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-cesophageal 
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, (1) 
ascending and descending bundles, which belong to the crura 
and cephalo-thoracic nerve-cord; these unite the metameral 
and supra-cesophageal 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 antennal, 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 cesophagus, 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 (1) a large bundle of fibres 
which enters the optic peduncle (PI. 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 (PI. XXXIIL., Figs. 1 and 2); 
31 
