igo LECTURE XII. 



the investigation of the internal structure of the organs, and, on the other hand, with 

 the exposition of those external influences by means of which this internal struc- 

 ture is impelled to the movements and performance of work specifically peculiar 

 to it. 



As an engineer who wishes to criticise a steam-engine must know exactly, on 

 the one hand its structure, and, on the other, the properties of heat and of steam ; 

 so it is the aim of the physiologist to explain the phenomena of life from the internal 

 structure of the organs, and the nature of the external influences acting upon them. 

 Only he meets, of course, with far greater difficulties than the engineer ; since the 

 organism is an incomparably more complicated machine than any, however perfect, 

 constructed by man. In the first place, the structure of the latter is relatively easy 

 to comprehend ; and it is set in motion by the well-understood activity of heat and 

 steam. On the other hand, our knowledge of the actual internal structure of the 

 organs of a plant is still in the highest degree incomplete; since in these matters 

 it concerns the coarser visible portions far less than the molecular structure and 

 atomic composition. This, since it depends upon invisible parts, can only be known 

 by indirect methods, and by means of conclusions slowly obtained. Moreover, 

 it is shown that the structure of the organism is not simply sensitive to one kind 

 of external influence, as is that of a steam-engine ; but that all known natural forces 

 may affect the living machine. Plants do not re-act only to the motion which is 

 communicated to them as heat, but also to the movements of the aether which 

 our eye perceives as light. They are, at the same time, sensitive to electrical changes, 

 and they re-act in a manner not yet understood to the influence of gravitation, and 

 even slight contact may produce very great effects. But above all — a matter which 

 does not come into consideration at all in a machine constructed by man — the 

 organism nourishes itself — i. e. it absorbs material into itself from without, and itself 

 joins this to the substance of the organs already present. Here, also, it again de- 

 pends upon the substance already existing, and its structure, that the nutrition in 

 a given organism takes place in a certain manner, and not otherwise. When 

 a seed or a bud becomes detached from a plant, in order to begin an in- 

 dependent life, the course of the latter is already traced out beforehand. The nature 

 and combination of the smallest particles in the seed are specifically determined by 

 the nature of the mother-plant — they are established beforehand. The question is 

 only whether the germ will develope ; whether it will, as a matter of fact, perform all 

 that which it can perform according to the laws prescribed for it by the mother-plant; 

 and this 'whether' depends upon the external influences of heat, light, gravitation, 

 electricity, nutrition and respiration, the contact of solid and fluid bodies, and so 

 forth. 



The difficult part of physiological investigation always lies in the explanation 

 of the structural relations of the organs, by which the latter, in consequence of the 

 influence of external forces, are put in motion in the way specifically determined ; 

 and, unfortunately, experience shows that the coarser visible structural relations only 

 play a more subordinate part in the matter, and that physiological investigation is 

 compelled to go back to the invisible, smallest particles of matter, the mutual 

 relations of which, again, can only be inferred from the entire working of all the 

 visible organs. We may therefore reserve until we enter upon the consideration of 



