And I have also shown in "Uric Acid," prev. refs., that a rise of acidity causes a rise of urea, because it clears the blood of uric acid which probably in colloidal form is obstructing the peripheral capillary circulation and preventing both the ready access of the blood albumens to the tissues, and the removal of waste products from these; and that the rise of urea which results is proportional, not so much to the greatness of the rise of acidity, but rather to the condition of the tissues at the time of the rise of acidity.

That is to say, the longer the tissue circulation has been exposed to the obstructing influence of colloid uric acid the greater may be the accumulation of waste products in those tissues and the greater will be the rise of urea when the circulation through them is once more freed. In illustration of this point I have remarked that while the rise of urea that follows the clearing of the blood of uric acid in physiological conditions, is only, as seen in these figures, a matter of a few grains; when the blood is cleared of uric acid in Bright's disease, where the obstruction to the circulation is both greater and more lasting, urea may rise as much as a hundred grains in a day.

Hence I conclude that the difference in the extent of the rise of urea in figs. 4 and 5 may be due to some difference in the quantity of waste products available in the tissues, and in conformity with this, we see that urea was only falling rather slowly as the result of the fast in fig. 4, while at the same hours in fig. 5 it was falling very quickly indeed, and my notes of the day corresponding to fig. 5 show that it was the day following a large excretion of urea after exercise, so that it is possible that the muscles were on this day rather poor both in stores of albumens and of waste products, and so the rise of acidity produced relatively little effect.

I have gone at length into this question of the effects of uric acid on circulation, nutrition and excretion of urea, because we here come in sight of our second great and important factor in nutrition, strength and power of endurance.

Our first factor, of which we have so far been talking, is a sufficient supply of albumens in a digestible form, to introduce into the blood stream sufficient albumens to produce in the tissues urea to the extent of 3.5 grains per lb. of body weight per day, and the corresponding amount of force.

We can now see that our second factor is the possibility of a satisfactory circulation through the tissues, bringing these albumens to the tissues and removing the corresponding waste products.

And this, as we can now also see, is dependent on the absence of any excess of uric acid from the blood stream.

From which it follows that, as some sources of albumens, such as animal flesh of all kinds, contain either uric acid or substances equivalent to it, such as the xanthins, these sources of albumen must be ruled out, for the blood cannot be kept properly free from this substance while it is being continually introduced with every mouthful that is swallowed; and, if the blood is not kept fairly free from this substance, the circulation will not be that best suited to the production of strength and endurance. A fact of which we shall see some practical proofs further on.

But to return to figs. 4 and 5, I give them to show that the effect of 1 oz. of sugar (containing no albumens) on the excretion of urea, is very slight compared with the effect of 1 oz. of cheese, and that this small effect is probably indirect and due to the sugar altering the alkalinity of the blood and so affecting the circulation in the tissues and the removal of waste products from them.

We have thus far learned, then, that the production of force requires:-

(1) A sufficient supply of albumens in a digestible form.

(2) A satisfactory interstitial circulation of the blood to carry these albumens to the tissues and remove waste products, and to make this possible the blood must contain little or no uric acid or similar substances, such as the xanthins.

Before I leave the subject, illustrated by the above figures of urea excretion, I will just point out that:-A small quantity of milk, cheese, or any food of similar albumen value causes a feeling that hunger is sated as the urea rises, and the feeling of hunger begins to return again as urea falls; but if a food poor in albumen is taken, such as fruit, though in much larger bulk than the milk or cheese, there is a feeling of fulness, but hunger is not sated and there is little or no rise of urea. So that the feeling of hunger is to a large extent a measure of the want of albumens in the body, and this can be sated only by albuminous food, and 1 oz. of cheese is worth in albumens more than 10 times its weight of fruit, so that hunger is not a matter of mere bulk.

Similarly, if sufficient albuminous food is taken, but something upsets digestion, there is little or no rise of urea, no increasing feeling of strength and power, but rather a feeling of languor and debility with desire to keep quiet.

But good digestion is accompanied, so long as urea is rising, by feelings of strength and power and a desire to be up and doing, so that it is quite possible to tell whether digestion is good, bad, indifferent or nil, by these feelings alone.

Thus it is not uncommon to meet with people who, soon after eating a hearty meal start for some exercise, but within half an hour are found in a condition bordering on collapse at the side of the road; and if you ask them, they will tell you that they have an intense feeling of emptiness referred to the epigastrium, as if all their good meal recently eaten had given them the slip.

What has happened here? Merely this, that the exercise has been too much for gastric digestion, which has been suspended, so that though the stomach is full, there is no absorption of albumens into the circulation, no rising urea, and no force available; and so in spite of a full stomach they have a feeling of more or less intense emptiness, which shows merely that the blood is poorly supplied with nutritive albumens, and we find that urea is falling, or at least, not rising.

A little rest puts matters right, the stomach circulation is restored, digestion goes on, albumen is absorbed and urea formed, and the man gets up and walks perhaps quite well for twenty miles, making almost as good use of his meal as if nothing had occurred.

On the other hand, it is possible that the temporary suspension of digestion may allow fermentative, putrefactive, and other processes to take place in the food mass, which continue and put digestion out for hours, days or weeks afterwards.

A similar suspension of digestion, absorption and urea formation may be produced, with exactly similar results, by other disturbing influences besides sides exercise, such as the circulation changes produced by the rush of a quantity of uric acid into the blood, producing, as I have elsewhere explained ("Uric Acid," prev. ref., p. 251), obstructive congestion of the whole digestive circulation, and so imperfection or suspension of gastric digestion, and the other results above mentioned.

And this is the causation of the gastric upset and vomiting, of the severe uric acid storm (or bilious attack, so called) and of the more or less sudden feelings of emptiness in cases of glycosuria and diabetes, associated with such and similarly caused circulation troubles and their effects on digestion.

As the readers of "Uric Acid" already know, every rise of uric acid in the blood and urine is accompanied by a fall in urea, which is no doubt partly due to defective circulation and metabolism in the muscles and other important tissues, the reverse effect to that shown in fig. 5, and also partly to defective circulation and digestion in the stomach, and consequent diminished absorption of albumen into the blood.

But if even at the end of the sixteenth, seventeenth, or eighteenth hours no food is taken, urea often begins to rise a little, and with this there comes a feeling of increased strength and power and the emptiness passes off or is greatly lessened.

This means, I believe, that the body has begun to feed on its own tissues, and that there is a rise of urea and of energy from this source; but prolong the fast for twenty-four to thirty-six hours and there is now a distinct loss of weight, as in the boy above mentioned, who was pat on a diet deficient in albumens; and this represents the albumen necessary to raise urea and produce the force required for the bodily functions, in the absence of supplies from without.

And this loss we know will be continued for weeks and weeks, and sufficient albumen will be taken each day to keep the chief functions of the body from coming to a standstill, and a corresponding amount of urea will be excreted.

We may sum up the main points in the physiology of the subject by saying that the first requisite for strength and power of endurance is a satisfactory and sufficient supply of albumens, that the body depends for these chiefly on the foods taken from day to day, but that there is also a small store of these substances in certain tissues which becomes available for use if prolonged exertion is called for in the absence of food; and further that, beyond this point in continued starvation, certain definite quantities of the tissues themselves are daily absorbed to produce the necessary albumens and urea.

My results are thus in complete accord with the teachings of those physiologists, who say that albumen is the sine qua non of a diet that sustains life; and my researches, some of which I have published in "Uric Acid" (prev. ref., p. 741), show that the excretion of urea bears a definite relation to the force produced, and that a diet on which urea is at or below 2.5 grains per pound of body weight per day will not suffice to produce much force, in spite of a liberal allowance of sugar, starch and fat.

Force is thus in all directions proportional to urea, whether gauged from hour to hour or day to day.

And the second requisite is a free circulation through the tissues to bring the albumens to them and remove their waste products, and this circulation is only possible in the comparative absence of uric acid from the blood stream; this absence explaining the rise of urea in fig. 5, and the still more marked rise if the blood can be cleared of uric acid in Bright's disease.