484 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



special sense ; and they evidently possess a power of directing and con- 

 trolling the movements of the entire body ; whilst the power of each 

 ganglion of the trunk is confined to its own segment. The longitudinal 

 ganglionic cord of Articulata occupies a position which seems at first 

 sight altogether different from that of the nervous system of Vertebrated 

 animals ; being found in the neighbourhood of the ventral or inferior 

 surface of their bodies; instead of lying just beneath their dorsal or 

 upper surface. There is reason, however, for regarding the whole of 

 the body of these animals as having an inverted position ; so that they 

 may be considered as really crawling upon their backs. On this view, 

 their longitudinal nervous tract corresponds with the spinal cord of 

 Vertebraf a in position, as we shall find that it does in function. 



856. We shall draw our chief illustrations of the structure of the 

 nervous system in the Articulated series, from the class of Insects ; in 

 which it has been particularly examined. In these animals, the number 

 of segments never exceeds twelve (exclusive of the head), either in their 

 larva, pupa, or imago states ; and the total number of pairs of ganglia, 

 therefore, never exceeds thirteen, including the cephalic ganglia. These, 

 in the larva, are nearly equal in size, one to another (Plate II., Fig. 2, 

 0, and 1-12) ; ihe functions of the different segments of the body being 

 almost uniform ; and the development of the organs of special sense not 

 being such, as to involve any considerable predominance in the size of 

 the cephalic ganglia. We observe, at the anterior extremity, the pair 

 of cephalic ganglia (a) ; from which proceeds, on each side, a cord of 

 communication to the first ganglion (1) of the trunk. This double cord, 

 with the ganglia above and below, thus forms a ring, which embraces 

 the oesophagus ; the cephalic ganglia being situated on the upper side 

 of it, whilst the ganglionic column of the trunk lies beneath the alimen- 

 tary canal along its whole length. In the Sphinx ligustri^ or Privet 

 Hawk-moth, the nervous system of whose larva is here represented, the 

 last two segments of the body are drawn together, as it were, into one ; 

 and instead of distinct llth and 12th ganglia, we find but a single mass, 

 nearly double the size of the rest, and obviously formed of the elements 

 that would have otherwise gone to form the two. 



857. When the structure of the chain of ganglia is more particularly 

 inquired into, it is found to consist of two distinct tracts ; one of which 

 is composed of nervous fibres only, and passes backwards from the 

 cephalic ganglia, over the surface of all the ganglia of the trunk ; 

 whilst the other includes the ganglia themselves. Hence every part of 

 the body has two sets of nervous connexions ; a direct one with the 

 ganglion of its own segment, and an indirect with the cephalic ganglia. 

 Impressions made upon the afferent fibres, which proceed from any part 

 of the body to the cephalic ganglia, become sensations when conveyed 

 to the latter ; whilst, in respondence to these, the influence of sensa- 

 tions received by the cephalic ganglia, and operating through them, 

 harmonizes and directs the general movements of the body, by means 

 of the communicating cords proceeding from them. For the reflex 

 operations, on the other hand, the ganglia of the ventral cord are suffi- 

 cient ; each one ministering to the actions of its own segment, and, to 

 a certain extent also, to those of other segments. It has been ascer- 



