92 THE COCKROACH : 



Median Nerve- Cord. 



Lyonnet,* Newport, f and LeydigJ have found in large 

 Insects a s} T stem 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 commis- 

 sures 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-cesophageal 

 connectives. 



Sto mato -gastric Net ves . 



In the Cockroach the stomato-gastric nerves found in so 

 many of the higher Invertebrates are conspicuous!} 7 developed. 

 From the front of each oesophageal connective, a nerve passes 

 forwards upon the oesophagus, outside the chitinous crura of the 

 tentorium. Each nerve sends a branch downwards to the 

 labrum, and the remaining fibres, collected into two bundles, 

 join above the oesophagus to form a triangular enlargement, the 



* Traite Anat., p. 201, pi. ix., fig. 1. 

 t Phil. Trans., 1834, p. 401, pi. xvi. 



I Vom Bau des Thierisclien Korpers, pp. 203, 262; Taf. z. vergl. Anat., pi. vi., 

 tig. 3. 



