408 VEGETABLE PHYSIOLOGY 
If, then, we admit that there is even a rudimentary 
nervous system in plants, we may proceed with an inquiry 
into the degree of its differentiation, and the completeness 
of the parallelism which it may be expected to show with 
the corresponding system in the animal kingdom. 
The latter, in the most completely organised beings, can 
be shown to possess certain distinct parts: one by means 
of which external stimulation ig received and appreciated ; 
another whereby movements, &c., are caused; and a third 
which is a regulating and controlling part, and which can 
co-ordinate the responses to stimulation, or can initiate 
movements, &c., in its absence. There are also definite 
paths or channels by which the three are brought into 
connection with each other, generally by impulses passing 
along such paths in definite directions. In the higher 
animals these are well differentiated from each other; we 
have the sense-organs, each devoted to and fitted for the 
appreciation of particular stimuli. We have various motor 
mechanisms, usually consisting of muscles or glands which 
are thrown into activity in consequence of the reception of 
impulses by sense-organs. It may appear to be straining 
matters somewhat to class these as part of the nervous 
system, but it does not appear wrong to do so in the sense 
that they are the means by which alone the working of the 
more particularly nervous elements of that system can be 
detected. The nervous and motor systems are indeed so 
closely connected that, for the purposes of this discussion, 
no inconvenience will result from classing them together. 
In the animal we have nerve-cells occurring singly or in 
groups, forming very large aggregatioas such as the brain, 
or smaller ones, the nerve ganglia. All such aggregations, 
or even single cells, are concerned in the task of co-ordinating 
stimuli and responses, or regulating the general life of the 
organism. Lastly, we have well-differentiated nerves which 
serve as the means of communicating between the three 
other factors already mentioned. Each nerve-fibre ends in 
one direction in a sense-organ or a motor mechanism, such 
