Thompson, Brains of Cerehratuhis. 649 



Eleventh ventral commissure. — The central part of the eleventh 

 commissure, fig. i, //, and figs. 7 to 8, //, is situated two sections 

 posterior to the tenth; it is two sections thick, and has a dorso- 

 ventral measurement of 2i/(. This is the second brain-oesophageal 

 commissure. The fibers of the central part, figs. 7 to 12, pass 

 above the oesophageal nerves, and are reinforced, especially on the 

 right side, by fibers that issue from the dorsal surface of the 

 nerves. The left root is slender and delicate but may be traced 

 almost to the cellular sheath of the left brain lobe. The right 

 root is stout and runs obliquely backward, through eight sections 

 into the cellular sheath of the right brain lobe, figs. 7 to 12. 



Twelfth ventral commissure. — The central part of the twelfth 

 commissure, fig. i, 12, lies four sections posterior to the eleventh; 

 it is present in two sections, and measures dorso-ventrally 4^2fj.. 

 This commissure, figs. 9 to 10, may be traced from the cellular 

 sheath of the left brain lobe across to and into the right oesophageal 

 nerve, but no fibers are distinguishable between the right oesopha- 

 geal nerve and the right brain lobe. The fibers pass above the 

 left oesophageal nerve, and as the nerve sheath is absent at this 

 point there is probably a mingling of nerve substance. In the 

 next two sections, figs. 11 to 12, a very stout bundle of fibers enters 

 the left oesophageal nerve on its lateral surface, and may represent 

 a second root of the same commissure. Fig. 10 is of interest 

 since it contains parts of three commissures, namely, the eleventh, 

 the twelfth and the thirteenth. The fibers at the more dorsal 

 level, 12, belong to the twelfth, the ventral ones, /j, to the thirteenth 

 and the root to the right, 11, to the eleventh. From this it is seen 

 that the twelfth and the thirteenth commissures are in contact 

 with each other, the ventral surface of the former adjoining the 

 dorsal surface of the latter. 



Thirteenth ventral commissure. — The anterior part of the thir- 

 teenth ventral commissure is in the same section with the posterior 

 part of the twelfth, but ventral to it, fig. 10. At its widest part 

 the dorso-ventral measurement is 60//. This commissure is pres- 

 ent in four sections, and in the last two sections, figs. 12 to 13, 

 the beginnings of the fibers of the fourteenth commissure are seen 

 ventral to the fibers of the thirteenth. The stout distinct roots are 

 several sections posterior to the central part, fig. i, and may be 

 traced into the fibrous core of the ventral lobes. The fibers from 

 the brain sweep beneath the oesophageal nerves and then upward. 



