18 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a much greater power of assimilation. Owing to this latter property the 

 plant-cell is able, with the aid of solar energy, to construct its protoplasm 

 from very simple forms of inorganic matter, such as water, carbon dioxide, 

 and inorganic salts. In this way energy is stored within the vegetable cell in 

 the form of complex organic compounds. Animal protoplasm, on the con- 

 trary, has comparatively feeble synthetic properties ; it is characterized chiefly 

 by its destructive power. In the long run, animals obtain their food from the 

 plant kingdom, and the animal cell is able to dissociate or oxidize the complex 

 material of vegetable protoplasm and thus liberate the potential energy con- 

 tained therein, the energy taking the form mainly of heat and muscular work. 

 We must suppose that there is a general resemblance in the ultimate structure 

 of animal and vegetable living matter to which the fundamental similarity in 

 properties is due, but at the same time there must be also some common dif- 

 ference in internal structure between the two, and it is fair to assume that 

 the divergent properties exhibited by the two great groups of living things 

 are a direct outcome of this structural dissimilarity; to make use of a figure 

 of speech employed by Bichat, plants and animals are cast in different moulds. 



It is difficult if not impossible to settle upon any one property which 

 absolutely shall distinguish living from dead matter. Nutrition, that is, the 

 power of converting dead food material into living substance, and repro- 

 duction, that is, the power of each organism to perpetuate its kind by the 

 formation of new individuals, are probably the most fundamental charac- 

 teristics of living things; but in some of the specialized tissues of higher 

 animals the power of reproduction, so far as this means mere multiplication 

 by cell-division, seems to be lost, as, for example, in the case of the nerve-cells 

 in the central nervous system or of the ovum itself before it is fertilized by 

 the spermatozoon. Nevertheless these cellular units are indisputably living 

 matter, and continue to exhibit the power of nutrition as well as other prop- 

 erties characteristic of the living state. It is possible also that the power 

 of nutrition may, under certain conditions, be held in abeyance temporarily at 

 least, although it is certain that a permanent lojss of this property is incom- 

 patible with the retention of the living condition. 



It is frequently said that the most general property of living matter is its 

 irritability. The precise meaning of the term vital irritability is hard to 

 define. The word implies the capability of reacting to a stimulus and usually 

 also the assumption that in the reaction some of the inner potential energy of 

 the living material is liberated, so that the energy of the response is many 

 times greater, it may be, than the energy of the stimulus. This last idea is 

 illustrated by the case of a contracting muscle, in which the stimulus acts as a 

 liberating force causing chemical decompositions of the substance of the muscle 

 with the liberation of a comparatively large amount of energy, chiefly in the 

 form of heat or of heat and work. It may be remarked in passing, however, 

 that we are not justified at present in assuming that a similar liberation of 

 stored energy takes place in all irritable tissues. In the case of nerve-fibres, 

 for instance, we have a typically irritable tissue which responds readily to 



