38 
MR. W. BE VAN LEWIS ON THE COMPARATIVE 
that which we find in animals which rank highest in the Mammalian series. The 
nerve-cells are distributed either in five or six layers, the five-laminated type being as 
characteristic of a certain constant area as the six-laminated arrangement is of another 
region of the brain. As in Primates and Carnivora, we find a limited portion of the 
cortex organically specialised by its disposition into five layers, and a more extensive 
realm distinguished by a six-fold lamination, so here in the Ungulata the same charac¬ 
ters are impressed upon the cortex. A still more striking resemblance is presented by 
the character of the individual layers, which are not alone identical in number with 
those of the brain in higher groups of animals, but are likewise, to a great extent, 
identical in composition. Thus we find in the cortex of the Pig the first or pale 
external layer, the layer of small angular cells, the third or large pyramidal layer, the 
fourth layer of small angular and pyramidal cells (absent in five-laminated areas), the 
ganglionic series of cells, and, lastly, the sixth or spindle layer. Neither in the 
number of layers nor in the individual constitution of these layers must we expect to 
find any great distinctions between these distant groups of the Mammalian class. It 
is in the essential characters of the individual cells of these layers, in the relationship 
of these anatomical units the one to the other, and in their general distribution, that 
we detect divergence from the type normal to the higher Mammalia. These diver¬ 
gencies, we shall find, appear very strongly pronounced as further on we enter into a 
close examination of the constitution of each individual layer of the cortex. It is 
important that we bear in mind the homologous constitution of the cortical layers in 
distant groups of Mammals, and that we are alive to the fact that minute differences 
in elementary distribution may afford suggestive hints as to any existing analogy in 
functional endowments. It will be apparent, upon the most superficial examination of 
this subject, that it is of the highest importance that we define as accurately as possible 
the regions characterised by these diverse laminar arrangements; and the contrast 
established between the comparative area of these realms in different orders must be 
regarded as highly desirable. What is the relative area borne by the five- to the six- 
laminated region in individual groups of animals, and what is the relative extent of 
similarly laminated regions in different orders ? These are questions which require 
careful and deliberate consideration, and towards the solution of which the following 
o 
researches are devoted. As I have elsewhere indicated/' the great distinctions betwixt 
the two chief laminated types of the cerebral vault centre about the mid-regions of the 
cortex, and hence the ganglionic layer and the layer superimposed will invite in these 
investigations especial attention. Let it then be understood at the outset, that the 
ganglionic cells of the cortex are arranged upon two distinct and readily recognisable 
plans. These may be termed—1st, the grouped, nested, or clustered arrangement; 
2nd, the laminar or solitary arrangement. The first is typical of a more limited area 
of the cortex than the latter, and is, as far as my investigations lead me, especially 
characteristic of the motor area of Ferrier— at least, as regards Man, the Monkey, the 
* “ On the Comparative Structure of the Cortex Cerebri.” ‘ Brain,’ part 1, April, 1878. 
