Histology of the Island of lie il. By Herbert C. Ma jor, M.D. 17 
my opinion, is the general plan and arrangement of the layers in the 
convolutions of the vertex, so as to be in a position to draw com- 
parisons between such arrangement and that found in the Insula in 
man and in the apes. 
The method of preparation of the brain tissue adopted in 
making these investigations, as well as the source from which 
sections of the normal Insula were obtained, should first be stated. 
With regard to the former point, it will be sufficient for me to say 
that Clarke’s method has been followed. The fresh method of pre- 
paration which my colleague, Mr. Bevan Lewis,* has done so much 
to improve and extend, is here, I regret to say, not available, or 
only to a very slight extent. The disadvantage of the fresh method 
is that by it the operator does not know exactly what he is in- 
vestigating — cannot, for instance, pick out the fourth cortical layer 
and examine it, excluding the admixture of other layers. Doubt- 
less this is a difficulty which patience and ingenuity will in time 
surmount, but at present it is fatal to such an investigation as the 
comparative structure of the cortical layers. 
The preparations which have served as my standards of com- 
parison for the healthy structure of the Insula were provided by 
the brain of a young man aet. 21, who was accidentally killed when, 
so far as could be ascertained, in a condition of full health. 
In a Thesis presented to the University of Edinburgh (1875), 
on the ‘ Histology of the Brain in Apes,’ I described six cortical 
layers as being the usual arrangement in the human brain. In the 
‘ Journal of Mental Science’ for January 1876, in a paper on the 
brain of the Chacma Baboon, I again showed that in the human 
subject the six-layer type of the cortex was the usual one. Now 
this is not the number as given by the majority of histologists, and 
it is necessary, therefore, that I should explain where it is that we 
diverge. 
An examination of the drawings I have given (Plates CLXXXV. 
and CLXXXVI.), and their comparison with that given by Meynert,! 
will make the point of divergence clear. It will be at once observed 
that with regard to the first, second, third, and fourth layers we 
are at one, but beneath the fourth layer Meynert figures one layer, 
while I give two ; and hence Meynert describes five layers, while I, 
following Baillarger, describe and figure six. Doubtless, modifica- 
tions in the general appearance of the layers are frequent. To 
illustrate this it is only necessary to pass successively in the field of 
the microscope, under a low power, the cortex of the summit of a 
gyrus and that in a sulcus, as figured in Plates CLXXXV. and 
CLXXXVI. Yet here is no alteration in the six-layer type, but 
merely a modification of it. 
* ‘ Monthly Micro. Journal,’ September 1876. 
t ‘ Strieker's Handbook,’ vol. ii. p. 2:14. 
c 2 
