PRIMITIVE SEGMENTS OF THE BRAIN 231 



The pair of subossophageal ganglia distributes nerves to the 

 mandibles, to the 1st and 2d maxillse, and to the salivary glands 

 (Fig. 248). 



Its general shape and relations to the walls and to the outer 

 organs of the head is seen in Figs. 247, 248. In all the winged 

 insects (Pterygota) its plane is situated more or less at right angles 

 to the horizontal plane of the ventral cord. On the dorsal and 

 anterior sides are situated the ocular lobes, and below these the 

 antenna! lobes. 



Viallanes first, independently of embryonic data, divided the 

 brain of adult insects into three regions or segments ; i.e. the "pro- 

 tocerebron," " deutocerebron," and " tritocerebron," which he after- 

 wards found to correspond with the three primitive elements 

 (neuromeres) of the brain and with the segments of the head of the 

 embryo. 



The brain of the locusts (Melanoplus and (Edipoda) being best 

 known will serve as the basis of the following description, taken 

 mainly from Viallanes, with minor changes in the name of the three 

 segments, and other modifications. 



I. The optic or procerebral segment is composed of a median por- 

 tion, i.e. two fused procerebral lobes (median protocerebrum), and 

 of two lateral masses, the optic ganglia (protocerebmim), and com- 

 prises the following regions fused together and forming the median 

 procerebral mass (Viallanes) : 



1. Procerebral lobes. 



2. Optic ganglia. 



3. Layer of postretinal fibres. 



4. Ganglionic plate. (Periopticon of Hickson.) 



5. External chiasma. 



6. External medullary mass. (Epiopticon of Hickson.) 



7. Internal chiasma. 



KKJ. -4s. 1, front view of the brain of Melinioplux femur-rulirt/ m : <>/it. tinny, optic ganglion ; 

 oc, ocelli and nerves leading to them from the two hemispheres, each ocellar nerve arising from the 

 region containing- the calices ; in. oc, median ocellar nerve ; opt. I, optic lobe sending off the optic 

 nerve to the optic ganglion ; ant. I, antennal or olfactory lobe ; ant. n, antcnnal nerve ; f. g, frontal 



gangli if sympathetic nerve; Ibr. n, nerve to labrum ; <e, cross-nerve or commissure between the 



two hemispheres; a>. c, oesophageal commissure to subresophageal ganglion. 2, side view of the 

 brain and subtesophageal ganglion (lettering of brain as in 1): ,s\ (/, stomatogastric or sympa- 

 thetic nerve; a.s.g, anterior, and />..(/, posterior, sympathetic ganglia; g 2 , suboesophageal 

 ganglion ; mil, nerve to mandible ; my, maxillary nerve ; In, labial nerve ; nl, unknown nerve, 

 perhaps salivary. 3, interior view of the right half of the head, showing the brain in its natural 

 position: an, antenna; cl, clypeus ; Ibr, labrum; ;/*, mouth-cavity; mil. mandible ; 1, tongue; 

 <B, (esophagus ; c, crop ; en, right half of the endocranium or X-shaped bonr, through the anterior 

 angle of which the (esophagus passes, while the great mandibular muscles play in the lateral angles. 

 The moon-shaped edge is that made by the knife passing through the centre of the X. 4, view of 

 brain from above (letters as before). 5, subcesophageal ganglion from above: t.c, commissure to 

 the succeeding thoracic ganglion (other letters as before). Fig. 3 is enlarged 8 times ; all the rest 25 

 times. Drawn from original dissections, by Mr. Edward Burgess, for the Second Report of the 

 U. 8. Entomological Commission. 



