ORIGIX OF THE CORPUS CALLOSUM. 57 



Figure 10 represents the appearance of a sagittal section through the corresponding 

 region in the brain of Phascolarctos clnereus, after stciining hy the Weigert method. 

 Between the small commlssiim dorsalis (c.n.) and the large elliptical commissura ventmlis 

 (c.v.) a large number of non-crossing fibres will be noticed in the " commissure-bed " and 

 "precommissural area." Of these the postcommissural form the columna fornicis (c.f.). 

 The precommissural area contains various sets of fibres. The largest group (a) connect 

 the alveus hippocampi and the basal region of the brain just behind the tuberculum 

 ■olfactorium (f.o.). In Oniif/ior/n/ncf/iis I distinguished these fibres as the " hippocampo- 

 basal association bundle" *. Other fibres (/3) pass backward below the ventral 

 commissure to enter the subthalamic region, wliere they become lost. Fibres entering 

 into the constitution of these tAVO series may be seen in the jjrecommissural area (in the 

 region marked jj.«.), proceeding from the most anterior i)art of the hijipocampus. Fibres 

 may also be seen proceeding directly from the olfactory peduncle (and probably bulb) 

 to enter the fascia dentata (f.d.). This is the olfactory bundle of the fascia dentata (y). 

 Other fibres belonging to the same series (o) may be seen proceeding backward above 

 the dorsal commissure as a well-defined tract, probably homologous with the nerve- 

 fibres of the stria medialis Lancisli of the higher mammalian brain. 



In the Marsupial, a typical hippocampal formation lies upon the dorsal aspect of the 

 commissura dorsalis. In tbis hippocampal formation there are two important series of 

 longitudinal fibres, Avhich are displayed most clearly in a horizontal section. Such a. 

 section of the cerebral hemisj^here of Trlchosurns culj)ecula is represented in fig. 11. 

 In this figure (as also in fig. 2) a large series of longitudinal association bundles are 

 visible in the stratmii lacunosum. Secondly, in the region of the subiculum (and 

 possibly in other regions of the hippocampus) large numbers of longitudinal fibres are 

 contained in the alveus, but in transverse section are not recognizable in the mass of 

 oblique fibres. 



Upon the ventral aspect of the hippocampus in its anterior part we have had occasion 

 to refer to a large mass of grey matter, the anterior paired region of which we have known 

 as the " corpus prsecommissurale," and its posterior region as the " commissure-bed." 

 This region has been the great source of confusion to comparative anatomists, more 

 especially those working at rej)tilian and amphibian forms, and it therefore demands 

 a very careful study. Most writers call the whole mass the " septum." 



A study of figs. 5, G, and 9, which are all from Perameles, shows that the " commissure- 

 bed," or matrix of the commissures, extends across the median plane uninterruptedly, 

 becomes continuous upon its dorsal aspect with the hippocampus of both sides, and 

 upon its ventral aspect with that thin band {I.e.) of the lamina terminalis which extends 

 from the ventral commissure to the optic tract (o.tr.). It is connected also in the median 

 line above with the thin epithelial roof of the third ventricle (a structure not sho^^ n in 

 the figures). It is clear, therefore, that this matrix, which includes the two commissures, 

 occupies the situation of the dorsal part of the lamina terminalis, and in part bomids 

 the third ventricle anteriorly. 



* Journal of Anatomy and Physiology, vol. xxs. p. 480. 



