STRUCTURE OF THE CORTEX CEREBRI. 
45 
around the Sylvian fissure. To speak generally, then, it may be stated that the 
arrangement of the elements of the fourth layer in clusters is more especially character¬ 
istic of the cortex in— 
a. The great fimbic lobe (anterior half of superior arc). 
b. The frontal lobe. 
c. The first and second parietal convolutions. 
Embraced within this territory we have the third and fourth parietal convolutions 
lying above the primary parietal sulcus (inter-parietal) extending from the ascending 
parietal convolution to the opposite pole of the hemisphere. These convolutions, 
forming the greater mass of the upper and posterior aspects of the hemisphere, present 
us with an arrangement of the ganglionic series very distinct from that just dealt with : 
I allude to the solitary or laminar arrangement of cells. This mode of distribution is 
highly characteristic of this region. When referring to the transition realms of the 
cortex it was stated that the passage of one form into that of another is never abrupt; 
the change is a gradual one, and hence no exact boundary line can be drawn betwixt 
the clustered cells of the fourth layer in the limbic lobe and the solitary elements of 
the same layer in the adjacent parietal gyri. Vertical sections taken through these 
adjacent convolutions constantly exhibit the grouped arrangement overstepping the 
boundary of the crucial sulcus and spreading towards the summit of the uppermost 
parietal convolution, but thinning out into the stratiform or solitary arrangement 
beyond. On the other hand, numerous nests of these same elements occupy that 
aspect of the third parietal gyrus which dips into the inter-parietal sulcus. Bearing 
this fact in mind, we may state that the crucial and inter-parietal sulci roughly map 
out the boundary lines between tw r o highly characteristic formations—the clustered 
and the solitary ganglionic series. We will now deal with the several regions alluded 
to seriatim, and describe the local variations in the conformation of the mid-regions of 
the cortex in—1, the limbic lobe ; 2, frontal lobe ; 3, parietal lobe. 
1. The great limbic lobe. —We shall limit our examination to the antero-superior 
arc of this lobe, or the convolution of the corpus callosum, and first inquire into the special 
arrangement of the fourth layer of the cortex in that anterior portion which lies in 
front of the sub-parietal sulcus, and whose limits extend from the commencement of 
the crucial sulcus to the internal root of the olfactory nerve, fig. 1 (6-19). The super¬ 
ficial portion of this lobe exposed on the surface of the hemisphere is represented in 
the diagram as mapped out by several oblique lines into six divisions (fig. 2). The 
ganglion cells of the fourth layer are most densely grouped in the divisions B and C. 
At this site the nests are large, oval, and deep, and are rich in densely-packed cells ; 
the groups themselves are so numerous and closely set as to be almost confluent. 
Reference to the table of measurements will show us that the cells of this immediate 
locality attain the greatest average dimensions — viz. : 48/a X 17^, the largest cell 
observed being 69/a X 32/a. At D the dense confluent groups are again met with, 
but over a limited area, the outer aspect exhibiting chiefly distant or discrete clusters. 
