Microscopic examination of the internal structure of the nerve-cord reveals a complex arrangement of cells and fibres. The connectives consist almost entirely of nerve-fibres, which, as in Invertebrates generally, are non-medullated. The ganglia include (1) rounded, often multipolar, nerve-cells; (2) tortuous and extremely delicate fibres collected into intricate skeins; (3) commissural fibres, and (4) connectives. The chief fibrous tracts are internal, the cellular masses outside them. A relatively thick, and very distinct neurilemma, probably chitinous, encloses the cord. Its cellular matrix, or chitinogenous layer, is distinguished by the elongate nuclei of its constituent cells.[102] Tracheal trunks pass to each ganglion, and break up upon and within it into a multitude of fine branches. [102] We have not been able to distinguish in the adult Cockroach the _double_ layer of neurilemmar cells noticed by Leydig and Michels in various Coleoptera. [Illustration: Fig. 41.--Transverse section of Third Thoracic Ganglion. _neu_, neurilemmar cells; _gc_, ganglionic cells; _tr_, tracheal tubes; _A_, ganglionic cells, highly magnified. × 75.] [Illustration: Fig. 42.--Longitudinal vertical section of Third Thoracic Ganglion. _n_, connective. The other references as in fig. 41. × 75.] Bundles of commissural fibres pass from the ganglion cells of one side of the cord to the peripheral nerves of the other. There are also longitudinal bands which blend to form the connectives, and send bundles into the peripheral nerves. Of the peripheral fibres, some are believed to pass direct to their place of distribution, while others traverse at least one complete segment and the corresponding ganglion before separating from the cord. [Illustration: Fig. 43.--Longitudinal horizontal section of Third Thoracic Ganglion. _n_, peripheral nerves. The other references as before. × 75.] 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. _Median Nerve-Cord._