150 COMPARATIVE PHYSIOLOGY. 



As levers, in proportion to their length, exaggerate all the 

 movements imparted to them, a constant process of correction 

 must he carried on in the mind in reading the records of the 

 graphic method, as in interpreting the field of view presented 

 hy the microscope. 



The student is epecially warned to carry on this process, 

 otherwise highly distorted views of the reality will become 

 fixed in his own mind ; and certainly a condition of ignorance 

 is to be preferred to such false knowledge as this may become. 

 But it is likewise apparent that movements that would without 

 such mechanism be quite unrecognized may be rendered visible 

 and utilized for inference. There is another source of possible 

 misconception in the use of the graphic method. The lever is 

 sometimes used to record the movements of a column of fluid 

 (manometer, Fig. 197), as water or mercury, the inertia of which 

 is considerable, so that the record is not that of the lever as 

 affected by the physiological (tissue) movement, but that move- 

 ment conveyed through a fluid of the kind indicated. Again, 

 all points, however delicate, write with some friction, and the 

 question always arises, In how far is that friction sufiicient to 

 be a source of inaccuracy in the record ? When organs are di- 

 rectly connected with levers or apparatus in mechanical rela- 

 tion with them, one must be sure that the natural action of the 

 organ under investigation is in no way modifled by this con- 

 nection. 



From these remarks it will be obvious that in the graphic 

 method physiologists possess a means of investigation at once 

 valuable and liable to mislead. Already electricity has been 

 extensively used in the researches of physiologists, and it is to 

 this and the employment of photography that we look in the 

 near future for methods that are less open to the objections we 

 have noticed. 



However important the methods of physiology, the results 

 are vastly more so. We next notice, then, the progress from 

 methods and observations to inferences, which we shall en- 

 deavor to make clear by certain cases of a hypothetical charac- 

 ter. Proceeding from the brain and entering the substance of 

 the heart, there is in vertebrates a nerve known as the vagus. 

 Suppose that, on stimulating this nerve by electricity in a rab- 

 bit, the heart ceases to beat, what is the legitimate inference ? 

 Apparently that the effect has been due to the action of the 

 nerve on the heart, an action excited by the use of electricity. 



