THE NERVOUS SYSTEM 1403 



fibres are prolonged as a narrow band of white matter, called the 

 fimbria, or taenia hippocampi, along the inner concave border of 

 the hippocampus major, to which it is attached, as far as the 

 uncus. As the two posterior pillars diverge from each other they 

 enclose between them a small triangular space on the under surface 

 of the corpus callosum posteriorly. This space is crossed by trans- 

 verse fibres, and is known as the lyra or psalteriion, from its 

 supposed resemblance to a lyre. The transverse fibres form a 

 commissure between the two hippocampi majores, and the lyra 

 is therefore known as the hippocampal commissure. Each lateral 

 half of the fornix is to be regarded as establishing a communication 

 between the hippocampus major, in which the majority of its 

 fibres originate, and the optic thalamus of the same side through 

 means of (i) the anterior pillar, {2) the corpus mammillare, and 

 (3) the bundle of Vicq d'Azyr. 



Development. — The fornix may be regarded as being developed from the 

 lamina terminalis. 



Anterior Commissure. — This is a round bundle of white fibres 

 which crosses the middle line immediately in front of the anterior 

 pillars of the fornix. Anteriorly its central portion is connected 

 with the lamina terminalis, and posteriorly the central portion 

 appears between the anterior pillars of the fornix, where it forms 

 part of the anterior boundary' of the third ventricle, and is covered 

 by the ventricular epend\Tna. On either side the commissure enters 

 the cerebral hemisphere, and divides into two parts, olfactory and 

 temporal. The olfactory portion is of small size, and enters the 

 corresponding olfactory tract. Some of its fibres serve to connect 

 the olfactory bulb of one side with that of the other side. The 

 other fibres connect the olfactory bulb of one side with the 

 temporal lobe of the opposite side. The temporal portion is of 

 large size, and its fibres disappear in the white matter of the 

 temporal lobe. 



The anterior commissure ser\^es to connect the olfactory bulbs 

 and the temporal lobes. 



Septum Lucidum. — ^This is a thin vertical partition which is 

 situated between the anterior comua of the lateral ventricles, as 

 well as between the front parts of the bodies of these ventricles. 

 It is triangular, being broad in front and narrow behind. Pos- 

 teriorly it is attached above to the imder surface of the corpus 

 callosum, and below to the upper surface of the body of the fornix. 

 Anteriorly it occupies the concavity behind the genu of the corpus 

 callosum, being attached above to the corpus callosum and below 

 to the rostnun of that body. The septimi lucidum is composed of 

 two delicate laminae. The outer surface of each lamina looks 

 into the corresponding lateral ventricle, and is covered by the 

 ventricular epend^Tna (epithelium). The inner surface faces that 

 of its fellow, a narrow interval, representing the fifth ventricle, 

 intervening between the two. Each lamina consists of white 



