64 DE. G. ELLIOT SMITH ON THE 



uaaltered " postcommissural " segment. This indentation soon increases and becomes a 

 very extensive bend, the portion of the hippocampus which surrounds the posterior 

 extremity of the dorsal commissure being carried backward and greatly stretched (fig,. 2). 

 In this way the obliquely-directed commissure fi.rst of all pushes upward and backward 

 the hippocampus at the junction of supra- and postcommissural parts, stretchiug and 

 carrying back the part of hippocampus that invests it above the rest of tlie j^ostcommis- 

 sural hippocmnpus. The " postcommissural hippocampus " (fig. 3, hip) thus appears tO' 

 become subcallosal and to be separated in the vertical plane from the splenium of the 

 dorsal commissure {s})!) and the circumsplenial portion of the hippocampus {hip') by a 

 process of pallium (figs. 2 and 3, *) ujjon which Zuckerkandl has laid so much stress 

 under the confusing name " Balkenwiudvmg." (It will be remem1)ered that it was 

 the lack of this feature which, in part, led him to group a bat with Marsupials and 

 Monotremes). 



As a result of the operation of the above-mentioned factors, therefore, a somewhat 

 reversed S-shaped bending is produced in tlie hij^pocanipus at the jimction of its middle 

 and posterior segments. The upper part of the S with its concavity looking forward is 

 formed by the attenuated hippocampus (fig. 3, hip') surrounding the splenium (fig. 3, spl), 

 while the lower part of the S, which is convex in front, is formed by a " sul)splenial " 

 bending of a plumper hippocampal region, whicli I have distinguished as the " hippo- 

 campal flexure " {fl^c)- 



Throughout all these changes tlie main mass of the ventral limb of the dorsal com- 

 missure (CD.") has remained unchanged. 



In the Marsupial and in a large number of lowlier Eutheria the plump ventral limb 

 extends obliquely upwards to meet the dorsal limb in a thick splenium. But when the 

 dorsal limb extends further l)ackward, the main mass of the ventral limb is left in 

 its old position (fig. 2, c.d."), its postero-supcrior extremity becoming greatly stretched 

 and correspondingly thinned by tiie backwardly-extending corpus callosum. 



The fimbria maintains its position unchanged (figs. 1 and 2,_7^") and always lies upon 

 the posterior or inferior aspect of the corpus callosum. In its backward growth the 

 corpus callosum carries back its matrix or " commissure-bed," in which it always lies. 



But although the elongation of tiie corpiis callosum is most obvious in tlie backward' 

 direction, it also groA^ s forward towards the anterior extremity of the brain. 



In the Amphibia and certain Reptiles, Avhere the olfactory bulb is placed in front and 

 is not overlapped by the hemisphere, the " corpus prgecommissurale " extends forward as 

 a horizontal baud from the lamina terminalis to the mesial wall of the olfactory peduncle.- 

 It is bomuled upon its dorsal aspect by the homologue of the hippocampus. In the 

 Marsupials with small pallia the upper margin of the precommissural body is slightly 

 oblique (PL 15. figs. 8 and D). As the pallium increases in extent it bulges over the 

 olfactory bulb (figs. 1 and 2, p. 63) more and more, and consequently the " precommis- 

 sural " hippocam])Us {hip") becomes more and more oblique, because it always extends 

 towards the olfactory peduncle. Thus the hippocampus comes to bend downward iu; 

 front of the commissures. Now it is obvious that the dorsal limb of the commissura 

 dorsalis (fig. 1), if it extends forward, must indent this precommissural segment of the 



