INTlt OD UCTIO N. 2 1 



formation of the pseudopodia of an amoeba or white blood-corpuscle, or in 

 the vibratile movements of ciliary structures, or in the powerful contractions 

 of voluntary muscle, the underlying mechanism by which the shortening is 

 produced is essentially the same throughout. However general the property 

 may be, it cannot be considered as absolutely characteristic of living struc- 

 ture. As was mentioned before, Engelmann ' has been able to show that a dead 

 catgut string when surrounded by water of a certain temperature and exposed 

 to a sudden additional rise of temperature will contract or shorten in a man- 

 ner closely analogous to the contraction of ordinary muscular tissue, and it is 

 not at all impossible that the molecular processes involved in the shortening 

 of the catgut string and the muscle-fibre may be esseutiallv the same. 



That conductivity is also a fundamental property of primitive protoplasmic 

 structure seems to be assured by the reactions which the simple motile forms 

 of life exhibit when exposed to external stimulation. An irritation applied 

 to one point of a protoplasmic mass may produce a reaction involving other 

 parts, or indeed the whole extent of the organism. The phenomenon is most 

 clearly exhibited in the more specialized animals possessing a distinct nervous 

 system. In these forms a stimulus applied to one organ, as for instance light 

 acting upon the eve, may be followed by a reaction involving quite distant 

 organs, such as the muscles of the extremities, and we know that in these 

 cases the irritation has been conducted from one organ to the other by means 

 of the nervous tissues. But here also we have a property that is widely 

 exhibited in inanimate nature. The conduction of heat, electricity, and other 

 forms of energy is familiar to every one. While it is quite possible that con- 

 duction through the substance of living protoplasm is something mi generis, 

 and does not find a strict parallel in dead structures, yet it must be admitted 

 that it is conceivable that the molecular processes involved in nerve conduction 

 may be essentially the same as prevail in the conduction of heat through a 

 solid body, or in the conduction of a wave of pressure through a liquid mass. 

 At present we know nothing definite as to the exact nature of vital conduction, 

 and can therefore affirm nothing. 



The four great properties enumerated, namely, nutrition or assimilation 

 (including digestion, secretion, absorption, excretion, anabolism, and katabolism), 

 reproduction, conduction, and contractility, form the important features which 

 we may recognize in all living things and which we make use of in distin- 

 guishing between dead and living matter. A fifth property perhaps should 

 be added, that of sensibility or sensation, but concerning this property as a 

 general accompaniment of living structure our knowledge is extremely im- 

 perfect; something more as to the difficulties connected with this subject will 

 •be said presently. The four fundamental properties mentioned are all ex- 

 hibited in some degree in the simplest forms of life, sueli as the protozoa. In 

 the more highly organized animals, however, we find thai specialization of 

 function prevails. Hand in hand with the differentiation in form that is 



displayed in the structure of tin istituent tissues there goes a specialization 



1 Ueber dt'n Uraprung der Muskelkraft, Leipzig, 1893. 



