98 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



the organism, as well as in the aggregate mass. And in all the vital, 

 as well as in the chemical actions, to which these structures are subser- 

 vient, the presence of liquid is essential. All nutrient materials must 

 be reduced to the liquid form, before they can be assimilated by the 

 solids ; and, again, the solid matters which are destined to be carried 

 off by excretion, must be again reduced to the liquid state, before they 

 can be thus withdrawn from the body. The tissues in which the most 

 active changes of a purely vital character are performed, namely, the 

 Nervous and Muscular, naturally contain a very large proportion of 

 water ; the former as much as 80 and the latter 77 per cent. On the 

 other hand, in tissues whose function is of a purely mechanical nature, 

 such as Bone, the amount of liquid is as small as is consistent with the 

 maintenance of a certain amount of nutrient action in its interior. By 

 the long- continued application of dry heat to a dead body, its weight 

 was found to be reduced from 120 pounds to no more than 12 ; so that, 

 taking the average of the whole, the amount of water, not chemically 

 combined, but simply interstitial, might be reckoned at as much as 90 

 per cent. It is certain, however, that much decomposition and loss of 

 solid matter must have taken place in this procedure ; and we shall pro- 

 bably estimate the proportion, more accurately, if we regard the weight 

 of the fluids of the human body as exceeding that of the solids by six 

 or seven times. 



150. There is a great variation in this respect, however, among dif- 

 ferent tribes of living beings. There are probably no highly organized 

 Animals, whose texture contains less liquid than that of Vertebrata 

 (unless, it may be, certain Beetles) ; but there can be no question that, 

 among some of the Zoophytes, the proportion of solids to liquids is just 

 the other way. In those massive coral-forming animals, which seem to 

 have been expressly created for the purpose of uprearing islands and 

 even continents from the depth of the ocean, we find the soft tissues 

 confined to the surface, and all within of a rocky hardness. It is not, 

 however, correct to say (as is commonly done), that the coral-polypes 

 "build up" these stony structures as habitations for themselves; for the 

 stony matter is deposited, by an act of nutrition, in the living tissue of 

 these animals, just as much as it is in the bones of Man. But the parts 

 once consolidated henceforth remains dead, so far as the animal is con- 

 cerned ; they are not connected with the living tissues by any vessels, 

 nerves, &c., their density prevents them from undergoing any but a very 

 slow disintegrating change, so that they require and receive no nutrient 

 materials ; and they might be altogether removed, by accident or decay, 

 without any direct injury to the still-active, because yet unconsolidated, 

 portions of the polype structure. 



151. There is a close correspondence, in this respect, between the 

 condition of the stony or horny stem of a Coral, and the heart-wood of 

 the trunk of a Tree ; for the latter, becoming consolidated by internal 

 deposit, for the purpose of affording mechanical support, is thenceforth 

 totally unconnected with the vegetative operations of the tree, and might 

 be removed (as it frequently is by natural decay) without affecting them. 

 In all the parts, in which the nutrient processes are actively going on, 

 do we observe that the tissue contains a large proportion of water ; and 



