ANATOMY AND PHYSIOLOGY 



8l 



The subcesophageal ganglion (Fig. 115) is al- 

 ways connected with the brain by a pair of nerve 

 cords (cesephageal commissures) between which 

 the oesophagus passes. This ompound ganglion 

 represents at most four neuromeres: (i) mandi- 

 bular, innervating the mandibles; (2) superlin- 

 gual, found by the author in Collembola, but not 

 yet reported in the less generalized insects; (3) 

 maxillary, innervating the maxillae; (4) labial, 

 which sends a pair of nerves to the labium. 



The minute structure of the brain, thgouh 

 highly complex, has received considerable study, 

 but will not be described here for the reason that 

 the anatomical facts are of no general interest 

 so long as their physiological interpretation re- 

 mains obscure. 



Sympathetic System. Lying along the 

 median dorsal line of the oesophagus is a recurrent, 

 or stomato gastric, nerve Fig. 116), which arises 

 anteriorly in a frontal ganglion and terminates 

 posteriorly in a stomachic ganglion situated at 

 the anterior end of the mid intestine. Con- 

 nected with the recurrent nerve are two pairs of 

 lateral ganglia, the anterior of which innervate 

 the dorsal vessel and the posterior, the tracheae 

 of the head. The ventral nerve cord may include 

 also a median nerve thread (Fig. 113) which gives 

 off paired transverse nerves to the muscles of 

 the spiracles. 



Structure of Ganglia and Nerves. A gang- 

 lion consists of (i) a dense cortex, composed of 

 ganglion cells (Fig. 117), each of which has a large 

 rounded nucleus and gives off usually a single 

 nerve fiber; and (2) a clear medullary portion 

 . (Punktsubstanz) derived from the processes of the 

 cortical ganglion cells and serving as the place 

 of origin of nerve fibrillae. There are, however, 

 ganglion cells from which processes may pass 

 directly into nerve fibrillae. 



A nerve fiber, in an insect, consists of an axis- 

 cylinder, composed of fibrillae, and an enveloping 

 membrane, or sheath. The axis-cylinder is the 

 transmitting portion and the ganglia are the 



sy-^ 



FIG. 113. Central nerv- 

 ous system of a thysanu- 

 ran, Machilis. The 

 thoracic and abdominal 

 ganglia are numbered in 

 succession. a, antennal 

 nerve; b, brain; e, com- 

 pound eye; /, labial nerve; 

 m, mandibular nerve; mx, 

 maxillary nerve; o, oeso- 

 phagus; ol, optic lobe; s, 

 subcesophageal ganglion; 

 sy, sympathetic nerve. 

 After OUDEMANS. 



