36 
BULLETIN OF THE LABORATORIES 
^ INTERNAL TOPOGRAPHY. 
In the following pages all statements apply both to Erethizon 
and to Geomys, unless otherwise indicated. Comparison was also 
made in many particulars with a series of sections of Fiber zibethicus. 
Ventricles. The third ventricle is shown in its relations in median 
longitudinal section by Fig. 3, Plate I. The aqueduct of Sylvius is 
wide and there is a slight indication of a ventricle of the mesencepha- 
lon. The third ventricle is largely occupied by the medicommissure, 
particularly dorsally. Below, it passes into the infundibulum in the 
center of the cinereum. Cephalad of the chiasm there is another de- 
pression, which is superficially bounded by little more than a mem- 
brane. Dorsad, the third ventricle enters the epiphysis for a short 
distance. Cephalad of the medicommissure are the foramena of 
Monro, or portae, shaded black in the figure. The lateral ventricles 
of the hemispheres envelop the striata on the dorsal and median as- 
spects. Thejpraecornua arch as far laterad as the cephalic ends of the 
striata. From their cephalo-ventral limits the aqueducts of the olfac- 
factory ventricles push out into the olfactory crura. The olfactory 
ventricles, though small, pass almost to the ends of the olfactory bulbs. 
The medicornua of the lateral ventricles are strong, enveloping the 
hippocampi on their lateral and dorsal aspects. 
Olfactory Lobe and Tracts. The minute structure of the olfactory 
lobe seems to be essentially that described for Arctomys in Vol. V 
of this Bulletin, page 81. The specific olfactory, or ganglion cells, 
however, are not usually pyramidal, but irregularly fusiform or flask- 
shaped, with the apices directed peripherad. 
The arrangement of the olfactory tracts is about the same in Ere- 
thizon, Geomys, Mus musculus, and Fiber zibethicus. It can best be 
described in Geomys. Transverse sections of the olfactory .bulb 
(Plate II, Fig. 4) show within the fibres of the olfactory nerves the 
usual glomerular layer containing sparse, small, pyramidal cells, a 
layer of ganglion cells in a single series, whose bases lie in a thin 
granular zone of Deiter’s corpuscles. This zone is separated by an- 
other thin band of the neuroglia layer from the dense central mass of 
laminated DeiteFs cells which fills up the rest of the bulb except a 
loose medullary portion about the ventricle. 
Fibres destined for the superficial olfactory tracts, or radices, 
gather well out in the olfactory bulb into a bundle which lies in the 
