INSECTA. 201 



chords {fig. 99.), which show as little trace of their median separation 

 as in the luli, swelling into two slight enlargements (^, g) opposite each 

 of the abdominal segments : two nerves are sent off from either side of 

 each enlargement, and the anterior of these four pairs of nerves 

 is directed at an acute angle forwards and outwards to the stigmata : 

 the remaining pairs supply the muscles of the segment and the legs, 

 and are of equal size. 



In the Centipede, a series of equal and equidistant 

 ganglia is developed upon the ventral surface of the two 

 abdominal chords. Only in the first and last of the ab- 

 dominal ganglions can any modification of size be de- 

 tected. The anterior, or sub-oesophageal ganglion, for 

 example, is larger than the rest, having to supply the 

 modified legs which perform the function of jaws and 

 under lip ; the chords, diverging as they escape on each 

 side of the oesophagus, enclose it by uniting with the 

 large bilobed ganglion, or brain above. The nerves 

 from this part supply the large antennae and the aggre- 

 gated ocelli. In the structure of the abdominal co- 

 lumns a tract less closely connected with the gangli- 

 onic nerves may be traced along their dorsal aspect. 

 This was first pointed out by Mr. Newport, who attri- 

 butes to it the motor function. A large, vascular 

 trunk, connected also with the dorsal aspect of the ner- 

 Poiydesmus. ^^"^ systcm, has been regarded as part of the nervous 

 system ; by some as a motor, by others as a respiratory 

 column : its true nature was detected by Mr. Lord.* With regard to 

 the ganglionic and non-ganglionic portions of the true nervous axis, 

 the same physiological reasonings will apply as have led to the 

 conclusions already given respecting their office in the crustaceous 

 animals. 



Of the four nerves which come off* from the sides of the ganglionic 

 portions of the columns, the second, which is principally distributed 

 to the muscles of the corresponding pair of legs, arises in a great pro- 

 portion from the ganglion itself. The first and third nerves, which 

 are smaller than the second, supply the muscles and integuments of 

 the segment. The fourth pair of nerves passes to the breathing 

 pore and to the integument. This, therefore, must be regarded as the 

 respiratory nerve. The stomato-gastric nerve is a distinct system 

 connected with the anterior ring or brain. 



Thus in the Myriapodous insects we find that although the prin- 



* Med. Gazette,. March 3d, 1838. 



