vn] Lhl>i<- Lobe and Olfactory Area 195 



SUMMARY AND CONCLUSIONS. 



(1) The parts studied in this section include all the constituents of Broca's " grande 

 lube limbique," viz. the olfactory lobe (excluding the olfactory bulb and peduncle), the whole 

 gyrus hippocampi (including the conm ainnionis and subiculum, the uncus and lobus pyri- 

 formis), the entire gyrus fornicatus, and other subsidiary structures. 



(2) Histology supports comparative anatomy in suggesting that, in the human brain, 

 the lobus pyriformis must be regarded as the principal cortical centre, although not the 

 si ilc one, governing the olfactory sense. Structurally the cortex of this lobe is not built 

 up on the usual plan, and its chief distinguishing features are (a) curious clusters or nests 

 of giant polymorphous cells which occupy a unique position close beneath the surface, (b) a 

 deep succeeding layer of pyramidal cells approximately equal to one another in size (S. Ramon 

 y Cajal's tassel cells), (c) a correspondingly peculiar arrangement of cortical nerve fibres, of 

 which the presence of projection bundles reaching right up to the zonal layer constitutes 

 a prominent feature. 



This type of cortex occupies a pear-shaped area practically corresponding with the lobus 

 pyriformis. It is a point of special phylogenetic interest that the rudimentary but constant 

 fissura rhinica constitutes an absolutely definite and fixed anterior boundary. Of other limits 

 the anterior division of the collateral sulcus is the lower one, and the lip of the fissura hippo- 

 campi, where the area merges with another type of arrangement, is the upper. 



In addition to the remarkable development of this lobe in osmatic animals the following 

 facts enhance its physiological importance : not only in the lower animals but in man also 

 it has been proved to be the terminus of the lateral olfactory root fibres ; in the developing 

 human brain it is one of the first, if not the first area to become medullated ; in the ape 

 and dog it has been shown that an experimental ablation of the lobe results in loss of 

 the sense of smell ; and, lastly, the balance of evidence derived from a study of natural lesions 

 of the lobe in the human brain supports experimental and other deductions. 



Ferrier's conclusion that common sensation is centred in the hippocampal region appears 

 to be unsupported. 



(3) On histological grounds it seems permissible to assign independence to the area of 

 cortex covering the wall and lip of the fissura hippocampi, a subdivision which receives 

 S. Ramdn y Cajal's acquiescence. Features peculiar to this cortex are the greatly-developed 

 zonal layer or lamina medullaris externa, the complicated arrangement of fibres in the deeper 

 parts, the existence of extensive islets of diminutive triangular cells situated immediately 

 beneath the zonal layer, and the deep subjacent layer of elongated large pyramidal cells, 

 which apparently differ from corresponding cells in the cornu ammonis and lobus pyriformis; 

 not to mention the absence of the superficial nests of large polymorphous cells found in the 

 lobus pyriformis. 



There is really no satisfactory evidence to guide us in a determination of the physiological 

 part played by this area ; that it has close internuncial relations with both the lobus pyri- 

 formis and the cornu ammonis is undoubted, but whether S. Ramo'n y Cajal is right in 

 regarding it as a tertiary olfactory centre the olfactory bulb and lobus pyriformis being 

 primary and secondary centres respectively or whether it is part of the gustatory centre, 

 remains to be proved. When we recall that the hippocampal fissure has extraordinary 



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