196 Summary and Conclusions [CHAP. 



phylogenic stability, being found in all mammalian brains from Ornithorhynchus to Homo, our 

 disappointment at being unable to say more about its function is accentuated. 



(4) The most important elements in the gyrus dentatus of the cornu ammonis are the 

 large pyramidal cells forming the prominent lamina, with which previous researches have 

 made us familiar. It is interesting to find that these cells extend forward into the hook 

 or uncus of the so-called uncinate lobe, and that in the posterior direction they are apparently 

 continued into the striae longituclinales mediales. 



Although possessing such a distinctive structure the physiological significance of the cornu 

 ammonis is but little understood. Some credit it with a gustatory function ; others regard 

 it as an additional, tertiary, olfactory centre ; and Alexander Hill's observation that it is the 

 only structure which is wholly wanting in the anosmatic brain renders the latter the more 

 credible assumption. 



(5) The cortical elements recognisable in the striae longitudinales mediales, the gyrus 

 subcallosus, and the olfactory trigone are all of a vestigial character. 



(6) In addition to other peculiarities of fibre arrangement and cell lamination the cortex 

 of the gyrus fornicatus is remarkable for being completely destitute of fibres, and also cells 

 of large size. 



In two parts the structure diverges from the common type : 



(a) A strip of cortex, 5 to 10 mm. broad, bordering the posterior half of the corpus 

 callosum, exhibits a pronounced superiority in general fibre wealth ; and it is especially note- 

 worthy that the cortex of the callosal fissure in this situation shows a dense and prominent 

 zonal layer, which not only resembles but seems to be continuous with the lamina medullaris 

 externa of the fissura hippocampi. 



(6) Another strip of cortex, 5 to 10 mm. broad, investing the genu of the corpus 

 callosum, can be differentiated by the presence of elongated pyriform cells of medium size, 

 which occupy the position usually taken by the internal layer of large pyramidal cells, and 

 are remarkable in' possessing an affinity for methylene dyes far greater than that exhibited 

 by any other cells in the whole cerebral cortex. The lower extent of this area is really- 

 continuous with the gyrus subcallosus, and is placed significantly close to the inner root of 

 the olfactory tract, hence giving rise to the idea that this root establishes connections with 

 the chromophilous cells above mentioned. There are three facts which favour this assumption : 

 many comparative anatomists have emphasised the relatively great development of the gyrus 

 fornicatus in osmatic mammals ; Zuckerkandl noticed mal-development of this particular area 

 in cases of congenital absence of the olfactory bulb ; and thirdly Flechsig shows that it 

 is one of the early medullated fields. S. Ramdn y Cajal, however, shakes our assurance in 

 this belief by stating that the gyrus fornicatus apparently has no direct olfactory associations, 

 and, although he does not seem over-confident on this point, further investigation will be 

 needed before the function of the part can be considered settled. 



(7) This research does not deal with the internuncial connections of the various 

 constituents of the limbic lobe, but from numerous investigations which have been carried 

 out we learn that connections of a most intricate nature do exist, and these suggest that 

 before an olfactory stimulus can be received and converted into a conscious sensation it must 

 traverse a pathway of bewildering complexity. 



(8) Definite information concerning gustatory localisation in the cortex cerebri is wanting. 



(9) In the limbic cortex of the anthropoid ape human characters are closely followed. 



