ANATOMY OF THE NERVE CENTRES. 463 



The Fibrous tracts of the Brain. The reticular medulla of the 

 brain is traversed by a great number of well-defined tracts or 

 bundles of parallel fibres. In the crura, infra-oesophageal 

 region and more central parts of the cerebron, these bundles 

 entirely, or almost entirely, replace the reticular substance. 

 Indeed the latter is formed by the decussation of radiating 

 fibres, connecting such definite tracts with various parts of the 

 cortex. 



The fibrous tracts may be divided into four groups, (i) 

 ascending and descending bundles, which belong to the crura 

 and cephalo-thoracic nerve-cord ; these unite the metameral 

 and supra-oesophageal centres ; (2) commissural bundles which 

 unite the opposite halves of the brain ; (3) the trabecular 

 system, or the tracts which terminate in the calices of the 

 corpora fungiformia ; and (4) inter-ganglionic tracts, which join 

 the antenna], optic and infra-cesophageal (crural) centres with 

 each other. 



The Crura consist mainly of fibres, which connect the cephalo- 

 thoracic nerve-cord, the maxillary nerves and the gray matter 

 of the infra-cesophageal centres, with the various parts of the 

 cerebron, and with the optic and antennal lobes. 



All these fibres apparently decussate in the infra-cesophageal 

 centre. This is very apparent in the case of those which 

 come from the maxillary nerves and from the ganglion cells of 

 the infra-cesophageal ganglia, and I shall subsequently show 

 that there is good physiological evidence that the right half of 

 the cerebron corresponds with the left half of the body, and 

 vice versa. 



The crura ascend on either side of the oesophagus, above 

 which they are separated into numerous distinct fasciculi by the 

 three great commissures which form the floor of the central 

 cavity of the cerebron. The most important of the fasciculi 

 into which the crura divide are (i) a large bundle of fibres 

 which enters the optic peduncle (PL XXIX., Fig. 2) ; (2) a 

 bundle which joins the trabecula ; (3) a fasciculus which enters 

 the antennal ganglion; (4) a great bundle of ascending fibres 

 which is lost in the mesocerebron (PL XXXIII., Figs, i and 2) ; 



