56 DK. G. ELLIOT SMITH ON THE 



commissura veiitralis (c.v.), oval in section and placed obliquely. Extending upward 

 from the ventral commissure there is a thick mass of grey matter — the "com- 

 missure-bed'" {s) — Avhich contains in its upper part the commissura dorsalis (c.D.), 

 shaped somewhat like an inverted obliquely placed V Avith the dorsal limb nearly 

 horizontal. 



Above the dorsal commissure the fascia dentata {f.d.) is easily recognizable and is 

 separated from the general cortex (jk) by the fissura Mppocamjpi {f.h.). Anteriorly the 

 hippocampal fissure gradually becomes shallower until it can no longer be traced, so that 

 the fascia dentata, the stratvim zonale of the hippocampus, which now of course lies upon 

 the surface (compare figure 8, hip.'"), and general cortex {p.) are indistinguishable. Upon 

 its ventral side the fascia dentata becomes directly continuous with the extensive pale 

 region which constitutes the " frecommissnral area" (p.a.). In the forward direction 

 the "• precommissural area " is directly continuous with the mesial wall of the extremely 

 short olfactory peduncle, which connects the hemisphere with the olfactory bulb {o.h.). 

 Upon its ventral side the " precommissural area " becomes continuous with the grey 

 boss generally known as the " tuherctihtm oJfactorinm " {t.o.). 



By means of this representation of the anterior portion of the mesial wall of the 

 hemisphere of Ferameles (fig. 9), and the semi-schematic representation of the corre- 

 sponding region in Dasyurus (fig. 8), we obtain a good general idea of the anatomy of the 

 commissural region. 



In Dasyurtis it will be observed that the dorsal commissure is not so distinctly 

 bilaminar as it is in Pe/'flrtK'/fs : and in Phascolarctos (PI. 16. fig. lOj it can hardly be 

 called bilaminar. Thus we have in the Marsupial all the intermediate stages between 

 the rounded dorsal commissure of Oriiithorhynchiis and Echidna and the distinctly 

 bilaminar form found in most mammals. There are certain important features of 

 this region which can only be demonstrated by sagittal section, which we will 

 consider almost immediately. But before leaving the consideration of figure 8, 

 I wish to call attention to the possible consequences of elongation of the commissura 

 dorsalis. 



The " commissura dorsalis " is contained in the mass of grey matter which I have 

 called the " commissure-bed," the separate anterior prolongations of which in the 

 two hemispheres are formed by the '^corpora j^'^ffcommissuralia'" — the grey masses 

 corresponding to the surface-region of each hemisphere, which is distinguished as 

 "precommissural area." Now this combined grey mass — " commissure- bed" and 

 " corpus prnecommissurale "■ — is separated from the general cortex (pallium) above it by 

 tlie hippocampus ; and the dorsal limb of the commissure separates the " commissure- 

 bed " from what we may call the " supracommissural hippocampus," while the " corpixs 

 prsecommissui'ale " becomes directly continuous with wdiat we may call tlie " pre- 

 commissural hippocampus." Now it is clear that if the dorsal commissure elongates in 

 the forward direction it must cut otf jiart of this " precommissural " body from the 

 precommissural hippocampus, which becomes " supracommissural." The structure 

 lying upon the ventral aspect of such an elongated dorsal limb of the commissura 

 dorsalis must be either " commissure-bed " or " corpus pra^commissiu'ale." 



