1^4 ANATOMY AND PHYSIOLOGY 
that is, in its external and material properties, for 
the manner in which it obeys the commands of the 
governing principle is just as subtle and mysterious in 
this as in any other branch of the animal kingdom. As 
the seat of sensation, it is the originating and animating 
cause of all activity and motion in the various organs. 
To it the senses convey their intimations of the 
different properties of external things, of which they 
are respectively adapted to take cognizance. From 
it the muscles derive the irritability which puts them 
in action ; by means of the nerves the intestinal canal 
is excited to action, and, by the impulse of the same 
organs, the sexual parts exercise the function appointed 
to them.* 
The nervous cords, on which such important duties 
are devolved, are composed, like those of mammals, 
of exceedingly minute globules, disposed in linear 
series, so as to form fibres of extreme tenuity. This 
matter constitutes the central nervous mass, pulp, or 
medullary matter, and it is contained within a darker- 
coloured cortical layer^ exterior to which there is an 
envelope of a fibrous nature, rather thick, and formed 
of two tunics, analogous to the dura-mater and the 
pia-mater. 
In regard to general form, the nervous system is 
disposed in a double cord running along the whole 
length of the body, which forms knots or ganglions 
at certain intervals, corresponding, in many cases, to 
the number of segments. These ganglia send forth 
Burmeister’s Manual, p. 474. 
