FUNCTIONS OF ORGANIC OR VEGETATIVE LIFE. 225 



they have no tendency to it except when dead, reason will hereafter he given 

 for the belief that no such distinction exists (Chap. XIV., Sect. 4). The 

 maintenance of the vital properties of all organized structure then, requires 

 either that this structure should be completely secluded from air, moisture, 

 warmth, and other agents which tend to its decomposition; or that it should 

 be renewed as fast as it decays. Now the exclusion of these decomposing 

 agents would prevent any vital actions from being called into operation; 

 since they are the ordinary stimuli which are necessary to them. For 

 instance, a seed which is buried so deep in the soil as to be excluded from 

 the contact of air, and from the warmth of the sun, will not vegetate, although 

 it may retain its power of germinating when placed in more favourable cir- 

 cumstances ; and it will not decay, because secluded from the air and warmth 

 which are necessary to its decomposition. But as a certain change of com- 

 position appears to be a necessary condition of its vital activity, it is obvi- 

 ously requisite that a provision should be made, for removing from the 

 organism all those particles which are manifesting an incipient tendency to 

 decay, and are thus losing their vital properties; and for replacing these by 

 newly-combined particles, which in their turn undergo the same process. 

 Thus we find that, in the softest parts of the Animal frame-work, as in those 

 of the Plant, there is much less permanency than there is in those harder and 

 more solid portions, which often seem altogether to defy the lapse of time. 

 Now it is in the former that the most active vital changes take place, those 

 of the nervous system, for example; whilst of the latter, the function is 

 chiefly, if not entirely, that of giving mechanical support to the structure. 

 The former organs are renewed many times, whilst the fabric of the latter is 

 not once completely changed; and thus a very interesting correspondence is 

 shown between the degree in which the action of any organized structure is 

 removed from, or is similar to, that of a mere inorganic substance, and the 

 amount of tendency to decomposition which that structure exhibits ; since 

 this constant renewal can scarcely serve any other purpose than that of 

 making up for the effects of decay. 



269. One of the most important purposes of the supply of aliment, there- 

 fore, which all living beings continually require, is the replacement of the 

 portions of the fabric that are thus lost. The effects of the process of decay, 

 when uncompensated by that of renovation, are remarkably seen in cases of 

 starvation ; for it is a very constant indication of this condition, that the body 

 exhales a putrescent odour, even before death, and that it subsequently passes 

 very rapidly into decomposition. This, it may be considered, is the reason 

 why a constant supply of aliment is still required for the maintenance of 

 every organic structure, though it may have arrived at its full growth; and it 

 also affords one source of explanation of the fact, that old people require less 

 food than adults, since their tissues are more consolidated, and thus become 

 at the same time unable to perform their usual actions with their pristine 

 energy, whilst their tendency to decomposition is less. In the growing state, 

 however, an additional important source of demand for food obviously exists, 

 in the extension which the tissues themselves are constantly receiving ; yet 

 this, perhaps, does not make so great a difference, as it appears to do, in the 

 supply which is requisite. For if the addition which is made by growth to 

 the body in any given time, be compared with the amount of exchange which 

 has taken place in the same time, the latter being judged of by the quantity 

 of matter excreted from the lungs, liver, kidneys, skin, &c., it will be found 

 to bear but a very small proportion to it; except during fetal life, when the 

 growth is very rapid, and a large proportion of the effete particles are com- 

 municated to the maternal blood, to be excreted from it. The real cause of 

 the increased demand for nutriment, during the early part of life, is rather 



