1909.] 



Cerebral Cortex of Echidna, etc. 



117 



Fisstira ■vallaris (f call) runs parallel to the border of the hemisphere in 

 the anterior part of the brain. Its length is about 10 mm. Its posterior 

 end lies above the anterior commissure, its anterior end 5 to 6 mm. from 

 the frontal pole. 



Radial Fissu7'es. — Three or four may be present, of which the anterior may 

 be a median prolongation of fasa. The second is sometimes independent, 

 sometimes a prolongation of fasjx The third is generally very sharply 

 defined, and is always in continuity with fpsa ; it may have the appearance 

 of being connected with FH. The fourth is often very weak and is usually 

 independent. 



Elliot Smith says that the fissure ■x/r is the deepest and most constant of 

 the radiating sulci in the mesial surface of the brain. In most brains it 

 crosses the dorsal edge and joins /Q {fasj)) in the dorsal surface. 



Description of Five Distinct Types of Cell Lamination, with Notes as 



TO Their Distribution. 



The sections A, B, and C, diagrammatically represented in fig. 2, are 

 intended to illustrate the distribution of the various types of cortex in the 

 middle region of the hemisphere. The position of these sections which are 

 cut transversely is shown in fig. 1. This figure also shows the distribution 

 of the types of cortex on the surface of the brain. From it and from the 

 foregoing descriptions the manner in which the various fissures are cut may 

 be understood. 



Type I. □ □. 



This cortex measures only 1 mm. in depth, and is not very highly differen- 

 tiated. The molecular layer occupies rather less than O'l mm. It is succeeded 

 by a layer of broad and irregular rectangular or pyramidal cells, which 

 stretch downwards to a depth of 0"5 mm. While in the upper region of this 

 layer the cells are small- (15 to 20 (jl in length), they become considerably 

 larger (20 to 27 /u.) in the neighbourhood of its lower boundary, and are there 

 more regular in shape and arrangement. The rest of the cortex is made up 

 of small polymorphic cells. A few granules are found in the polymorphic 

 layer and in the lower part of the pyramidal layer. 



Type I may be found in the median wall of the hemisphere, lying between 

 the Fissura vallaris {F vail) and the Fissura hippocampi {FH). It extends 

 backwards some way behind the posterior end of the former. It may be 

 seen in practically the same position in sections A, B, and C. In section A 

 the Hippocampal fissure is quite shallow, but posterior to this it gets much 

 deeper, and as it deepens the cortex originally lying above it becomes 

 invaginated so as to clothe its upper wall. 



