66 



DR. G. ELLIOT SMITH ON THE 



cerebra as those of the Iguana, Ornithorhynclnis^ Phascol- 

 arctos, Perameles, Nyetophilus, Dasypus, and Talpa, with 

 more highly-developed forms. 



In this process of forward extension the dorsal com- 

 missure has carried forward its oAvn matrix, derived from 

 the original " commissure-bed." It is obvious that if this 

 takes place withovit a corresponding growth of the bridge 

 of grey matter {s) which lies between the dorsal and ventral 

 commissiu'es, we shall have pai't of the great longitudinal 

 fissure (botmded on either side by the precommissural area) 

 roofed in by the corpus callosum (contained, of course, in its 

 proper matrix). This roofed space will be bounded posteriorly 

 by the ventral limb of the " commissura dorsalis." This 

 enclosed part of the great longitudinal fissure is the so- 

 called "fifth ventricle^'' or " cavitm septi pellucUli." 



By means of the accompanying five schemes (in which the 

 commissure-bed is shaded) T have graphically represented 

 different stages in this process. From these schemes it 

 will be readily recognized how the backward extension of 

 the dorsal commissure (fig. 8) stretches not only the matrix 

 in which it lies but also the sheet of the " precommissiiral 

 body " {p.a.) which fills up the large angle between the two 

 limbs of this commissure. Thus a portion of the pre- 

 commissural body of each hemisphere becomes one of the 

 leaves of the septum pellucidura. In liis valuable memoir 

 Paul Martin speaks of a fusion taking place (in the brain 

 of the cat) between the opposed walls of the hemisphere 

 on the ventral side of the corpus callosum. In tliis manner, 

 in the more highly-developed macrosmatic Eutherian 

 cerebra, the massive corpora praecommissuralia below the 

 corpus callosum may meet and fuse in the median plane 

 so as to obliterate the cavum septi. But the commissure 

 itself is not situated in anv such secondary fusion of the 

 hemisphere walls, so that it does not aff'ect the main 

 argument advanced above. 



But though the coi-pus praecomraissui'ale thus takes an 

 important share in the formation of the septum pellucidum, 

 part of it always remains distinct in the higher mammalian 

 cerebrum as a vertical band, lying parallel to the lamina 

 terminalis and separating the latter from the general pallium. 

 This band is the "gyrus subcallosus " of Zuckerkandl. In 

 order to show the last stage which this process reaches, I 

 have represented in PI. 16. fig. 18 this region of the cerebmm 



/="/c. -?- 



Fie. 5. 



nc.e. 



Fic. 7. 



F'<^ 



Figs. 4-8. — Schemes to explain the 

 evolution of the " septum pellu- 

 cidum." 



Fig. 4 is a type of a Reptile. 

 „ 5 ,, „ Monotreme. 

 „ 6 „ „ Marsupial. 



