Page:Jardine Naturalist's library Entomology.djvu/180

174 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