ISO Structure of the Lobns Pi/rf/ormis [CHAP. 



rudimentary fissura rhinica (present according to Zuckerkandl in 86 per cent, of human brains); 

 I may further mention that in man the homologue of a lobus pyriformia has long been 

 recognised ; and I would here say that the distribution of the histological area bearing the 

 type of cortex just described is closely determined by the extent and limits of this lobe, 

 but with two exceptions, the account of which must be prefaced by an anatomical explanation 

 (see figure 16). 



First, Retzius, of Stockholm, has pointed out that on the upper part of the human 

 pyriform lobe there exists a small, shallow and apparently insignificant sulcus of crescentic 

 form with the convexity pointing upwards ; this he names the sulcus semilunaris, the small 

 stretch of substance between it and the anterior end of the hippocampal fissure he calls the 

 gyrus semilunaris, and the larger, laterally-placed and more prominent part investing the gyrus 

 semilunaris he designates the gyrus circumambiens. Satisfied as to the constant presence of these 

 small gyri I have paid special attention to the microscopic characters of the part, and my 

 conclusion is that while the gyrus circumambiens forms a portion of the hippocampal area, 

 the smaller gyrus semilunaris is to be excluded. My reasons are as follows : its surface 

 structure is of quite a special kind and bears little resemblance to cortex ; the nerve fibres 

 it contains are arranged in all ways, chiefly in irregular whorls; the nerve cells are all of 

 small size, irregular in shape and scattered about in an aimless fashion, indeed they suggest 

 the appearance seen in the basal nuclei rather than that of cortex ; and lastly, none of the 

 superficial cell nests which we must regard as essential constituents of the hippocampic cortex 

 are discernible. On the whole I am inclined to believe that this gyrus semilunaris is nothing 

 more than the surface projection of the subjacent nucleus amygdalaris. 



The second exception to which I have to refer is the cortex of the uncus proper, and 

 by the uncus proper I mean the small, free projection known as the gyrus Giacomini, which 

 is divided from the gyrus circumambiens by the annular band of fibres representing the 

 anterior extremity of the gyrus dentatus (Luschka, Giacomini). This free projection contains 

 cells like those to be described in the cornu ammonis, and hence has to be excluded from the 

 area under consideration. 



So much then for the anterior extent of this area : let us next turn to the lower border. 

 In describing this, notice has to be taken of some anatomical points bearing on the collateral 

 fissure ; and following those who recognise three different constituents in this fissure, an anterior, 

 a middle, and a posterior portion, I would state that the anterior portion, or rather that 

 part of it which lies below the lobus pyriformis, is an important and constant boundary for 

 our area ; behind this the lower limit gradually slopes upwards, and while at first the 1 mi- 

 may touch the middle division of the collateral fissure it soon leaves it to pass obliquely 

 across the gyrus hippocampi, and making for the anterior extremity of the calcarine fissure 

 it is there arrested by coming into contact with the visuo-psychic area ; or the area may be 

 described as coming to a point and terminating on the isthmus of the gyrus fornicatus. 



As to the sharpness of this boundary, I may say that the superficial islets of large 

 stellate cells lose in distinctness, the arrangement of fibres begins to assume a common 

 temporal appearance, and the chromophilic deeply-placed cells undergo a reduction in number 

 as the lower border is approached ; hence the change from the hippocampal to the common 

 temporal type is not abrupt. 



It only remains for me to describe the extent of this type of cortex in the upward 

 direction, that is in the direction of the fissura hippocampi. The determination of this limit 



