272 INSECTA. 



organs of a very singular character that we shall examine more mi- 

 nutely hereafter. Two other cords of variable length {Jig- 122, 

 g, g) are given off from the inferior aspect of the brain, and serve 

 to connect it with the anterior ganglion of the ventral chain (j%. 

 122, A), to which some writers have thought proper to give the 

 name of cerebellum, though upon what grounds it is difficult to 

 conjecture ; the mass last mentioned gives off various nerves to 

 supply the parts connected with the mandibles, maxilla, and other 

 organs of the mouth. 



The rest of the ventral chain of ganglia forms a continuous series 

 (fig. 121, 2, 3, 4, 5, 6, 7, 8) of nervous centres arranged in pairs, 

 and united to each other by double cords of communication, but they 

 vary much in number and relative magnitude in different families. 

 Those situated in the thorax are usually of the greatest proportion- 

 ate size, inasmuch as they furnish the nerves that supply the mus- 

 cles of the wings and legs ; the succeeding ganglia give branches to 

 the abdominal segments ; and the last, which is commonly of consi- 

 derable bulk, supplies the sexual organs and the extremity of the 

 colon. 



(313.) It is the general opinion of modern physiologists that the 

 intimate composition of the nervous apparatus described above is by 

 no means so simple as it appears to ordinary observation ; and, since 

 the experiments of Sir Charles Bell and Majendie demonstrated 

 the existence of distinct columns or tracts in the spinal axis of ver- 

 tebrate animals, various anatomists have endeavoured to show that 

 corresponding parts may be pointed out in the ventral chain of ar- 

 ticulated animals. There can, indeed, be no doubt that this por- 

 tion of the nervous system of an insect corresponds in every parti- 

 cular with the medulla spinalis ; and if, in the one case, the nerves 

 which preside over the general muscular movements arise from a 

 different column to that whence the nerves that correspond with 

 the periphery of the body originate, while those which regulate the 

 motions of respiration emanate from a distinct tract, we might rea- 

 sonably suppose a similar arrangement to exist in the structure of 

 the nervous system we are now examining. It has, in fact, been 

 well ascertained that the nerves given off to the muscular system of 

 the Homogangliata are not derived from the ganglionic masses them- 

 selves, but from the cords which connect them together, while the 

 nerves distributed to the integument and external parts of the body 

 communicate immediately with the ganglia. These different modes 

 of origin give presumptive evidence that at least two distinct tracts 



