210 ANIMAL PHYSIOLOGY, 
ance (load), as the contraction is also a function of the stimulus, 
The separation of a muscle from its nerve by section of the lat- 
ter leads to certain changes, most rapid in the nerve, which 
show that the two are so related that prolonged independent 
vitality of the muscle is impossible, and make it highly proba- 
ble that muscle is constantly receiving some beneficial stimulus 
from nerve, which is exalted and manifest when contraction 
takes place. 
The study of the development of the electrical cells of cer- 
tain fishes shows that they are greatly modified muscles in 
which contractility, etc., has been exchanged for a very decided 
exaltation of electrical properties. It is likely, though. not 
demonstrated, that all forms of protoplasm undergo electrical 
changes—that these; in fact, like chemical phenomena, are vital 
constants. 
The phases of the contraction of smooth muscular tissue are 
all of longer duration; the contraction-wave passes in different 
directions, and may spread into cells devoid of nerves, which 
we think not unlikely also to be the case, though less so, for all 
forms of muscle. 
The smooth muscle-cell must be regarded as a more primi- 
tive, less specialized, form of tissue. Variations in all the phe- 
nomena of muscle with the animal and the circumstances are 
clear and impressive. Finally, muscle illustrates an evolution 
of structure and function, and the law of rhythm. 
5 
THE NERVOUS SYSTEM.—GENERAL CONSIDERATIONS. 
Since in the higher vertebrates the nervous system is domi- 
nant, regulating apparently every process in the organism, it 
will be well before proceeding further to treat of some of its 
functions in a general way to a greater extent than we have yet 
done. 
Manifestly it must be highly important that an animal shall 
be able to place itself so in relation to its surroundings that it 
may adapt itself to them. Prominent among these adaptations 
are certain movements by which food is secured and dangers 
avoided. The movements having a central origin, a peripheral 
mechanism of some kind must exist so as to place the centers 
in connection with the outer world. Passing by the evolution 
of the nervous system for the present, it is found that in verte- 
brates generally there is externally a modification of the epi- 
