THE BLOOD. 147 
disordered action resulting from gross or molecular injury 
(disease), gives a basis for certain conclusions as to the normal 
functions of the human body or those of lower animals. This 
information is especially valuable in the case of man, since he 
can report with a fair degree of reliability, in most diseased 
conditions, his own sensations. 
It is hoped that this brief treatment of the methods and 
logic of physiology will suffice for the present. Throughout 
the work they will be illustrated in every chapter, though not 
always with distinct references to the nature of the intellectual 
process followed. 
Summary.—There are two methods of physiological observa- 
tion, the direct and the indirect. The first is the simplest, and 
is valuable in proportion to the accuracy and, delicacy and 
range of the observer; the latter implies the use of apparatus, 
and is more complex, more extended, more delicate, and precise. 
It is usually employed with the graphic method, which has the 
advantage of recording and thus preserving movements which 
correspond with more or less exactness to the movements of 
tissues or organs. It is valuable, but liable to errors in record- 
ing and in interpretation. 
The logic of physiology is that of the inductive sciences, It 
proceeds from the special to the general. The conclusions of 
physiology never pass beyond extreme probability, which, in 
some cases, is practically equal to certainty. It is especially 
important not to make generalizations that are too wide. 
THE BLOOD. 
It is a matter of common observation that the loss of the 
whole, or a very large part, of the blood of the body entails 
death ; while an abundant hemorrhage, or blood-disease in any 
of its forms, causes great general weakness. 
The student of embryology is led to inquire as to the neces- 
sity for the very early appearance and the rapid development 
of the blood-vascular system so prominent in all vertebrates. 
An examination of the means of transit of the blood, as 
already intimated, reveals a complicated system of tubes dis- 
tributed to every organ and tissue of the body. These facts 
would lead one to suppose that the blood must have a tran- 
scendent importance in the economy, and such, upon the most 
minute investigation, proves to be the case. The blood has 
