174 ANATOMY AND PHYSIOLOGY 



that is, in its external and material properties, fur 

 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 cana 

 is excited to action, and, by the impulse of the same 

 organs, the sexual parts exercise the function appointee 

 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 formec 

 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. 



