At the same time it must be allowed that the body is not entirely a passive agent subject to the controlling action of food, for no supply could prevent the vital actions subsiding at night, or make them equal both by night and day.
There is a power inherent in the body which accepts or rejects food as to amount, as well as to quality, and which might at length act through the appetite, and refuse the kind supplied. Moreover, the wants of the body vary from many other well-known influences, and cause an increase or decrease in the vital actions which proceeds pari passu with the consumption of the transformed or stored-up food in a degree proportionate to the cause, but such effects are often more rapid and transitory than that of food.
The variations in the requirement for food are induced by age, climate, season, and degree of exertion, and will be more fully discussed in the work on Dietaries; but it may now be desirable to give a glance at some of them.
In reference to age, there can be no doubt that all vital processes, including the action of foods, are greater and more rapid in early, and less and slower in later, than in mature life, and in both the former a more frequent administration of food is necessary. In early life, more-over, there is the important function of growth, which demands a large and more frequent supply of food, not only for daily wants, but to promote a due increase in the bulk of the structures of the body. I have also shown that the season of the year has also a decided influence over the vital actions, so that they are the greatest in the spring and the least at the end of summer.
The action of climate is similar to that of season, and shows that the vital actions are greater in cold than in hot climates, and in the uplands than in close valleys.
The influence of exertion over vital changes is immediate and proportionate, while the subsidence with the rest is less rapid than the increase. The following table of experiments upon myself shows the proportionate effect of exertion of varying degrees on the basis of the increased volume of air inspired:
No. 6.
| The lying posture | being | 1 |
| The sitting posture | is | 1.18 |
| Reading aloud or singing | " | 1.26 |
| The standing posture | " | 1.33 |
| Railway travelling in the 1st class | " | 1.40 |
| """2d class | " | 1.5 |
| ""upon the engine, at 20 to 30 miles per hour | " | 1.52 |
| """"50 to 60"" | " | 1.55 |
| ""in the 3d class | " | 1.68 |
| ""upon the engine, average of all speeds | " | 1.58 |
| """"at 40 to 50 miles per hour | " | 1.61 |
| """"30 to 40"" | " | 1.64 |
