Fig. 43.—Longitudinal horizontal section of Third Thoracic Ganglion. n, peripheral nerves. The other references as before. × 75.
Many familiar observations show that the ganglia of an Insect possess great physiological independence. The limbs of decapitated Insects, and even isolated segments, provided that they contain uninjured ganglia, exhibit unmistakable signs of life.
Median Nerve-Cord.
Lyonnet,[103] Newport,[104] and Leydig[105] have found in large Insects a system of median nerves, named respiratory (Newport) or sympathetic (Leydig). These nerves do not form a continuous cord extending throughout the body, but take fresh origin in each segment from the right and left longitudinal commissures alternately. The median nerve lies towards the dorsal side of the principal nerve-cord, crosses over the ganglion next behind, and receives a small branch from it. Close behind the ganglion it bifurcates, the branches passing outwards and blending with the peripheral nerves. Each branch, close to its origin, swells into a ganglionic enlargement. The median nerve and its branches differ in appearance and texture from ordinary peripheral nerves, being more transparent, delicate, and colourless. They are said to supply the occlusor muscles of the stigmata. In the Cockroach the median nerves are so slightly developed in the thorax and abdomen (if they actually exist) that they are hardly discoverable by ordinary dissection. We have found only obscure and doubtful traces of them, and these only in one part of the abdominal nerve-cord. The stomato-gastric nerves next to be described appear to constitute a peculiar modification of that median nerve-cord which springs from the circum-œsophageal connectives.
Stomato-gastric Nerves.
