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Ventral nerve cord


The Ventral Nerve-Cord

Although the suboesophageal ganglion, like the brain, consists of three fused ganglia, these have become less modified and are clearly discernible in sagittal sections (Fig. 27). The rule that each ganglion of the central nervous system innervates only the segment in which it originated in the embryo also holds good of the stlboesophageal ganglion. We find that it sends off three pairs of nerves, containing both motor and sensory fibers. The first pair (nin), which is stollter than the two others, innervates the sense organs and muscles of the mandibles, and the second ( mltx) and third (W) the corresponding parts of the maxillae and labium respectively. The three thoracic ganglia, owing to the voluminous and complicated leg and wing muscles which they innervate, are much larger than the abdominal ganglia. Each gives off a pair of crural nerves to the legs and the prothoracic ganglion also supplies a chordotonal organ near its antero-ventral end ( cho ) .

From the mesothoracic ganglion arises a pair of so-called alar nerves, which innervate the great longitudinal and transverse vibratory muscles of the wings. The musculature of the epinotum, petiole and postpetiole is supplied by the first to third abdominal ganglia, the two first of which are fused with the metathoracic ganglion. The fourth abdominal (first gastric in the Myrmicidae) remains in the segment to which it belongs, but lies at its extreme anterior edge. As both this and the succeeding gastric ganglia have been secondarily drawn forward, the pairs of nerves which they give off run obliquely backward, to their innerva­tions. Janet (1902) has found that each of the two nerves arising from each of the four anterior gastric ganglia divides into a dorsal and a ventral trunk.

The former sends off a sensory nerve to the corre­sponding dorsal quadrant of the segment and three motor nerves to its three muscles, the latter a sensory nerve to tha ventral quadrant and six motor nerves to as many muscles. The sensory nerves go to the sense-hairs of the integument.

The terminal (fifth gastric) ganglion, formed, as we have seen, by a fusion of the eighth to tenth abdominal ganglia, sends off four pairs of nerves, the first to the sense organs and muscles of the stylets of the sting, the second to the sense organs and muscles of the gorgeret, the third to the anal sphincter and papilla and the fourth to the walls of the hind gut.