PHYSIOLOGICAL. RESEARCH AND REASONING. 149 



must depend on practice — that is to say, the education of the 

 senses. The hand may hecctme a most delicate instrument of 

 observation ; the eye may learn to see what it once could not ; 

 the ear to detect and discriminate what is quite beyond the 

 uncultured hearing of the many. But it is one of the most 

 conAdncing evidences of man's superiority that in every field of 

 observation he has risen above the lower animals, some of which 

 by their unaided senses naturally excel him. So in this science, 

 instruments have opened up mines of facts that must have other- 

 wise remained hidden ; they have, as it were, provided man 

 with additional senses, so much have the natural powers of 

 those he already possessed been sharpened. 



But the chief value of the results reached by instruments * 

 consists in the fact that the movements of the living tissues can 

 be registered; i. e., the great characteristic of modern physiol- 

 ogy is the extensive employment of the graphic method, which 

 has been most largely developed by the distinguished French 

 experimenter Marey. Usually the movements of the point of 

 lever are impressed on a smoked surface, either of glazed 

 paper or glass, and rendered permanent by a coating of some 

 material applied in solution and drying quickly, as shellac in 

 alcohol. The surface on which the tracing is written may be 

 stationary, though this is rarely the case, as the object is to get 

 a succession of records for comparison ; hence the most used 

 form of writing surface is a cylinder which may be raised or 

 lowered, and which is moved around regularly by some sort of 

 clock-work. It follows that the lever point, which is moved by 

 the physiological effect, describes curves of varying complexity. 

 That tracings of this or any other character should be of any 

 value for the purposes of physiology, they must be susceptible 

 of relative measurement both for time and space. This can be 

 accomplished only when there is a known base-line or abscissa 

 from which the lever begins its rise, and a time record which is 

 usually in seconds or portions of a second. The first is easily 

 obtained by simply allowing the lever to write a straight line 

 before the physiological effect proper is recorded. Time inter- 

 vals are usually indicated by the interruptions of an electric 

 current, or by the vibrations of a tuning-fork, a pen or writer 

 of some kind being in each instance attached to the apparatus 

 so as to record its movements. 



* Illustrated in the sections on muscle physiology and others. 



