CONSIDERATIONS. 139 



Part VII. — Nervous System. 



Of all the invertebrated animals, the Articulata are those 

 which present the most developed nervous system. In them, 

 as in the Mollusca, this system of organs differs chiefly from 

 that of Vertebrata, in being placed, with the exception of the 

 first pair of ganglia, below the alimentary canal ; whilst in the 

 Vertebrata it is always above : and although some of the 

 lowest Vertebrata approach so near to the Annelida in other 

 respects, we find no approach in the form of the nervous 

 system. 



Some have supposed the nervous system in the Annu- 

 losa to be the analogue of the only great sympathetic nerves 

 of higher animals ; but this can hardly be the case ; for these 

 furnish nerves almost solely to the vital organs, whilst the 

 spinal marrow of the Annulosa furnishes nerves to nearly the 

 whole of the body. 



By comparing the nervous system to the other systems of 

 organs with which it is in relation, we arrive at the conclusion, 

 on which we can establish the following laws : — 



First Law. — When the body is composed of similar seg- 

 ments, the spinal marrow has as many ganglia as there are 

 sterna {sterna with the muscles which are repeated with them), 

 varying in size according to the mass of organs of animal life 

 which each segment contains, and the greater or less degree of 

 activity of these organs. 



The ganglia are commonly placed in the middle of the sternal 

 pieces, at the intersection of the axes of the coxce. 



The length of the chords of the spinal marrow, being deter- 

 mined by the distances of the ganglia, is here equal in all the 

 segments. 



The terminal part of the chords placed beyond the last 

 ganglia are distributed, after the manner of the principal 

 nerves, to the posterior part of the body. When the body is 

 composed, commonly of two, or, rarely of three parts, besides 

 the head, distinguished by the form of the segments of which 

 they are composed, one of these parts, which may be called 

 the trunk, and which is always the anterior, retains the prin- 

 cipal organs of animal life, as the feet ; whilst in the posterior, 

 those organs subject to the will are more or less reduced, 



