38 BULLETIN OF THE LABORATORIES 



fibres beyond them. It has been suggested by my brother, who first 

 observed this relation in the opossum, that this is not necessarily a 

 true olfactory tract, but may be the tract of this portion of the cortex.* 

 It passes caudad, sinking into the substance of the brain ventrad of 

 the striatum, finally enters the base of that body, and continues its 

 course, as a large well defined bundle, to the anterior commissure 

 (Plate II, Figs. 2, 7, and 8, fie.) 



The Anterior Commissure contains two sets of fibres. The cepha 

 lie and slightly larger bundle has been already described in connection 

 with the olfactory tracts. The caudal branch arches backward, its 

 fibres passing caudo-laterad along the ental border of the lower portion 

 of the striatum. They can be followed as far back as the chiasm. In 

 Geomys these two bundles fuse so that in the body of the commissure 

 they cannot be distinguished. On the other hand in Mus musculus 

 and Fiber zibethicus they remain distinct through the whole length of 

 the commissure, and can be easily distinguished in transverse section 

 when the brain is cut perpendicular longitudinally {cf. Plate II, Fig. 1.) 

 The commissure, after the union of its two portions, curves gently dor- 

 sad, so that its highest point is in the meson. In Erethizon the same 

 relation prevails as in Geomys, though somewhat obscured by the en- 

 veloping fibres of the striatum at the point of separation of the two 

 bundles (Plate II, Fig. 3.) 



The Callosum is as usual among rodents. It is, however, consid- 

 erably larger in Erethizon than in Geomys, in the former case being 

 thicker than the fornix commissure, in the latter scarcely as thick. It 

 extends caudad to a point as far back as the superior commissure. In 

 Erethizon its cephalo-caudad length is 14 mm, in Geomys, 6 mm. 



The Striatum is a pear-shaped body with the more convex surface 

 entad and the larger end cephalad. The dorsal aspect is arched and 

 quite free from the surrounding organs. The ventral surface is not so 

 sharply defined, but passes into the substance of the hemisphere. It 

 is connected with the thalamus on the caudo-ventral aspect by the 

 pyramidal tracts from the peduncle. The anterior commissure crosses 



*See a paper on the Brain of the Opossum, by C. L. Herrick, in the forth- 

 coming number of the Journal of Comparative Neurology. According to this 

 view, the various layers of the olfactory bulb, constituting the pero of Professor 

 Wilder, are mechanically superposed upon the pes, or cortical and ventricular 

 portion of the olfactory, without sustaining any vital relation to it. 



I am also indebted to this paper for all comparative data with the opossum 

 which occur in this article. 



