“We at present consider that by the passage of the continuation of the ganglion-cells into the punktsubstanz this continuation becomes lost in the fine threads, and on the other side of the punktsubstanz the similar fibrillar substance forms the origin of the axis-cylinders arranged parallel to one another; so it is quite certain that the single axis-cylinder derives its fibrillar substance as a mixture from the most diverse ganglion-cells.”
d. The visceral (sympathetic or stomatogastric) system
This system in insects is composed (1) of a series of three unpaired ganglia (Fig. 249, gv1, gv2, gv3), situated over the dorso-median line of the œsophagus, and connected by a median nervous cord or recurrent nerve (nr, vagus of Newport). The first of these ganglia is the frontal ganglion, which is connected with the œsophageal ganglia by a pair of roots (rvt), which have an origin primitively common with that of the labral nerves (Fig. 248, fg and lbr).
Fig. 255.—Anterior portion of the paired and unpaired visceral nervous system of Blatta orientalis seen from above. The outlines of the brain (g) and the roots of the antennal nerve (na), which cover a portion of the sympathetic nervous system, are given by dotted lines. Lettering as in Fig. 247. nsd, nerve to salivary gland. The nervus recurrens (nr) enters an unpaired stomach ganglion farther back.—After Hofer, from Lang.
2. Of two pairs of lateral ganglia (Fig. 255, ga, gp) situated two on each side of the œsophagus. They are connected both with the antennal lobes by a nerve (rvd), and to the chain of unpaired ganglia by a special connective. The first pair of these ganglia sends nerves to the heart and aorta; the second pair to the tracheæ of the head.
The unpaired median or recurrent nerve (nr) extends back from under the brain along the upper side of the œsophagus, and (in Blatta), behind the origin of the nerves to the salivary glands, enters an unpaired ganglion, called the stomachic ganglion (ganglion ventriculare), situated in front of the proventriculus. The number of these stomachic ganglia varies in different orders of insects.
In Blatta, Küpffer and also Hofer have shown (Fig. 255) (Müller, Brandt, ex Kolbe) that the nerve to each salivary gland arises from three different centres: the anterior end situated under the œsophagus is innervated by the paired visceral nerves from the hinder paired ganglia; the remaining part by nerves arising from each side of the recurrent nerve; and thirdly by a pair of nerves arising from the subœsophageal ganglion which accompanies the common salivary duct, and ends in branches which partly innervate the salivary glands and in part their muscles.
Hofer considers that the function of this complex system of paired and unpaired ganglia, with their nerves, is a double one, viz. serving both as a centre for the peristaltic action of the œsophagus, and as innervating the salivary glands.
Besides these a second portion of the visceral system arises from the thoracic and abdominal ventral cord. It may be seen in the simplest condition yet known in the nervous system of Machilis (Fig. 239 s). It consists of a fine, slender nerve, which extends along the surface of the ventral chain of ganglia, and sending off a pair of branches (accessory transverse nerves) in front of each ganglion. These accessory nerves receive nerve-twigs from the upper cord of the ventral chain, dilating near their origins into a minute elongated ganglion, and then passing partly outwards to the branches of the tracheæ and the muscles of the spiracles, uniting in the middle line of each segment of the body behind the head, i.e. of those segments containing a pair of ganglia.