case of the ascending current the stimulus has to pass through a district of diminished irritability, which with a very strong current acts as a block, but with a weak only slightly affects the contraction. As the current is stronger recovery from anelectrotonus is able to produce a contraction as well as kathelectrotonus, a contraction occurs both at the make and the break of the current. The absence of contraction with a very strong current at the break of the ascending current may be explained by supposing that the region of fall in irritability at the kathode blocks the stimulus of the rise in irritability at the anode.

Thus we have seen that two circumstances influence the effect of the constant current upon a nerve, viz., the strength and direction of the current. It is also necessary that the stimulus should be applied suddenly and not gradually, and that the irritability of the nerve be normal, and not increased or diminished. Sometimes (when the nerve is specially irritable?) instead of a simple contraction a tetanus occurs at the make or break of the constant current. This is especially liable to occur at the break of a strong ascending current which has been passing for some time into the preparation; this is called Ritter's tetanus, and may be increased by passing a current in an opposite direction or stopped by passing a current in the same direction.

CHAPTER XV.

NUTRITION; THE INCOME AND EXPENDITURE OF THE HUMAN BODY.

THE various physiological processes which occur in the human body have, with the exception of those in the nervous and generative systems, which will be considered in succeeding chapters, now been dealt with, and it will be as well to give in this chapter a summary of what has been considered more at length before.

The subject may be considered under the following heads. (1). The Evidence and Amount of Expenditure. (2). The Sources and Amount of Income. (3). The Sources and Objects of Expenditure.

1. Evidence and Amount of Expenditure,-There is complete evidence of Expenditure by the living body.

From the table (p. 238) it will be seen how the various amounts of the excreta are calculated.

a. From the Lungs there is exhaled every 24 hours,

In the account of Expenditure, must be remembered in addition the milk (during the period of suckling), and the products of secretion from the generative organs (ova, menstrual blood, semen); bnt, from their variable and uncertain amounts, these cannot be reckoned with the preceding.

Altogether, the expenditure of the body represented by the sum of these various excretory products amounts every 24 hours to

The matter thus lost by the body is matter the chemical attractions of which have been in great part satisfied; and which remains quite useless as food, until its elements have been again separated and re-arranged by members of the vegetable world. It is especially instructive to compare the chemical constitution of the products of expenditure, thus separated by the various excretory organs, with that of the sources of income to be immediately considered. It is evident from these facts that if the human body is to maintain its size and composition, there must be added to it matter corresponding in amount with that which is lost. The income must equal the expenditure.

2. Sources and Amount of Income.-The Income of the body consists partly of Food and Drink, and partly of Oxygen. Into the stomach there is received daily :

:

8,000 grains (520 grms.)

35,000-40,000,, (2,444 )

The average total daily receipts, in the shape of food, drink and oxygen, correspond, therefore, with the average total daily expenditure, as shown by the following table.

These quantities are approximate only. But they may be taken

The absolute identity of the tables is of course diagramaccount occurs in any living

as fair averages for a healthy adult. two numbers (in grains) in the two matic. No such exactitude in the body, in the course of any given twenty-four hours. But any difference which exists between the two amounts of income and expenditure at any given period, corresponds merely with the slight variations in the amount of capital (weight of body) to which the healthiest subject is liable.

The chemical composition of the food (p. 239) may be profitably compared with that of the excreta, as before mentioned. The greater part of our food is composed of matter which contains much potential energy; and in the chemical changes (combustion and other processes) to which it is subject in the body, active energy is manifested.

3. The Sources and Objects of Expenditure.-The sources

of the necessary waste and expenditure in the living body are various and extensive. They may be comprehended under the following heads :

(1) Common wear and tear; such as that to which all structures, living and not living, are subjected by exposure and work; but which must be especially large in the soft and easily decaying structures of an animal body.

(2) Manifestations of Force in the form either of Heat or Motion. In the former case (Heat), the combustion must be sufficient to maintain a temperature of about 100° F. (37.8° C.) throughout the whole substance of the body, in all varieties of external temperature, notwithstanding the large amount continually lost in the ways previously enumerated. In the case of Motion, there is the expenditure involved in the (a) Ordinary muscular movements, as in Prehension, Mastication, Locomotion, and numberless other ways: as well as in (b) Various involuntary movements, as in Respiration, Circulation, Digestion, &c.

(3) Manifestation of Nerve-force; as in the general regulation of all physiological processes, e.g., Respiration, Circulation, Digestion; and in Volition and all other manifestations of cerebral activity.

(4) The energy expended in all physiological processes, e.g., Nutrition, Secretion, Growth, and the like.

The total expenditure or total manifestation of energy by an animal body can be measured, with fair accuracy; the terms used being such as are employed in connection with other than vital operations. All statements, however, must be considered for the present approximate only, and especially is this the case with respect to the comparative share of expenditure to be assigned to the various objects just enumerated.

The amount of energy daily manifested by the adult human body in (a) the maintenance of its temperature; (b) in internal mechanical work, as in the movements of the respiratory muscles, the heart, &c.; and (c) in external mechanical work, as in locomotion and all other voluntary movements, has been reckoned at about 3,400 foot-tons. Of this amount only one-tenth is directly expended in internal and external mechanical work; the remainder being employed in the maintenance of the body's heat. The latter amount represents the heat which would be required to raise 484 lb. of water from the freezing to the boiling point; or if converted into mechanical power, it would suffice to raise

the body of a man weighing about 150 lb. through a vertical height of 8 miles.

To the foregoing amounts of expenditure must be added the quite unknown quantity expended in the various manifestations of nerve-force, and in the work of nutrition and growth (using these terms in their widest sense). By comparing the amount of energy which should be produced in the body from so much food of a given kind, with that which is actually manifested (as shown by the various products of combustion, in the excretions) attempts have been made, indeed, to estimate, by a process of exclusion, these unknown quantities; but all such calculations must be at present considered only very doubtfully approximate.

Sources of Error.-Among the sources of error in any such calculations must be reckoned, as a chief one, the, at present, entirely unknown extent to which forces external to the body (mainly heat) can be utilised by the tissues. We are too apt to think that the heat and light of the sun are directly correlated, as far as living beings are concerned, with the chemico-vital transformations involved in the nutrition and growth of the members of the vegetable world only. But animals, although comparatively independent of external heat and other forces, probably utilise them, to the degree occasion offers. And although the correlative manifestation of energy in the body, due to external heat and light, may still be measured in so far as it may take the form of mechanical work; yet, in so far as it takes the form of expenditure in nutrition or nerve-force, it is evidently impossible to include it by any method of estimation yet discovered; and all accounts of it must be matters of the purest theory. These considerations may help to explain the apparent discrepancy between the amount of energy which is capable of being produced by the usual daily amount of food, with that which is actually manifested daily by the body; the former leaving but a small margin for anything beyond the maintenance of heat, and mechanical work.

In the foregoing sketch we have supposed that the excreta are exactly replaced by the ingesta.

Nitrogenous Equilibrium and Formation of Fat.-If an animal, however, which has undergone a starving period, be fed upon a diet of lean meat it is found that instead of the greater part of the nitrogen being stored up, as one would expect, the chief part of it appears in the urine as urea, and continuing with the diet