98 Anatomy of the Nervous System 



as a whole is relatively poorly developed. Between the two 

 parts there is an almost contiiluous series of fibres connecting 

 the olfactory areas all along the hemisphere (Gurdjian). 



The arrangement of the tertiary olfactory cortex, hippo- 

 campus or Amnions horn {cornu Ammonis) is a little difficult 

 to understand at first, and as it is explained by the phylo- 

 genetic history of the hemisphere, a brief summary of the 

 latter is introduced here. 



The hippocampus is derived from what was originally the dorsal half 

 of the medial wall of the cerebral hemisphere. Ventrally, this region met 

 the dorsal edge of what was to become the septum, while dorsally, it 

 came into contact with the part which was to become the pyriforrn lobe 

 (Fig. 5). Between it and the last-mentioned part, the neopallium later 

 developed in the dorsal wall of the hemisphere, separating these two 

 olfactory regions. As the neopallium grew in extent longitudinally as 

 well as transversely, it pushed the posterior parts of the hemisphere 

 downwards, so that the originally straight primordium of the hippocampus 

 was bent down posteriorly, curving round until its posterior end pointed 

 antero- ventrally in the temporal region of the hemisphere (Figs. 6, 7). 

 At the same time, the transverse growth of the neopallium forced the 

 primordium of the pyriform lobe down laterally until it reached the ventral 

 position where we have seen it in the rat and finally the ventro-medial 

 location which it occupies in man. The hippocampal area, however, 

 could not be thus displaced and consequently became folded inwards so 

 that most of it lay at the bottom of a deep groove, the hippocampal fissure. 

 It appears in this condition in the marsupials, but in the placental mammals 

 the increasing pressure of the neopallium and the development of the corpus 

 callosum have resulted in the degeneration of the anterior and dorsal part, 

 so that only the posterior part which curves down into the temporal lobe 

 remains well developed (Fig. 8). The remainder is represented only by 



of the two hemispheres. There are also commissural and decussating 

 fibres from a pair of small bed nuclei of the anterior commissure and the 

 stria terminalis component which is mentioned on p. 95. 



Kappers, in his recent text-book, calls the anterior part of this 

 commissure pars olfactoria and the posterior part pars neocorticalis, main- 

 taining that the latter is made up of neocortical commissural fibres like 

 those of the corpus callosum. He describes four different kinds of fibres 

 in the pars olfactoria (one of these being the commissural bundle of the 

 stria terminalis), but denies that any of these arise in the olfactory bulb. 



