STRUCTURE OF THE BRAIN IN RODENTS. 
703 
He also casually refers to a peculiar constitution of the olfactory area, but enters 
into no detailed account of the same, stating that it still awaits a monographical 
exposition.* 
Since, in accordance with the rule which I find it essential to adopt as to what is to 
be understood by a divergence in the laminar type of the cortex, I should exclude from 
his list the third or Sylvian, the diversities of structural type embraced in Meyneet’s 
category will be four only. This, however, but very inadequately expresses the truth 
as regards the cortex of the Mammalian brain, for I find at least eight char act eristic 
cortical areas distinguished by their laminar structure. These we may enumerate as 
follows :— 
1 . Type of upper limbic arc. 
2 . Modified upper limbic type. 
3 . Outer olfactory type. 
4 . Inner olfactory type. "1 Comprised within the limits of the lower and 
5 . Modified olfactory type. J anterior limbic arcs.' 
6 . Extra-limbic hype. 
7 . Type of cornu ammonis. 
8 . Type of olfactory bulb. 
In enumerating these types I have distinguished them by terms having special 
reference to their regional distribution, not that I believe this to be the better method 
ultimately to adopt, but the simplest in our present state of defective knowledge upon 
these points. With advancing knowledge of the intimate structure and peculiarities 
of the various regions of the cerebral cortex it will be found advisable to term these 
varied types after some important and essential structural peculiarity. The same 
remark applies to the enumeration of the individual cortical layers which are usually 
denominated by their relative positions as first, second, or third layers, &c. This 
method cannot fail to mislead and introduce much confusion, since we are constantly 
meeting with intercalation of fresh layers or the absence of others, and as a result of 
this method the same denomination becomes applied to layers which are wholly different 
in constitution. It is, therefore, on this account necessary to term each layer after 
some characteristic structural feature, more especially one which finds its counterpart 
in all the various members of the Mammalian series, and to subordinate the numbering 
of layers to the expression of their relative depth. 
Upper Limbic Arc (Plate 49, fig. 1). 
Adopting the above considerations, I find it necessary to regard the cortex typical 
of the upper limbic arc as consisting of four well differentiated layers, which may be 
termed respectively— 
* “The Brain of Mammals,’* Stricker’s ‘Human and Comp. Histology,’ Sydenham Soc. Trans., 
vol. 2, p. 414. 
MDCCCLXXXII. 4 X 
