of smell. The air of a room in which respiratory CO, does not exceed 2 per 10,000 is 'fresh'; when the respiratory CO2 is between 2 and 4 per 10,000 the room begins to feel close, between 4 and 6 per 10,000 it is decidedly 'close,' between 8 and 10 the air is foul,' and beyond this limit intolerable for any length of time. 2 per 10,000 or 1 per 5,000 is thus the limit of expired CO, admissible in perfect ventilation, and should never be exceeded. But although this comparatively high standard of purity is desirable in the air of permanently occupied chambers, we frequently, and without discomfort, remain for short periods in air with a much higher percentage of respiratory CO,-e.g. in theatres or in class-rooms. CO, is not of itself specially injurious-an ordinary gas-light gives off nearly as much CO, as ten people, but the air-although spoiled to some extent is not nearly as much spoiled by one gas-burner as by ten people; thus, in gauging chamber-air it is mainly the respiratory CO, which should be estimated.

2

2

From the data given above we may at once calculate the

A 100 c.c. measuring tube graduated in tenths of a c.c. between 75 and 100. A filling bulb. Two gas pipettes. The measuring tube communicates with the inlet and with the gas pipettes by three tubes guarded by clips 1, 2, 3. It is first charged with acidulated water up to the zero mark by raising the filling bulb, tap 1 being open; it is then filled with 100 c.c. of expired air, the filling bulb being lowered until the fluid in the burette has fallen to the 100 mark. Tap 1 is now closed, the measuring tube containing 100 c.c. of expired air with unknown quantities of CO, and of O. The amount of CO, is ascertained as follows: Tap 2 being opened, the air is expelled into a gas pipette containing KHO by raising the filling bulb until the fluid has risen to the zero mark of the measuring tube. Tap 2 is now closed, and the air left in the gas pipette for about a minute, during which the CO, present is entirely absorbed. The air is then drawn back into the measuring tube by lowering the filling bulb while tap 2 is open. The volume of air (minus the CO2, which is absorbed) is read, the filling bulb being adjusted so that its contents are at the same level as the fluid in the burette. The amount of O, is next ascertained in a precisely similar manner by sending the air into a second gas pipette containing sticks of phosphorus in water, and measuring the loss of volume (due to absorption of O) in the air when drawn back into the tube. A gas pipette works thus: fluid in its lower half is displaced into its upper half when air is driven in from the measuring tube, and returns to its original place when air is drawn back. If desired, the apparatus can be connected with a vessel in which a frog, or mouse, or excised muscle has been placed, and the consequent alterations of the gases O, and CO, measured in a similar manner. The analysis of a single sample of expired air is not a reliable indication of respiratory activity. It is necessary to know the total amount of air expired in a given time, and to analyse a mixed sample of that total amount corrected to standard temperature and pressure. These conditions are fulfilled by Zuntz’