FUNCTIONS OF THE NERVOUS SYSTEM. 



from the efferent fibres ; but the distinctness of the excito-motor system from 

 the sensori-volitional, is not so clearly made out. Here, however, the afferent 

 and efferent fibres cannot be readily distinguished ; but it is obvious that the 

 reflex actions, which manifest themselves when the communication with the 

 cephalic ganglia is cut off, are to be attributed to those fibres which enter the 

 cord under the afferent character, pass into the edge of the ganglion as the 

 fibres of reinforce?nent, or cross it as commissural fibres, and then emerge 

 again as efferent fibres, either in the nerves of the same segment, or in those 

 of another more or less distant. By traversing the cord along a part of its 

 length, and thus placing the several segments in communication with each 

 other, the fibres of reinforcement thus constitute a part of the longitudinal 

 filaments of the cord, the remainder consisting of the fibres continuous with 

 the cephalic ganglia. 



147. Without describing -in minute detail the forms which the nervous 

 system presents in the higher classes of Articulata, or tracing that interesting 

 series of changes which it undergoes during the metamorphosis of Insects, a 

 few particulars may be stated on these subjects as having an immediate bear- 

 ing on our present object. The nervous system of the Larva, like that of the 

 Annelida, or Myriapoda, presents an obvious relation to the means and extent 

 of locomotion possessed by the animal. Each segment is equally concerned 

 in locomotion; and with each is associated a pedal ganglion. None of the 

 movements of the animal are very energetic; simple and slow progression is 

 all for which its structure is adapted ; and the uniformity in the actions of its 

 legs would render it easy to combine them at the will of the animal, even 

 though their respective centres remain much isolated from one another. But, 

 in the perfect Insect, the whole locomotive apparatus is concentrated in the 

 thorax. The six legs (which are now all that remain), and the single or 

 double pairs of wings, are all developed from its three segments; and a much 

 greater variety of action is required, as well as more complete consentaneous- 

 ness, on account of the increased number and velocity of the movements of the 

 animal. We accordingly find that the ganglionic matter of the ventral cord of 

 perfect Insects, is more or legs concentrated in the thoracic region ; whilst the 

 ganglia of the abdomen are usually few and small ; the nerves to its segments, 

 however, being given off as before at regular intervals. In some of the Cole- 

 optera and Hemiptera, the concentration of the thoracic ganglia takes place to 

 such an extent, that they seem to form but one mass ; and this is the case also 

 among some of the Crustacea, the different forms of whose nervous system are 

 exactly parallel to those of their congeners among the inhabitants of the air 

 and land. The nerves which supply the wings of Insects are found, in all 

 stages of the development of these organs, to have a double origin. One root 

 arises from the fibrous tract alone ; whilst the other takes its origin from both 

 tracts at the point of enlargement. When the ganglionic centres which sup- 

 ply the anterior and posterior pairs of wings remain distinct, there is a curious 

 plexiform arrangement of their nerves ; more or less intricate, according as 

 the wings are destined to act with greater or less consentaneous energy ; and 

 absent when the anterior pair serve only as elytra, and do not assist in flight.* 

 This would remind us of the circular filament, which connects the nerves of 

 the arms in the naked Cephalopoda. Besides these nerves, the wings are 

 supplied from the respiratory system next to be described, from which scarcely 

 any branches go to the legs. This will be readily understood, when it is con- 

 sidered that the wings are developed, as it were, out of an extension of the 

 respiratory apparatus,! and that their movements are closely connected with 

 its actions. 



* Newport, in Phil. Trans., 1834, p. 394, 5. 



j- See Principles of General and Comparative Physiology, 2J ed. 465. 



