Thompson, Brains of Cerehratulus. 647 



central part and the roots are not distinguishable. At first it 

 seemed possible that the roots might be merely entering fibers 

 from large cells in the cellular sheath, and this view was supported 

 by the presence of a group of cells of type III, just medial to each 

 of the brain lobes. A further study of succeeding sections showed, 

 however, that the fiber bundles, or roots, may be traced outside 

 the cellular sheath and slightly beyond the medial side of the 

 oesophageal nerves. The question then arises, whether the central 

 part may not be merely the anterior part of the sixth commissure, 

 which begins in the following section. My final conclusion, based 

 upon the careful study of successive sections with the immersion 

 lens, is that the fiber bundles in question are either the roots of a 

 separate, very delicate brain commissure, the fifth, which immedi- 

 ately adjoins the sixth, or the roots of a compound brain-cesopha- 

 geal commissure, the fifth and the sixth, the anterior fibers of 

 which come from the brain, the posterior fibers from the oesopha- 

 geal nerves. Since further study of the sections makes the former 

 view slightly more probable, the fifth commissure is represented, 

 fig. I, as separate from the sixth. 



Sixth ventral commissure. — The sixth ventral commissure, fig. 

 I, 6, is found one section posterior to the central part of the fifth, 

 and is the first oesophageal commissure, running only between the 

 oesophageal nerves. This commissure is three sections thick; in 

 the first two sections the dorso-ventral measurement is 53/i, but 

 is much less in the last section. It is a very clear and well defined 

 commissure, and the passing out of the fibers from the oesophageal 

 nerves is distinctly seen in several sections, as the nerve sheaths 

 are wide open on their medio-ventral surfaces owing to the breadth 

 of the bands of fibers. As the left root originates a few sections 

 posterior to the right, the entire commissure is not in the same 

 frontal plane. 



Seventh ventral commissure. — The seventh ventral commissure, 

 fig. I, 7, is found four sections posterior to the central part of the 

 sixth; its central mass is present in two sections, with a dorso- 

 ventral measurement of 42/<. The left root is a sharply defined, 

 rather broadband of fibers, and may be traced beneath the left 

 oesophageal nerve through the cellular sheath into the fibrous core 

 of the brain. The right root is delicate and rather indistinct and 

 can be traced only as far as the cellular sheath of the right ventral 

 lobe. Some of the more ventral fibers of the left root seem also 



