BULLETIN OF THE LABORATORIES 
38 
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 largd well defined bundle, to the anterior commissure 
(Plate II, Figs. 2, 7, and S, pc.) 
The Anterior Conimissiire contains two sets of fibres. The cepha- 
lic 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. i.) 
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. 
