OF THE NERVOUS SYSTEM. 377 



the alimentary canal. Each pair of ganglia is connected with the 

 integument and muscles of its own articulation by sensitive and 

 motor filaments ; and with those which precede and follow by a 

 double cord of longitudinal commissural fibres. In the first articu- 

 lation, moreover, or the head, the ganglia are larger than elsewhere, 

 and send nerves to the antenna and to the organs of special sense. 

 This pair is termed the cerebral ganglion, or the " brain." 



A reflex action may take place, in these animals, through either 

 one or all of the ganglia composing the nervous chain. An im- 

 pression received by the integument of any part of the body may 

 be transmitted inward to its own ganglion and thence reflected 

 immediately outward, so as to produce a movement of the limbs 

 belonging to that articulation alone ; or it may be propagated, 

 through the longitudinal commissures, forward or backward, and 

 produce simultaneous movements in several neighboring articula- 

 tions ; or, finally, it may be propagated quite up to the anterior pair 

 of ganglia or " brain," where its reception will be accompanied with 

 consciousness, and a voluntary movement reflected back upon any 

 or all of the limbs at once. The organs of special sense, also, com- 

 municate directly with the cerebral ganglia ; and impressions con- 

 veyed through them may accordingly give rise to movements in 

 any distant part of the body. In these animals the ventral ganglia, 

 or those which simply stand as a medium of communication be- 

 tween the integument and the muscles, are nearly similar through- 

 out ; while the first pair, or those which receive the nerves of special 

 sense, and which exercise a general controlling power over the rest 

 of the nervous system, are distinguished from the remainder by a 

 well-marked preponderance in size. 



In the centipede it will be noticed that nearly all the organs and 

 functions are distributed in an equal degree throughout the whole 

 length of the body. The organs of special sense alone, with those 

 of mastication and the functions of perception and volition, are 

 confined to the head. The ganglia occupying this part are there- 

 fore the only ones which are distinguished by any external pecu- 

 liarities; the remainder being nearly uniform both in size and 

 activity. In some kinds of articulated animals, however, particular 

 functions are concentrated, to a greater or less extent, in particular 

 parts of the body; and the nervous ganglia which preside over 

 them are modified in a corresponding manner. In the insects, 

 for example, the body is divided into three distinct sections, viz : 

 the head, containing the organs of prehension, mastication, tact 



