3<D BULLETIN OF THE LABORATORIES 



marked. The lateral columns are greatly enlarged ventrad to the 

 clava, forming the area ovalis of Wilder. The relations of the dorsal 

 columns and the structures on the floor of the fourth ventricle are 

 shown by Plate I, Fig. 3. The funiculus gracilis and the funiculus 

 cuneatus fuse at the region of the calamus scriptorius to form the 

 clava, which is very large. The obex is also large, lying ventrad of 

 the preceding. On the floor of the ventricle the fasciculi teretes are 

 divaricated caudad by the projecting genua of the seventh nerve. The 

 eminentia acustica are large and double, being divided into two por- 

 tions by a longitudinal fissure. The strire medullares are hardly visi- 

 ble in Erethizon; in Geomys they are very large. The posterior 

 peduncle of the cerebellum passes into the restiform tract in the usual 

 way. 



The cranial nerves are illustrated by Plate I, Fig. 6. The optic 

 nerves are smaller than usual, 1.2 mm. in diameter, 1 cm. from their 

 insertion. The third nerves are about the usual size, inserted 1 cm. 

 caudad of the chiasm on the ventral aspect of the crura. The fourth 

 nerves are very small, pursuing the usual course. The fifth is large, 

 3 mm. in diameter at insertion, rapidly flattening out to much wider. 

 It has two roots. The median is ventral and smaller and crosses the 

 other at the Gasserian ganglion. The sixth is small, inserted just 

 caudad of the pons about 3 mm. from the meson. But one root is ap- 

 parent. The seventh is inserted on the caudal margin of the pons 

 laterad of the sixth. The eighth is inserted 1 to 2 mm. caudo laterad 

 of the seventh. There are two roots, which lie in very close contact. 

 The ninth, tenth and eleventh are closely associated in their exit. The 

 ninth is small and more easily separated from the other two. The 

 tenth has several large roots which are hard to separate from those of 

 the eleventh. The eleventh itself can be followed as a large strand as 

 far back as the third spinal, and for at least two thirds of this distance 

 gives off frequent smaller fasciculi into the medulla and cord. The 

 twelfth arises on the caudal aspect of the pyramids by about six roots. 



Measurements. Total length, tip of olfactories to end of medulla, 

 46.5 mm.; olfactory to cerebellum, 33 mm., to pons, 29 mm., to hy- 

 pophysis, 21 mm., to chiasm, 17.5 mm.; width of the two hemi- 

 spheres, median, 35 mm., one-fourth distance from caudal end, 33.5 

 mm. three-fourths distance from caudal end, 32 mm.; length of crura 

 exposed, 8 mm.; length of tuber cinerum, 7 mm.; width of tuber cin- 

 erum, 6.5 mm ; length of corpora quadrigemina, 7.5 mm.; width of 



