458 PROF. 6. ELLIOT SMITH ON THE MORPHOLOGY OF THE 



ventricle. It is the rouuded buudle of nerve-fibres forming the aberrant commissure 

 lying' upon the epithelivim of the velum (c). 



If a series of coronal sections of the brain of Sphenodoii be examined, it will be found 

 that 01 the mesial structures only the lamina terminalis and that i:)ortion of the epithelial 

 roof -nhich lies in front of the commissiira aherrans stand in direct connection with the 

 cerebral hemis2)here. That portion of the roof -which is placed on the caudal side of the 

 commissiira aherrans, and which forms the dorsal sac (the so-called " Zii'belpolster " of 

 Edinger), is connected laterally with the dorsal lips of the optic thalami. In other words, 

 it forms the roof of the third ventricle, and is not directly connected with any part of the 

 cereb)'al hemisphere. This simple relationship of the epithelial dorsal sac to the optic 

 thalaini is shown in the accompanying diagram (fig. 3). 



kM 



A simple scheme to illustrate the relationships bf the epithelial roof (•' dorsal sac '") behind the 



commissura aberriDis. 



But the lateral connections of the forward continuation of this epithelial roof undergo 

 sudden and most significant changes immediately in front — i.e., on the cephalic side — 

 of the^ commissura aherrans. This fact (which, so far as I am aware, has never hitherto 

 been definitely formulated) might be diff'erently stated by saying that the commissura 



ABEKRANS INDICATES IN THE MESIAL PLANE THE CAUDAL LIMIT OF THE ATTACHMENT OP 



THE CEREBRAL HEMISPHERE to the rest of the neural tube. 



In endeavoiiring to arrive at some explanation of the meaning of tliis ambiguous 



cojmmissure, as I shall attempt to do later on in this memoir, its limbic situation will be 



fc»und to be perhaps its most significant feature. 



, Before investigating the relationships of the cephalic portion of the roof of the median 

 ventricle, it is expedient to study the relations of the lamina terminalis and the cerebral 

 liemisphere. 



The examination of a series of horizontal sections through the brain of any Reptile or 

 Amphibian will reveal the lamina terminalis as a narrow band connecting two large 

 vertical masses of grey substance, which I propose to call the ^' paraterminal bodies" 

 Each corpus paraterminale forms the inner wall of the anterior portion of the lateral 

 ventricle, and extends forward from the lamina terminalis as far as the olfactory bulb or, 

 in those cases in which the bulb is pedunculated, to the olfactory peduncle. These two 



Varge masses of neural substance are linked together by the lamina terminalis, which 



