260 FUNCTIONS OF THE NERVOUS SYSTEM. 



sented to it, but not in immediate contact with it. But reflex movements 

 alone existed in the posterior division, in which the legs were very slowly 

 moved, even when the animal was not progressing. Brisk actions were now 

 more easily excited in them than at first, either by contact with the segments, 

 by irritation of one or two of the legs themselves, or by a sudden current of 

 air. By these means, when the animal was lying still, actions were imme- 

 diately excited in all ihe legs of the posterior parts of the body, anterior and 

 posterior to those which were irritated ; and these actions were induced in 

 those of both sides of the body, but appeared to commence on the opposite 

 side, in the legs corresponding to those which were first irritated. In eighteen 

 hours, the anterior part of the body was quite dead, so that no motions what- 

 ever could be excited in it, either voluntary or reflex; but reflex actions were 

 then readily excited in the posterior, and also slightly so by mechanical irrita- 

 tion, even at twenty-four hours." It would appear, then, that we may obtain 

 more decided proof, in the Articulated series, of the real character of reflex 

 actions, and of their dependence upon a distinct system of nerves, than we 

 can draw from any other class of animals. In the Vertebrata, it is easy to 

 distinguish the sensory from the motor the afferent from the efferent 

 fibres; but the distinctness of the excito-motor system from the sensori-voli- 

 tional, 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 ihe fibres of rein- 

 forcement, or cross it as conVmissural 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. 



330. Hitherto we have spoken only of that division of the nervous system 

 of the Articulata, which may be regarded as corresponding with the sensory 

 and locomotive ganglia of the Mollusca ; we have next to inquire what we 

 find corresponding with the branchial ganglion. It is to be recollected, that the 

 respiratory apparatus of Insects is diffused throughout the whole body, so that its 

 presiding system of nerves must be proportionally extended; and we are, there- 

 fore, prepared to find the branchial ganglion of the Mollusca repeated, like 

 the pedal, in each segment. Besides the nervous trunks proceeding from the 

 ventral cord at its ganglionic enlargement, we find, in most of the Articulated 

 classes, a series of smaller nerves, given off at intermediate points, without 

 any apparent swelling at the points of divergence. The connections of these 

 are most distinctly traced in the thoracic region, just as the Larva is passing 

 into the Pupa state ; for the cords of the ventral column then diverge, so that 

 an additional tract may be seen which occupies the central line. By a close 

 scrutiny, this tract may be found in the perfect Insect, on the superior or vis- 

 ceral aspect of the cord ; and its nerves are given off from minute ganglionic 

 enlargements upon it. It seems to be quite unconnected, along its whole 

 course, with the column upon which it lies. Its nerves, however, communi- 

 cate with those of the sensori-motor system ; but they have a separate distri- 

 bution, being transmitted especially to the tracheae, on the parietes of which 

 they ramify minutely, and also to the muscles concerned in the respiratory 

 movements. (The latter, however, being a part of the general locomotive 

 apparatus, are also supplied from the principal ganglionic column.) These 

 nerves, then, which are evidently analogous to those of the gills and siphonic 



