PHYSIOLOGICAL RESEARCH, PHYSIOLOGICAL REASONING. 143 
of some kind being in each instance attached to the apparatus 
so as to record its movements. 
As levers, in proportion to their length, exaggerate all the 
movements imparted to them, a constant process of correction 
must be carried on in the mind in reading the records of the 
graphic method, as in interpreting the field of view presented 
by the microscope. 
The student is especially 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. 207), 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 sufficient 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 modified 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 
