304 JoURNAL OF COMPARATIVE NEUROLOGY. 
thermal sensations, muscle sensations, etc., but enough has 
been cone to enable us to lay down the general plan or pattern 
of the periphera) nervous system asa whole and to define 
the main pathways by which stimuli of different modalities 
reach the brain and are reflected back to the responsive organs. 
Our anatomical knowledge of these pathways is sufficiently well 
controlled by precise physiological experimentation to enable us 
to state with confidence that each of the nine systems mentioned 
above is a real functional unit. 
The fibers composing these systems may reach the central 
nervous system through a series of many nerve roots arranged 
in a segmental way, like the general cutaneous nerves of the 
spinal cord, or they may all be represented in a single large 
nerve, like the optic and olfactory. Thus it happens that some 
nerves, like those last mentioned, are ‘‘pure’’ nerves, while 
others, like the facialis or vagus, are ‘‘mixed,’’ containing in 
some cases as many as four anatomically distinguishable com- 
ponents. 
It is a general rule that in the body the components tend 
to be distributed among a large number of nerves in a more or 
less segmental way, while in the head they tend to be concen- 
trated into a few pathways, or only one, into the brain, an adap- 
tation which presents obvious advantages for the simplification 
and unification of the secondary reflex paths from these pri- 
mary centers. 
Now, the central nervous system is, as we have already 
seen, primarily a mechanism to facilitate the reaction of the 
animal to impressions from without, in other words, to put the 
body in correspondence with the environment. Its structure is 
directly determined by the avenues of sense through which 
these stimuli come in and by the character of the responses to 
these stimuli which are necessary for the conservation of the 
organism. In view of the fact that we already possess a de- 
tailed knowledge of these peripheral nervous pathways, it is 
manifest that we have here a most favorable avenue of approach 
in an analysis of the inconceivable complexity of cerebral 
structure. 
