498 ANIMAL PHYSIOLOGY. 
paths that have been shown by pathological and clinical inves- 
tigation to-be best marked out in the spinal cord are those for 
voluntary motor impulses. So far as the functions of the hu- 
man organ are concerned, clinical and pathological facts have 
thrown the greatest amount of direct light on the subject; but 
the inferences thus drawn have been modified and supple- 
mented by the results of experiments on certain other mam- 
mals. 
It is especially important to bear in mind that, while certain 
conducting paths are usual, they are not invariable; in like 
manner, reflex impulses may not be confined to usual groups of 
cells, but may extend widely, and so bring into action a large 
number of muscles. The resulting reflex in any case is depend- 
ent on the character, intensity, and location of the stimulus, 
and especially on the condition of the central cells involved. 
In the whole functional life of the cord the influence of higher 
centers in the organ itself and especially in the brain is to be 
considered. The cord is rather a group of organs than a single 
one. 
THE BRAIN. 
The methods of investigating the functions of the brain are 
analogous to those employed for the cord, and may be classed 
as physiological proper and pathological, though, as a matter 
of fact, neither one nor the other is now considered as reliable 
when taken alone. With the pathological is generally in- 
cluded the clinical method; and the conclusions thus derived, 
are corrected and supplemented by the results obtained by sec- 
tion, removal, or other operative procedure affecting parts of 
the brain. The difficulties are still greater than in the case of 
the cord, on account of the extreme complexity of the organ, 
especially in the higher mammals and man. 
At the outset we may remark that the whole subject will 
be studied more profitably if it be borne in mind that—1. The 
brain is rather a collection of organs, bound together by the 
closest anatomical and physiological ties than a single one; in 
consequence of which it is quite impossible to understand the 
normal function of one part without constantly bearing in 
mind this relationship. This aspect of the subject has not re- 
ceived the attention it deserves. No one regards the aliment- 
ary tract as a single organ; but it is likely that the dependence 
functionally of: one part of the digestive canal upon another 
