394 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



This free border is rolled inwards like the letter $ throughout 

 the greatest part of the length of the cornu Ammonis. The 

 commencement of the inrolling is (fig. 236) the sigmoid con- 

 volution, or subiculum cornu Ammonis. The free margin of 

 the median cortex, reflected back to its point of origin, is 

 applied to it as the fascia dentata Tarini (Tar) (the rostrum 

 of the S). 



Between the subiculum (inner sharp edge of gyrus unci- 

 natus) and the fascia dentata is the continuous grey substance 

 of the inrolled layer (stratum convolutum, c c), the first fold 

 of which (next the subiculum), covered by the medullary 

 fibres of the trough-like lamina (alveus, Muldenblatt, A A), 

 projects on the inner wall of the descending cornu, as the 

 cornu Ammonis. The fibres of the membranous medulla of 

 the trough-like layer (alveus) collect themselves into a band 

 (F\ the fimbria, the chief origin of the fornix. This is the 

 third of the four longitudinal elevations of the cornu Ammonis 

 that here appearing in section clearly develop morphologically 

 from the process of involution. The subiculum obtains a 

 plexiform medullary investment, analogous to the medullary 

 element in all other parts of the cortex, in the form of the far 

 thicker substantia reticularis (Arnold) (fig. 236, rt). This 

 medullary investment is developed also upon the region of 

 the stratum involutum, corresponding to the free surface of 

 the central cortex, which, as is clearly shown in fig. 236, 

 coincides with the involuted fold between the subiculum and 

 fascia dentata, and there forms the nuclear lamina medul- 

 laris (m). 



This involution of the cortex in a transverse plane, forming the 

 cornu Ammonis, cannot, it is obvious, correspond to a longitudinal 

 fold, the so-called hook (see p. 392). This illusory form arises from the 

 circumstance that at the most anterior extremity of the cornu Ammonis 

 the sigmoid-formed layer seen in fig. 236 spreads out, and only pre- 

 serves a slight undulation, the summits of the waves of which project 

 as the so-called claws (digitationes). Thus expanded, the free border 

 of the involuted layer (seen from within) projects beyond the subiculum 

 as the point of the hook, whilst the compressed amygdaloid nucleus in 

 front of the cornu Ammonis (fig. 233, H A), rounds off the contour 

 to the apparently recurved structure of the hook convolution. 



