STRUCTURE OF THE BRAIN IN RODENTS. 
701 
poles, as indicative of the anterior and posterior extremity of the hemisphere, it 
being understood that these terms do not imply the existence of a frontal or occipital 
lobe. The inner margin of the hemispheres bordering upon the great longitudinal 
fissure may also be conveniently spoken of as the sagittal border. Along this 
border, in the hinder half of the hemisphere, there is seen a strip of cortex, much 
paler than the remaining portion of the hemisphere, about 2 mm. wide behind, but 
beco mi ng gradually narrower as it extends forwards. This pale strip is not marked off 
by any linear depression, yet it forms an important region characterised by a peculiar 
type of cortex. In the Rabbit, on the other hand, the same sagittal region is mapped 
off from the extra-limbic mass outside it by a constant though shallow depression 
(Plate 49, fig. 1, K K), the representative of the primary parietal sulcus of higher 
animals, and which, as in the Pig and Sheep, separates the sagittal from the Sylvian 
gyri of the parietal lobe. Moreover, the upper aspect of the brain in the Rabbit is of 
more pyriform contour, and remarkably attenuated in front, so that the pointed frontal 
poles contrast strangely with the broader frontal extremities of certain animals 
included in the same category (fig. 8, Beaver, op. cit., Broca.). The hemispheres 
of a four-months’ old Rabbit measured 28 mm. long, and weighed (together with 
corpora quadrigemina) just six grammes. The extra-limbic lobe is traversed by 
three or four delicate channels for blood-vessels, which course upwards and backwards 
from the limbic fissure, and very slightly indent its surface. At the widest portion 
the cerebrum has a diameter of 30 mm., whilst in the Rat it is but 15 mm. across. 
The brain of the Rat displays the same cordate outline at its base, the hemispheres 
widely separated behind by the descent of the cerebral peduncles. The lower limbic 
arc forms so prominent a feature as wholly to conceal from view the lateral por¬ 
tions of the extra-limbic lobe (Plate 49, fig. 2, N). The base of this cordate area is 
therefore formed by the gyrus hippocampi, in front of which extends the olfactory lobe. 
Along the course of each olfactory lobe the superficial olfactory medulla (external root) 
runs back, rapidly narrowing as it approaches the gyrus hippocampi, in which it loses 
itself (Plate 49, fig. 2, M). Betwixt the two superficial olfactory bands, and in 
front of the optic commissure, lie two pyriform grey areas, the olfactory field of 
Gratiolet (Plate 49, fig. 2, P). They are bounded externally by the fasciculus just 
named, and lie in contact with one another on the inner side, the optic nerves being 
here placed superficial to them. The optic nerves terminate in a well-marked optic 
commissure (Plate 49, fig. 2, R), from which the optic tracts diverge, and are lost 
beneath the gyrus hippocampi. The tuber cinerum and infundibulum forms a 
prominent mass of grey matter behind the optic commissure (Plate 49, fig. 2, L), and 
are crossed behind the optic tracts by a distinct white band of fibres extending across 
the angular interval left betwixt the diverging tracts. Stellate pigment cells of a jet- 
black hue accumulate more or less thickly along the whole lower limbic arc and olfactory 
lobe. They are especially frequent over the basal aspect of the olfactory lobe and 
along the limbic fissure, back even as far as the posterior extremity of the latter. 
