94 THE COCKROACH : 



plexus with a pair of nerves which proceed from the back of 

 the brain. Each nerve forms two ganglia, one behind the 

 other, and each ganglion sends a branch inwards to join the 

 recurrent nerve. Fine branches proceed from the paired nerves 

 of the ocsophageal plexus to the salivary glands. 



The stomato-gastric nerves differ a good deal in different 

 insects ; Brandt* considers that the paired and impaired nerves 

 are complementary to each other, the one being more elaborate, 

 according as the other is less developed. A similar system is 

 found in Mollusca, Crustacea, and someVermes (e.#.,Nemerteans). 

 When highly developed, it contains unpaired ganglia and 

 nerves, but may be represented only by an indefinite plexus 

 (earthworm). It always joins the cesophageal ring, and sends 

 branches to the oesophagus and fore-part of the alimentary canal. 

 The system has been identified with the sympathetic, and also 

 with the vagus of Vertebrates, but such correlations are hazar- 

 dous ; the first, indeed, may be considered as disproved. 



Internal Structure of Brain. 



The minute structure of the brain has been investigated by 

 Leydig, Dietl, Flogel, and others, and exhibits an unexpected 

 complexity. It is as yet impossible to reduce the many curious 

 details which have been described to a completely intelligible 

 account. The physiological significance, and the homologies 

 of many parts are as yet altogether obscure. The comparative 

 study of new types will, however, in time, bridge over the wide 

 interval between the Insect-brain and -the more familiar Verte- 

 brate-brain, which is partially illuminated by physiological 

 experiment. Mr. E. T. Newton has published a clear and 

 useful description f of the internal and external structure of the 

 brain of the Cockroach, which incorporates what had previously 

 been ascertained with the results of his own investigations. He 

 has also described J an ingenious method of combining a 

 number of successive sections into a dissected model of the 



* "Mem. Acad. Petersb.," 1835. 



f "Q. J. Micr. Sci.," 1879, pp. 340-350, pi. xv., xvi. 



I " Jouvn. Quekett Micr. Club," 1879. 



