176 Structure of Lobus Pyriformis [CHAP. 



to Ramon y Cajal, while they break up into collaterals in this superficial part of the cortex, 

 the impulses they convey proceed further to the deep pyramidal cells, via the long apical 

 shafts of these cells. Barker is of opinion that these fibres are principally destined for the 

 gyri semilunaris et circumambiens of Retzius, but he does not support his statement with 

 strong reasons, and as I shall show later he seems to overestimate the importance of the gyms 

 semilunaris. 



Ramcfa y Cajal attributes no importance to the second set of fibres above-mentioned, 

 nor does he consider the occasional middle root proceeding to the tuberculum olfactorium of 

 any consequence. In his opinion the lateral root is of main importance. 



Structure of tie Cortex covering the Lobus Pyriformis. 



The cortex of the Lobus Pyriformis is rightl}' regarded as the chief destination of the 

 lateral olfactory root, and although in its passage backwards this root first comes into relation 

 with the posterior olfactory lobule or anterior perforated space, we need not discuss this 

 crossway seriously, because in the human brain the surface is so broken up by entering 

 blood-vessels and a confusion of strands of fibres that it can hardly be described as cortex 1 ; 

 a little further back, however, where the root strikes the inner edge of the tip of the temporal 

 lobe, there exists a fissuret coupled with the name of Zuckerkandl and known as the " scissura 

 limbica," or " fissura rhinica." This fissuret will call for further remarks when we discuss 

 the comparative anatomy of this region ; here I desire to mention that for the histologist 

 it constitutes a most valuable and important landmark, for on studying the cortex of this 

 part in a series of transverse, or better still, horizontal sections, it may be seen that while 

 that anterior to the fissura rhinica resembles the common temporal type, that behind it is 

 entirely different, and in fact exhibits the type of formation which I now wish to describe in 

 detail. 



Type of Fibre-Arrangement. (Plate XVII, Figure 1.) 



The superficial fibreless layer calls for no special comment. 



The zonal layer is well-developed and composed of a dense band of fine and coarse 

 varicose fibres, and it is important to notice that it is joined by numerous fibres which are 

 continuous with the radiations of Meynert. We have seen fibres running into the zonal layer 

 elsewhere and referred to them under the name of fibres of Martinotti, but I may here affirm 

 that in no part of the brain are such fibres so numerous as in this region. 



In describing the cell lamination of this part I shall have occasion to mention that 

 curious groups or nests of cells arei to be found on the surface of the cortex at the point 

 where the plexiform and small pyramidal layers join, and in sections stained for nerve fibres 

 it may be seen that circular areas, sparsely supplied with fibres, mark the position of these 

 nests; it may further be noticed that the upward continuations of the radiations are forced 

 to circumvent these clusters in their passage to the zonal layer, and the resulting appearance 

 is very curious and unusual. 



1 This is the part which in certain lower animals stands out prominently and is called the tuberculum olfactorium ; 

 in such brains it has a more definite structure, being characterised, as Calleja and Ramon y Cajal have pointed out, by 

 curious islets of nerve cells, hut in man its structure is certainly rudimentary. 



