134 Mr. P. A. Buxton on the 



the organ is a reflex centre not connected with any one 

 motor or sensory function. 



(The ocellarv glomeruH. which are sometimes considered 

 with the central body, are described on p. 135.) 



V. The Bridge. 



(Die Hirnbriicke — Kuhnle, etc. I^e Pont — Viallanes. 

 Fibrillar Arch — Kenyon.) 



The Bridge (6r.) is a protocerebral structure found in all 

 insect brains. In Micropfeiifx it occupies its usual position 

 as a transverse band of axonic material on the sujDerior side 

 of the protocerebrum. It lies behind the nerve to the 

 ocellus, and between the heads of the mushroom bodies 

 (PI. VIII, IX and X). 



The bridge consists in part of Punktsubstanz ; this is 

 found at either end. The middle of the bridge is much 

 attenuated and appears to consist entirely of nerve fibres 

 passing from one side to the other. The organ, then, is 

 dumb-bell shaped, the swollen ends, or " heads," being 

 presumably centres, the narrow waist a decussating tract. 

 The bridge is covered by some layers of ganglion cells, 

 which appear to belong to the normal type.* 



The axons of these apparently " normal " cells pass down- 

 wards to several parts of the brain. Some which proceed 

 from the more lateral cells pass in front of, or behind, or 

 round the end of the bridge, and enter the dorsal surface 

 of the protocerebral lobes in a diffuse manner. Other 

 axons pass through the heads of the bridge and so on- 

 wards to the lobes of the protocerebrum ; it is probable that 

 these give off a collateral while they are within the sub- 

 stance of the bridge. Other cells, again, send their axons 

 into the bridge itself, where the fibre is lost to sight. It 

 is probable that some of these fibres cross the middle line. 

 The whole matter requires investigation by the Golgi 

 method. We have, then, a number of similar cells, some of 

 which appear to be associated with the bridge, some with 

 the dorsal part of the protocerebrum, some, again, with both. 

 From this I am inclined to argue that the bridge is of less 

 importance as a physiological entity than some authorities 

 have beheved; that it is rather of anatomical than of 



* In Forficula Kiihnle distinguishes several types of cell in this 

 region, but in every respect the ganglion cells of Micropteryx appear 

 to have very little tendency to be differentiated into types. 



