54 DE. G. ELLIOT SMITH ON THE 



series may take place. The longitudinal series belonging to the margin of the general 

 cortex or pallium is the so-called cingulum of higher forms (figs. 6 & 11, cing., clngJ). 

 There are, however, definite bonds of union between hippocampus and general cortex 

 which are found in their junction. Thus from the stratum zonale and stratum lacunosum 

 of the hippocampus fibres extend not only into the " neutral ground," so to speak, of 

 the " subiculum cornu ammonis " but well into the molecular layer of undoubtedly true 

 cortex. This is a definite association bundle between hippocampus and pallium, which 

 may be called tractus hippocamfi ad pallium, (fig. 2,?/). 



As the hippocampal region is successively examined in a series of coronal sections, the 

 main features of the hipj)0campal formation itself remain imchauged for some time, but 

 its commissure — the commissura dorsalis — undergoes marked changes. 



Thus the depth of the commissure rapidly increases at the same time that the 

 prominence of the fimbriae is diminishing (fig. 3). The meaning of this is that an 

 increased number of fibres from the fimbria are crossing in the dorsal commissure. The 

 scheme (fig. 1) of the commissures as they are seen ia sagittal section explains the 

 meaning of these changes. 



In figure 5 a new factor for the first time becomes evident. Upon the lateral aspects 

 of the commissure, more especially upon the right side of the figure, a mass of grey 

 matter {s) will be observed to enclose the dorsal commis.sure and to become continuous 

 ventrally with a mass enclosing a second commissu.re — the commissura ventralis. This 

 grey mass enclosing the two commissures will for the jiresent be distinguished as the 

 " commissure-bed." On either side it will be noticed to become continuous by a narrow 

 bridge of grey matter with the corjms striatum (est.). In this narrow bridge there are 

 a number of longitudinally-coursing nerve-fibres (s.t.). These are the uncrossed fibres 

 of the stria ter'minalis, the other fibres of which may l)e seen entering the ventral 

 commissure {s.t!.) in figure 3 to cross to the other side. In figure 5 the dorsal 

 commissure has reached its greatest depth. 



Figure 6 shows the appearance which is presented by a section after all the 

 commissural fibres of the fimbria have crossed. The thin commissura dorsalis (c.d.) is 

 derived from the corresponding region of the hippocampal formation. The huge 

 commissura ventralis (c.v.) appears to become continuous with the capsula externa [ce.], 

 which, after being joined by fibres of the capsula interna (cL), becomes the corona 

 radiata. The dorsal and ventral commissures lie in the large mass of grey matter which 

 forms the commissure-bed. A thin lamina (?') of this grey mass may sometimes be 

 distinguished upon the dorsal as^jcct of the commissura dorsalis. This may be 

 distinguished as the " indusium vertim." 



The " commissure-bed " (s) contains large numbers of uncrossed fibres (d.f.), which are 

 derived from the alveus of the hippocampus (which is situated above them) and from 

 the fimbria. These fibres are divided into two groups by the ventral commissure. The 

 p)ostcommissural fibres (columna fornicis, c.f.) collect upon the dorsal aspect of the 

 ventral commissure and incline backward and downward to enter the thalamic region. 

 The precommissural fibres proceed downward and forward in front of the ventral 

 commissui'e. 



