246 



HARDWICKE'S SCIENCE-GOSSIP. 



an indefinite plexus (earthworm). It always joins 

 the oesophageal ring, and sends branches to the 

 t!^.sophagus and fore-part of the alimentary canal. 

 The system has been identified with the sympathetic, 

 and also with the vagus of vertebrates, but such cor- 

 relations are hazardous ; the first indeed may be 

 considered as disproved. 



Between and above each pair of connectives of the 

 ventral cord runs a transparent nerve, composed 

 largely of ganglionic cells, which is often termed the 

 sympathetic, though its physiological relations are 

 iminvestigated. These branches proceed from the 

 middle of the connective, on the right and left side 

 alternately, and pass backwards to the neighbourhood 

 of the succeeding ganglion. Here the nerve forks, 

 each half forming a spindle-shaped enlargement, and 

 joining the lateral branches from the ventral cord. 



For our knowledge of the internal structure of the 

 ventral nerve-cord of insects we are chiefly indebted 

 to Leydig. The connectives consist of nerve -fibres 

 only, which, as in invertebrates generally, are non- 

 medullated. The ganglia include (i) rounded, 

 usually unipolar nerve-cells ; (2) tortuous and ex- 

 tremely delicate fibres collected into intricate skeins 

 {punkt-stihstanz) ; {3) commissural fibres, and (4) 

 connectives. The chief fibrous tracts are internal, 

 the cellular masses outside them. A double invest- 

 ment protects the cord. Closely embracing the 

 nervous structures is a transparent, structureless, 

 chitinous sheath, within which a matrix of branched 

 mother-cells may be here and there distinguished. 

 Outside the proper sheath is a peritoneal layer of 

 loose and irregular cells. Tracheal trunks pass to 

 each ganglion, and break up upon and within it into 

 a multitude of fine branches. 



Two bundles of commissural fibres connect the 

 ganglia of the same pair.* Of the peripheral fibres 

 some pass direct to their place of distribution, others 

 traverse at least one complete segment and the cor- 

 responding ganglion before separating from the cord. 



Many familiar observations show that the ganglia 

 of an insect possess great physiological independence. 

 The limbs of decapitated insects, and even isolated 

 segments, provided that they contain uninjured 

 ganglia, exhibit unmistakable signs of life. Yersin 

 and Baudelot's experiments imply that the ventral 

 cord is divisible into an upper motor tract and a 

 lower sensory, the centres of both motion and sensa- 

 tion lying in the ganglia exclusively. 



The minute structure of the brain has been investi- 

 gated 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 comi)letely intelligible 

 account. The physiological significance, and the 



* Michels finds in Oryctes that such transverse commissures 

 do not exist, but that there are numerous transverse fibres con- 

 necting the ganghonic cells of one side with the peripheral 

 nerves of the other ("Zeits. f. wiss. Zool.," 13d. xxxiv.. p. 6q6, 

 1881). 



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 physio- 

 logical experiment. Mr. E. T. Newton has published 

 a clear and useful description * of the internal and 

 external structure of the brain of the cockroach, 

 which incorporates what had previously been ascer- 

 tained with the results of his own investigations. 

 He has also described f an ingenious method of 

 combining a number of successive sections into a 

 dissected model of the brain. Having had the 

 advantage of comparing the model with the original 

 sections, we offer a short abstract of Mr. Newton's 

 memoir as the best introduction to the subject. He 



<^-T 



(con 



Fig. 147. — A, Lobes of the brain of the Cockroach, seen from 

 within; c, cauliculus ; /, peduncle; t, trabecula. B, ditto, 

 from the front ; ocx, outer calyx ; icx, inner calyx. C, ditto, 

 from above. [Copied from E. T. Newton.] 



describes the central framework of the cockroach 

 brain as consisting of two solid and largely fibrous 

 trabecular, which lie side by side along the base of 

 the brain, becoming smaller at their hinder ends ; 

 they meet in the middle line, but apparently without 

 fusion or exchange of their fibres. Each trabecula is 

 continued upwards by two fibrous columns, the 

 cauliculus in front, and the peduncle behind ; the 

 latter carries a pair of cellular disks, the calices (the 

 cauliculus, though closely applied to the calices, is 

 not connected with them) ; these disks resemble two 

 soft cakes pressed together above, and bent one 

 inwards, and the other outwards below. The 

 peduncle divides above, and each branch joins 

 one of the calices of the same hemisphere. 



* " Q. J. Micr. Sci.," 1879, pp. 340-356, pi. xv. xvi. 

 t " Journ. QuekettMicr. Club," 1879. 



