282 ANATOMY. 



and partly to the coxae. Between the branches of this process, or 

 when it is distinctly furcate between the fork, the nervous cords pass, 

 proceeding over the connecting membrane of the pro- and mesothorax, 

 running closely to it, and thus proceed into the mesothorax, again form- 

 ing the second ganglion in front of the internal process of its sternum. If 

 the branches of the first sternal process be united in an arch the nervous 

 cord runs beneath this arch, and above, the muscles affix themselves to 

 the process of the arch (Locusta viridissima, Termes fatalis, Calli- 

 chroma moschaium). The branches of the second sternal process are 

 not in general closed, the ganglion and cord consequently lie here 

 freely, which is the case also in the third process. This, however, is 

 higher than the preceding, often as it were pediculated, so that the 

 ventral cord must raise itself that it may pass over this process into the 

 abdomen. In front of this elevation the third ganglion then lies, imme- 

 diately upon the surface of the sternum : it is the largest, and sends 

 off the thickest nerves, and the second ganglion lies nearer to it than it 

 does to the first, and thus, even in the nervous system, the more 

 intimate connexion of the two posterior thoracic segments is clearly 

 shown. 



The nerves which originate from this ganglion vary in number ; the 

 first thoracic ganglion sometimes sends off two and sometimes three 

 branches on each side. In the first case the first branch runs to the 

 legs, the second to the muscles in the prothorax ; in the second case 

 both the first and third on each side are nerves of muscles, whereas the 

 central one is the leg-nerve. Three branches are also found on each 

 side of the second ganglion, the central one of which is a nerve of a 

 leg, and the first and third pass on to muscles. It is probable that 

 the anterior one gives off fine nerves for those contained within the 

 hollow cavities of the ribs of the wings. The third thoracic ganglion 

 also sends off three branches, which distribute themselves in a like 

 manner. Of these the central or leg nerve is always the thickest, and 

 most deeply seated, in as far as the direct muscles of the thorax, or 

 the connecting muscles of the thoracic processes, pass over it; the 

 others, on the contrary, raise themselves over these muscles. 



The number of the abdominal ganglia varies considerably in the 

 different groups. Insects with an imperfect metamorphosis, as the 

 Locustce, Termites, and Libellula:, exhibit as many ganglia as segments, 

 viz., from seven to eight, the two last of which, however, are so closely 

 contiguous that they form one ganglion of a figure of eight. In the 



