THE NERVOUS SYSTEM. 13 



Section IV. The Nervous System. 



The nervous system, or rather, that part which is analogous 

 to the cerebro-spinal system of vertebrates, is next to the in- 

 tegumentary in point of importance as far as regards type. 

 Insects belong to that division of the animal kingdom which 

 Professor Owen terms Homogangliata, that is, animals in which 

 the nervous system is represented by a pair of nervous cords 

 which traverse the ventral portion of the body, and lie next to the 

 integument, uniting and forming a ganglion for each segment, 

 from which the proper nerves of the segment are given off. 

 No insect that has yet been examined departs from this type 

 so much as the fly and its allies ; for, even in the larva state, 

 the whole of the nervous system is collected in the anterior 

 segments, and the pair of ventral cords do not exist ; whilst, in 

 the imago, with the exception of two small ganglia in the pro- 

 boscis, there are but two nerve centres, one situated in the head, 

 and. one in the thorax. 



The first of these, called the cephalic ganglion on account of 

 its position, is the homologue, that is, the anatomical represent- 

 ative of the oesophageal ganglion or ring of the lower forms of 

 life ; it surrounds the esophagus and gives off four pairs and one 

 single nerve, which are distributed to every part of the insect's 

 head. It is connected with the great thoracic nerve centre by 

 a thick cord which represents the double ventral cord, typical of 

 the insecta. 



The nerves given off from this ganglion, or more correctly 

 speaking, collection of ganglia, for it is really composed of at least 

 six, may be properly divided into two classes, those which consist 

 entirely of nerve fibres having their origin in its substance, and 

 those of which some of the nerve fibres at least, probably the 

 greater portion, merely pass through it and have their real 

 origin in the thoracic ganglion. 



