THE BRAIN IN THE EDENTATA. 313 



Behind the liippoeampal fissure, in the section we are considering, we find the general 

 cortex forming' the posterior lip ol" the fissure. If we trace this cortex toward the 

 hippocampus, we find that its superficial layer becomes directly continuous with that of 

 the hippocampus. The change from general cortex to hippocampus takes place opposite 

 the bend in the hippocampal fissure, and consists essentially in the levelling down of 

 the scattered cell-elements of the general cortex into a regular column of cells of peculiar 

 shape, which characterizes the hippocampal formation. We may call this region the 

 hippocampo-pallial junction. From this point the hippocampus proper extends forward, 

 its deeji surface coated with alveus forming the mesitil wall of the ventricle, its real 

 surface being in apposition Avith that of the fascia dentata. The hippocampus, still 

 maintaining these relations, extends forward, then curves around so that its "deep" 

 alveus-coated surface sweeps beyond the fimbria and thus makes its appearance upon the 

 surface of the hemisphere. In this manner the morphologically deep as^iect of the 

 hippocampus actually forms part of the surface area of the hemisphere ; in other words, 

 part of the hippocampus becomes completely invested, with its " superficial " area 

 excluded from the actual surface and its " deep " aspect exposed. This area of the 

 surface of the brain, which is placed between the fascia dentata and tlie fimbria, and 

 is covered by extraventricular alveus, may be termed the " hipjiocampus inversus." The 

 whole of the " morj)hological surface" of the hippocampus proper is hidden from view in 

 tliis section because it is submerged in the depths of the hippocampal tissui-e. 



After these considerations we may better appreciate the appearance of the mesial 

 surface of the hemisphere. 



In a mesial view of the hemisphere of Orycteropus (fig. 4) we see the fimbria beginning 

 below at a point just beliind the optic chiasma as a very narrow band and extending obliquely 

 upward and backward, then curA'ing upward and ultimately horizontally forward to the 

 situation of the psaltcrium ventrale. It rapidly increases in breadth during its course from 

 below upward. If, instead of examining this region from its lateral aspect, we look at it 

 from below (fig. 21), we find tliat for its upper half the fimbria follows a very oblique 

 course forward and inward, so that anteriorly it approaches close to the mesial plane, 

 and a large proportion of its fibres extend across to the other side of the Ijvain, thus 

 forming the 'psalterlam ventrale. The fimbria presents analogous features in all the other 

 Edentates, in common with most lowly mammals. Lying behind the fimbria, we find in 

 Orycteropus the area of inverted hippocampus which separates the fascia dentata from the 

 fimbria. As we trace these structures upward, we find that the area of inverted hipjDO- 

 campus rapidly tapers, and at the same time the fascia dentata In-oadens and approaches 

 the fimbria. The exact arrangement of the upper endings of these three bands will be 

 discussed after the inferior' endings are considered. In Orycteropus the inverted hij)po- 

 campus and the fascia dentata appear to end suddenly below in a deep arcuate furrow 

 (fig. 1) which marks the upper limit of the peculiar hippocampal tubercle, to which a 

 bj-ief reference has already been made. As this hippocampal tubercle exists among the 

 Edentata in Orycteropnis only, we may witli advantage consider the mode of termination 

 of the hippocampus in the other forms first. In all Edentates we find the three super- 

 ficial bands— ;;^"/«Z//7Vr, hippocanqjits inversus, and fascia dentata — which we have noted 



