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 portse, shaded black in the figure. The lateral ventricles 

 of the hemispheres envelop the striata on the dorsal and median as- 

 spects. The 'praecornua 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 Deiter's 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 



