STRUCTURE OF THE BRAIN IN RODENTS. 
735 
ventricular portions placed in front of tlie cornu ammonis. In the median line 
separating the two striate bodies lie the double leaflets of the septum pellucidum. 
The callosal fibres are seen to meet those ascending from the internal capsule and 
upper pole of the ganglia. If the extra-ventricular nucleus he pressed gently out¬ 
wards away from the caudate nucleus, we observe numerous white fasciculi beneath its 
surface arching upwards parallel to the latter to enter the projection mass at the 
upper pole, whilst running parallel with the converging tract of the fornix, and closely 
connected with the whole inner border of the caudate nucleus, is the fairly broad white 
band—the tsenia semicircularis. By means of a fine dissecting needle and camel-hair 
brush the whole remaining dissection may be completed. First, the septum pellucidum 
is gently raised from behind and thrown forwards, the hemispheres pressed asunder so as 
to expose the descending pillar of the fornix, in front of which the anterior commissure 
crosses from one to the other hemisphere. By means of the needle the continuation of 
this commissure, with the olfactory lobe of one side, may be most readily traced, and 
upon cautiously teazing out the substance of the striate body on one side, the con¬ 
tinuation from the commissure outwards through its substance may be quite as easily 
* - 
traced. We now see gleaming deeply through the basal structures of this region a 
pale band which may be regarded as the outer side of a remarkable triangle, the 
inner side being constituted by the central olfactory fasciculus, and the base by the 
anterior commissure and its posterior extension. Now this triangular area is the 
deeper portion of the pyriform region recognised at the base as Gratiolet’s olfactory 
area. Closely examined by a hand magnifier we now see the very peculiar structure 
alluded to as the “ olfactory lyre.” Gently raise by the needle the posterior olfactory 
fasciculus and examine also with a lens. From its anterior border arise numerous 
delicate fibres constituting more or less fine fasciculi, the largest arising near the 
anterior commissure, whilst a few delicate offsets are given off from the commissure 
itself. Whilst these fasciculi curve inwards and appear continuous with the olfactory 
tract into and through the commissure, other fasciculi may be seen passing through the 
structure of the posterior olfactory band directly backwards, separating its strands and 
emerging behind to enter this region of the corpus striatum. Traced forwards these 
delicate fasciculi lie parallel to and on a plane with the central olfactory fasciculus, 
forming as it were a floor for this triangular space (the strands of the lyre), and in fact 
sharply defining the upper limits of the olfactory area from the structure of the 
caudate nucleus proper. With the scissors divide these fibres along their line of 
connexion to the posterior olfactory tract, and teaze out with the needle the structures 
lying immediately beneath. We now see by means of a low power, and even by the 
naked eye, the coarse fasciculi enclosing betwixt their strands the oval grey nuclei of 
the olfactory area. Next, by a delicate dissection with needle and moistened brush, 
the posterior olfactory fasciculus may be followed from the outer margin of the striate 
nucleus backwards ; its brush-like division here sending a small detachment of fibres 
upwards along the outer aspect of the corpus striatum to reach the vertex. The 
MDCCCLXXXII. 5 B 
