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MR. W. BEVAN LEWIS ON THE COMPARATIVE 
cortex. In the brain of the small Rodents now under consideration there is also a 
notable development of the olfactory organ and its central connexions, but on the 
other hand the great extra limbic or parietal mass is wholly devoid of gyri. The 
Mammals possessing such smooth, non-convolnted brains have been grouped in an 
artificial class by Owen, who terms them Idssencephales; the animals possessing 
convoluted brains are termed Gyrencephales. Broca, on the other hand, has adopted 
the terms Osmatic" and Anosmatic Mammals,! as indicative of a highly-developed 
olfactory apparatus in the one, as contrasted with the defective, rudimentary, or 
wholly absent olfactory lobes of the other class. Both these terms are extremely 
useful in such investigations as we at present are concerned with, and I shall therefore 
freely adopt them, together with the nomenclature of the convolutions advocated by 
Broca. From what has been stated it is obvious that Osmatic Mammals may have 
smooth or convoluted brains, but the Rat and Rabbit present us with typical brains of 
the Osmatic Lissencephales. The cerebrum in these animals is mapped out into two 
grand divisions :— 
1. A median portion, encircling the basal ganglia and peduncular mass as it leaves 
the cerebrum, forming the great limbic lobe. 
2. The more exposed hemispheric mass forming the outer and upper aspect of the 
brain corresponding to the parietal lobe of higher animals, and which I shall 
term, after Broca, the extra-limbic mass. 
The former, or great limbic lobe, is divisible into three portions— 
a. An upper arc corresponding to what has been called the convolution of the 
corpus callosum (gyrus fornicatus). 
b. A lower arc, also called the gyrus hippocampi. 
c. An anterior arc, formed by the olfactory lobe, which unites the upper and lower 
limbic arcs. 
Looked at from above, the brain of the Rat is heart-shaped, with the apex directed 
forwards. The hemispheres diverging behind reveal a minute, semi-translucent, pale- 
grey body, ovoid in form, projecting betwixt them, the pineal body, posterior to which 
the corpora quadrigemina appear. The surface of the extra-limbic mass is almost 
perfectly smooth, being marked only by a few extremely delicate venules. There is no 
distinct frontal, occipital, or temporal lobe differentiated from this extra-limbic mass, 
which must be regarded as entirely constituting a parietal lobe. We therefore find 
no trace of a fissure of Rolando, as is distinctly seen in Osmatic Gyrencephales 
(e.g.. Pig and Sheep)..]; We shall, however, retain the terms frontal and occipital 
* Includes the large majority of Mammalian brains. 
f The Cetacea, Amphibious Carnivora, and Primates. 
J Vide Plate 9, fig. 2, op. eit. 
