638 CEREBRAL COMMISSURES OF THE MAMMALIA, 



The ventricular aspect of the hippocampus is covered in its whole 

 extent by the alveus, a layer of fine medullated nerves, which 

 arise as axis-cylinder processes of the large pyramidal cells of the 

 hippocampus (especially those near the fimbria) and from the 

 polymorphous cells, which form the nucleus fasciaj dentatse. 

 " Collaterals " derived from the same sources also pass into the 

 fimbria to become "commissural" fibres. All these fibres converge 

 towards the thickened lamina terminalis, which is the homologue 

 of the sept^im lucidum of higher Mammals. In order to reach 

 this point, above the foramen of Monro, the fibres coming from 

 the anterior extremity of the hippocampus are directed backwards 

 and inwards, and those coming from the posterior part of the 

 hippocampus are directed forwards and inwards with varying 

 degrees of obliquity. Since the greater part of the entire 

 hippocampal formation is placed behind the foramen of Monro, 

 including the whole of the descending horn, which alone is present 

 in Eutheria, there is, consequently, a large mass of fibres coursing 

 forwards and increasing as it goes, from fresh accessions of alveus 

 fibres. This mass of fibres becomes collected to form a distinct 

 ridge which constitutes the fimbria, which is mainly derived from 

 the descending horn. The fimbria, therefore, will vary in size 

 with the length and degree of development of the hippocampus 

 behind the foramen of Monro, more especially with the size and 

 extent of the descending limb of the hippocampus. In Platypus, 

 and to a less marked degree in Echidna, the descending part of the 

 the hippocampus is short, and in neither is the layer of alveus fibres 

 thick in this region. As a consequence neither Monotreme has a 

 well-formed fimbria. In all Marsupials, however, there is an 

 extensive part of the hippocampus lying posterior to the foramen 

 of Monro and a well developed descending limb, so that the cut 

 fimbria projects as a prominent spur in transverse section. Since 

 the hippocampus extends forwards beyond the foramen of Monro, 

 no part of the fimbria ever becomes free from the hippocampus to 

 form posterior pillars or body of the fornix as in placental 

 mammals. The fimbria, or its representative, lies in the whole of 

 its extent along the margin of the cortex, where that is formed 



