SUPPLY OF FOOD REQUIRED BY MAN. 353 



and that the reception of any other substances into their interior, must he 

 looked upon as resulting simply from the permeability of their walls. This 

 statement applies to the not unfrequent occurrence of the absorption of bile 

 and other fluids, from the walls of the cavities in which they were collected ; 

 with regard to the absorption of pus, however, which has been occasionally 

 noticed to take place, both from internal collections, and from open ulcers, it 

 may be remarked, that the lymphatic vessels were not improbably laid open 

 by ulceration ; since in no other way can be understood the entrance of glo- 

 bules so large as those of pus into their interior. If this view of the function 

 of the Lymphatics be correct, it follows that we must attribute to the Blood- 

 vessels the absorption of the truly effete particles ; and in this there would 

 seem no improbability. We know that venous blood contains the elements of 

 two important excretions, that of the lungs and that of the bile, in a far higher 

 amount than does arterial blood ; and we shall hereafter see, that there is a 

 certain portion of the fluid which consists of "ill-defined animal principles" 

 that seem ready to be thus thrown off. Moreover, the materials of a large 

 part of the excretory products are probably derived from the processes of 

 assimilation themselves ; each tissue appropriating' the principles which it 

 needs, and leaving the remainder in the fluid as superfluous matter. It may 

 be further remarked, that the reciprocal part which Hunter imagined the 

 Arteries and Lymphatics to perform in the function of Nutrition, is quite 

 inconsistent with what is now known of the nature of that process ; for, as will 

 subsequently appear, it entirely consists in a reaction between the tissues and 

 the nutritious fluid, in which the vessels have no share save as the channels 

 of supply. When these channels are obstructed, or the supply of new matter 

 is cut off in any other way, the removal of the old by interstitial absorption 

 becomes evident ; and that this is accomplished at least as much by the veins 

 as by the lymphatics, appears from the fact, that in some tissues, in which it 

 may take place with rapidity, lymphatics do not exist. 



VIII. Supply of Food required by Man. 



470. The quantity of food required for the maintenance of the Human 

 body in health, varies so much with the age, sex and constitution of the indi- 

 vidual, and with the circumstances in which he may be placed, that it would 

 be absurd to attempt to fix any standard, which should apply to every particu- 

 lar case. The appetite is the only sure guide for the supply of the wants of 

 each ; but its indications must not be misinterpreted. To eat when we are 

 hungry, is an evidently natural disposition ; but to eat as long as we are 

 hungry may not always be prudent. Since the feeling of hunger does not 

 depend so much upon the state of fulness or emptiness of the stomach, as upon 

 the condition of the general system, it appears evident that the ingestion of 

 food cannot at once produce the effect of dissipating it, though it will do so 

 after a short time ; so that, if we eat with undue rapidity, we may continue 

 swallowing food long after we have taken as much as will really be required 

 for the wants of the system ; and every superfluous particle is not merely use- 

 less but injurious. Hence, besides its other important ends, the process of 

 thorough mastication is important, as prolonging the meal, and giving time to 

 the system to become acquainted (as it were) that the supply of its wants is in 

 progress ; so that its demand may be abated in due time to prevent the inges- 

 tion of more than is required. It is very justly remarked by Dr. Beaumont, 

 that the cessation of this demand, rather than the positive sense of satiety, is 

 the proper guide. " There appears to be a sense of perfect intelligence con- 

 veyed to the encephalic centre, which, in health, invariably dictates what 

 quantity of aliment (responding to the sense of hunger and its due satisfaction) 



30* 



