the normal. No definite or conclusive cause can be assigned to this peculiarity of rhythm; it is not, as was once believed to be the case, characteristic of fatty degeneration of the heart, but makes its appearance in a variety of diseases, or in the absence of any disease at all; during normal sleep, particularly in children, a waxing and waning respiratory rhythm is of common All we can say in explanation is to point to the fact that the Cheyne-Stokes rhythm is to the respiratory system what the Traube-Hering rhythm is to the vasomotor system; both rhythms are originated by bulbar centres and are of about

Occurrence.

the same frequency, viz. 1 to 3 per minute; indeed, the association is sometimes most definite and exact; on the rabbit, for instance, after hæmorrhage, phases of increasing and diminishing amplitude of respiration coincide with rise and fall of arterial blood-pressure; they are instances among many others of the common tendency towards 'pulsatile or rhythmic activity' manifested by all living matter. The light is apt to flicker, and especially so when it is going out.

Variations of respiratory activity under different conditions.-We have learned that the depth and frequency of respiratory movements, and consequently the amount of CO, excreted, vary under different conditions; these variations of the external yield are the consequence of variations of tissue-activity, and therefore afford an index to the variations of the true or internal respiratory activity of the entire body. It should be clearly recognised that the magnitude of the external respiratory exchange is determined by the degree of internal respiratory activity, and that the converse event-a modification of internal by external respirationis comparatively slight or of accidental occurrence. We shall take as our point of departure and standard of reference the average respiratory activity of a normal adult. Given that the

average frequency of respirations is 15 per minute, that the average amount of tidal air is 500 c.c., and that expired air contains 5 volumes per cent. less oxygen and 4 volumes per cent. more carbon dioxide-a simple calculation gives the average hourly or daily absorption of oxygen and exhalation of carbon dioxide as follows:

Thus a normal adult (weighing 70 kg.) excretes per diem 432 litres CO, (weighing 852 grammes, and containing 232 grammes of carbon); he absorbes per diem 540 litres O2 (weighing 773 grammes). Or, otherwise expressed, the average excretion of CO, is litre (weighing gramme) per kilo per hour, or about 4 c.c. per kilo per minute.

2

It should be particularly observed that these are average numbers, subject to considerable variation. Thus Pettenkofer and Voit's man, weighing 70 to 73 kilos, yielded daily amounts of CO, fluctuating between 695 and 1,038 grammes. Moreover, they are subject to considerable fluctuations in varying conditions of health and of activity; thus an excretion of CO, between 3 and 5 c.c. per kilo per minute, i.e. 180 to 300 c.c. per kilo per hour, may be regarded as normal.

2

The respiratory activity of different animals under different conditions is usually estimated by measuring the excretion of CO, rather than the absorption of O,; for observations in which it

2

2

is desired to ascertain the respiratory quotient (p. 125) both measurements must be simultaneously carried out.

The subjoined tables contain illustrative numbers from which we may draw certain further conclusions.

The respiratory activity of cold-blooded is less than that of warm-blooded animals. In cold-blooded animals it varies with temperature, being much greater at a high than at a low temperature: in warm-blooded animals the relation is more complex ;, so long as their body temperature remains constant, their respiratory activity diminishes with a rise and increases with a fall of temperature; but if their body temperature is raised or lowered, then the respiratory activity, as measured by CO2, varies in the same sense, being greater at a high than at a low temperature; with the high temperature of fever the excretion of CO, is increased, but the respiratory quotient is not diminished. The variations with age, food, and muscular exercise are particularly instructive. During incubation or pregnancy, and immediately after birth, the respiratory activity is low-new-born animals resist asphyxia for many minutes; during infancy and childhood respiratory activity is greater than during adult life; during adult life it is greater than in old age; the average excretion of CO, per kilo per hour being

In utero

In childhood
In adult life.

In old age

2

30 c.c.

500

above

Muscular exercise at once raises the excretion of CO2 the normal of ordinary life; perfect quietude, on the contrary, lowers it; the excretion of CO2 by an adult per kilo per hour

averages

2

2

This relation is confirmed by experiments on animals; the CO, excreted by a dog or rabbit with the posterior extremities paralysed by division of the spinal cord, is below normal; that of the same animal when the posterior extremities are tetanised, is above normal. Again, the respiratory excretion of CO, by a curarised animal is diminished, that of a strychninised animal is increased. The diminished production of CO,, from paralysed as.

compared with quiescent muscle will be referred to again when we come to consider the respiration of muscle.

Food increases the production of CO,, carbohydrates being particularly effective in this direction. The excretion of CO, is greater after than before a meal. Light is favourable to respiratory activity; it has been shown by experiments on frogs, birds, and mammals, that the excretion of CO, is greater in the light than in darkness, even if the animals have been blinded. The difference is due to an accelerated or retarded tissue-change, as illustrated in the fattening of cattle; the balance of evidence is to the effect that fattening proceeds better in dimly-lighted than in well-lighted stalls.

Variations in the respiratory quotient are of some theoretical importance, but are difficult to determine with accuracy. From the numbers given below it will be seen that the ratio is nearest to unity with carbohydrate diet, and furthest from it in the hibernating condition.

It is also increased by muscular exercise, and diminished during quiescence; the respiratory quotient of a rabbit, for instance, is raised by tetanisation, depressed by curarisation. According to Pettenkofer and Voit, it is greater during the day than during the night, day-time being favourable to discharge of CO,, night-time to absorption of O.

Respiration of muscle.-Direct data relating to internal respiration are most readily obtained from muscle, and the following plans have been followed:-1. Examination of the gaseous exhalation of muscle enclosed in air, or in an indifferent gas such as nitrogen, or in vacuo. 2. Extraction of the gases of muscle itself. 3. Examination of the gaseous exchanges which take place between muscle and the blood by which it is traversed. The last method is specially applicable to warm-blooded animals; in the first two methods the excised muscles of the frog have been exclusively employed.

To examine the gaseous exchanges between muscle and air the posterior extremities of a frog are left suspended in a closed vessel for several hours, at the end of which the air is analysed. It is found to have lost oxygen and gained carbon dioxide. We are not, however, justified in at once attributing the change to a respiratory activity of living muscle, because a control experiment made under similar conditions with dead muscle gives a similar result, the effect in this case being due to putrefaction, which, like respiration, is attended by a consumption of oxygen and a production of carbon dioxide. As regards the consumption of oxygen, it has not been found possible to distinguish a respiratory from a putrefactive effect; moreover, the consumption of oxygen bears no constant relation to the production of carbon dioxide. The oxygen item is, therefore, discarded from further study by this method. But as regards the production of carbon dioxide, it is possible to distinguish a true respiratory from the subsequent putrefactive discharge, and to estimate differences of the respiratory discharge coinciding with differences of muscular activity. Attention is therefore concentrated upon the carbon dioxide item.

Evidence of the above statements is supplied by the comparison of the gaseous exchanges of muscle at rest with those of tetanised muscle, as well as by the gas discharge of muscle in nitrogen or in vacuo. There is no demonstrable difference in the amounts of oxygen absorbed by resting and by tetanised muscle, whereas there is a marked difference in the amounts of carbon dioxide exhaled in the two cases. Fresh minced muscle subjected for many hours at a temperature of 20° to 30° to the vacuum of a mercurial pump, yields a continuous discharge of gas; during a first period the discharge is considerable, it then slackens, and again becomes considerable. The first discharge is regarded as respiratory, it consists almost entirely of carbon dioxide; the second discharge is the effect of putrefaction, it consists of carbon dioxide and of nitrogen, and it continues for an indefinite period. Muscle suspended in vacuo as above, or in an indifferent atmosphere of nitrogen, may thus discharge carbon dioxide with no less activity than when it is in an atmosphere containing oxygen. An entire frog washed free of blood by the injection of salt solution, and enclosed in nitrogen or in vacuo, continues equally to discharge carbon dioxide. These facts show conclusively that carbon dioxide is not due to the immediate action of oxygen, but that it is formed in the dissociation of