610 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



each other by sensitive columns. A tract has 

 also been recently described by Mr. Newport,* 

 passing along the dorsal surface of these co- 

 lumns, and giving off lateral nervous branches ; 

 this has been regarded as a molar tract. Mi- 

 nute anatomy has also unfolded to us what 

 may be considered as the analogue of the re- 

 spiratory system of nerves, and a par vagum. 



These points we will now notice in detail, 

 commencing with the Hemipterous Insecta, in 

 which the nervous system is least perfectly or- 

 ganized. In the perfect state of the Ranatra 

 Itneuris the nervous system consists (besides 

 the supra-cesophageal nervous accumulation) 

 of a small round ganglion (Jig. 345, a), situated 

 below the oesophagus at its very commence- 



Fig. 345. 



Ven'ral nervous cord of Ranatra linearis (perfect 

 state), magnified to about twice the natural size. 



n, small round sub-cesophageal ganglion, b, 

 large quadrilobate thoracic ganglion, c, c, fila- 

 ments passing down the lengthened abdomen. 



rnent; from this two longitudinal commissures 

 pass to join with a large quadrilobate ganglion 

 (/>), situated at the further extremity of the 

 thorax. From each side of this ganglion there 

 are given off three nervous threads, passing 

 superiorly, transversely, and inferiorly ; and 

 from the lower part of the ganglion, which is 

 slightly fusiform in shape, there are given off two 

 bundlesof most minute and delicate nervous fila- 

 ments (re), each containing five branches, which 

 pass downwards into the lengthened abdomen to 

 supply the parts situated in that region; there is 

 also a supra-cesophageal nervous accumulation. 

 In the Orthoptera the nervous system presents 

 a certain degree of concentration worthy of no- 

 tice. In the perfect state of a species of 



Acrydium there are two comparatively large 

 thoracic ganglia, very near each other, and 

 connected, as usual, by commissures; in the 

 abdomen there are five ganglia of much smaller 

 size, connected in a similar manner, and giving 

 off lateral filaments: the first and second of 

 these abdominal ganglia are some distance from 

 each other; the three last are much more 

 closely approximated, and are rather larger 

 and more distinct ; the cerebral ganglion is of 

 small size. 



Proceeding with the Coleoptera, we find that 

 many of the Lamellicornes, in their perfect 

 state, have a singular and rather unusual mode 

 of developement of their nervous system ; the 

 ganglia are but few in number, closely approxi- 

 mated, and the two posterior ones give off nume- 

 rous radiating filaments : this is the case with 

 the larva of the Oryctes nasicornis, according 

 to a dissection by Swammerdam.* In the 

 Geotrupes stercorarius (fig. 346), perfect 



Fig. 346. 



Ventral nervous cord of Geotrupes stercorarius (per- 

 fect state), magnified to about three times the 

 natural sise. 



a, bilobate thoracic ganglia, b, abdominal gan- 

 glia, c, c, filaments to the intestine, d, intestine. 



state, another of the Lamellicornes, there are 

 found in the thorax two distinctly bilobate 

 ganglia (a), connected by longitudinal com- 

 missures, and one smaller ganglion at the junc- 

 tion of the thorax with the abdomen, immedi- 

 ately contiguous with which are one large 

 and three smaller ganglia, very closely approxi- 

 mated to each other (b); the last was the 

 longest, of rather a fusiform shape, and gave 

 off radiating nervous filaments, particularly two 

 long branches to the abdominal viscera and 

 adjacent parts (c, c). The more usual form, 

 however, of the nervous system, is such as 

 will be described in the subsequent species. 



Philosophical Transactions for 1832 and 1834. 



Biblia Nature. 



