170 THE GROWTH OF SCIENCE IN THE NINETEENTH CENTURY. 



infiuei;ces which fashion the garuieiit of life. Whether the problems 

 of life are looked upon from the one point of view or the other, we 

 to-day, not biologists onl}^ but all of us, have gained a knowledj^e hid- 

 den even from the philosophers a hundred years ago. 



Of the pro])lems presented by the living body viewed as a machine, 

 some ma}^ be spoken of as mechanical, others as physical, and yet 

 others as chemical, while some are, apparently at least, none of these. 

 In the seventeenth century William Harvey, laying hold of the central 

 mechanism of the blood stream, opened up a path of inquiry which 

 his own ag(> and the century which followed trod with marked success. 

 The knowledge of the mechanics of the animal and of the plant 

 advanced apace, but the phj^sical and chemical problems had yet to 

 wait. The eighteenth century, it is true, had its physics and its chem- 

 istry; but, in relation at h^ast to the problems of the living being, a 

 chemistry which knew not oxygen and a ph3'^sics which knew not the 

 electricity of chemical action were of little avail. The philosopher of 

 171>1>. when he discussed the functions of the animal or of the plant 

 inv<»lving chemical clianges, was fain for the most part, as were his 

 predecessoi's in the centurv before, to have recourse to such vague 

 terms as " fermentatioir* and (lie like: to-day our treatises on physi- 

 ology are largely made up of ])rt'cise and exact expositions of the pla}^ 

 of physical agencies and chemicid bodies in the living organisms. He 

 made use of the words '* vital force '^ or ""vital principle'" not as an 

 occasional, but as a common explanation of the phenomena of the 

 living ])()dy. During the i)resent centurv. especially during its latter 

 half, the idea embodied in those words has been driven away from one 

 seat after another; if we use it now when we are dealing with the 

 chemical and physical events of life, we use it w ith reluctance, as a deus 

 ex macluua to be appealed to only when everything else has failed. 



Some of the problems — and those, perhaps, the chief problems — of 

 the living body have to be solved neither by physical nor chemical 

 methods, but by methods of their own. Such are the problems of 

 the nervous system. In respect to these the men of 1799 were on the 

 threshold of a pregnant discovery. During the latter part of the 

 present century, and especialh' during its last quarter, the analysis of 

 the mysterious processes in the nervous sj^stem, and especially in the 

 brain, which issue as feeling, thought, and the power to move, has 

 been pushed forward with a success conspicuous in its practical, and 

 full of promise in its theoretical, gains. That analysis may be briefly 

 described as a following up of threads. We now know that what 

 takes place along a tin3^ thread which we call a nerve fiber ditiers 

 from that which takes place along its fellow-threads, that di fleering 

 nervous impulses travel along dift'erent nervous fibers, and that nerv- 

 ous and psychical events are the outcome of the clashing of nervous 

 impulses as they sweep along the closely woven web of living threads 



