The hippocampus 



CHAPTER L\ I 



JOHN D. GREEN 



Di'partmint oj Analomy, University oj California at Los Angeles, 

 Los Angeles, California 



CHAPTER CONTENTS 



Connections of the Hippocampus 

 Terminology 

 Phylogeny 

 General Anatomy 

 Projections 

 Cortex 

 Thalamus 

 Other areas 

 Cell Terminations 

 Fimclions of the Hippocampus 



Theories of Hippocampal Function 



Spontaneous Electrical Activity and Relation to "Arousal" 



Mechanisms 

 Field Potentials and Unit Activity 

 Seizure Discharges 

 General Conclusions on Fimctional Role of the Hippocampus 



CONNECTIONS OF THE HIPPOCAMPUS 



Terminology 



The hippocampus has attracted the attention of 

 anatomists since Thomas Henry Huxley and Bishop 

 VVilberforce disputed its evolutionary significance, and 

 C^harles Kinajsley parodied them in The Water Babies. 

 Many anatomists have studied this functionally enig- 

 matic part of the brain, and in the confusion many 

 terminological disagreements have occurred. The gen- 

 eral location and contours of this region in man are 

 shown in figure i . Figure 2 shows many of the gross 

 anatomical features and some of the connections of 

 the hippocampus of the cat. To some, hippocampus 

 means the hippocampal gyrus, and to others the 

 archicortex which is folded into the lateral ventricle. 

 The latter sense is used here. Entorhinal cortex and 

 pyriform cortex will be used interchangeably to mean 

 the cortex lying on the ventral aspect of the temporal 



lobe medial to the rhinal fissure. The junctional 

 region between the entorhinal cortex and the hippo- 

 campus, lying on the medial aspect of the temporal 

 lobe beneath the choroidal fissure, is called the su- 

 biculum. The single-layered cortex which is enfolded 

 into the lateral ventricle is subdivisible into two 

 interlocking gyri formed by the hippocampal pyra- 

 mids and gyrus dentatus. Here it is collectively called 

 Amnion's horn or hippocampus. The term hippo- 

 campal gyrus will be avoided since most American 

 and British authors consider entorhinal cortex and 

 hippocampal gyrus to be more or less identical. By 

 hippocampal formation is understood Amnion's 

 horn (hippocampus plus gyrus dentatus) with its ad- 

 jacent and continuous regions of the brain, and its 

 chief afferent and efferent pathways. Grossly, these 

 structures include the subiculuni and entorhinal 

 cortex, the gyrus cinguli and amygdala, the psalterium 

 (hippocampal commissure), the septum lucidum and 

 the fornix. It also includes certain embryological 

 rudiments, the striae longitudinales and the indusium 

 griseum. The fornix is considered to comprise, first, 

 the postcommissural fornix extending to the mamniil- 

 lary body, .second, the precommissural fornix which 

 connects the hippocampus and the septum (within 

 which lies the related diagonal band of Broca and 

 radiation of Zuckerkandl) and, third, the dorsal 

 fornix, by which is meant the band of white fibers 

 extending rostrally above the septum lucidum and 

 beneath the corpus callosum, and grossly traceable 

 caudally toward or into the colonne horizontalc of 

 GerebtzofT (39) or'spheno-cornual' bundle of Ramon y 

 Cajal (90). The term dorsal hippocampus is used to 

 imply the dorsal subcallosal hippocampus seen in 

 many mammals, and the dorsal supracallosal hippo- 

 campus will be referred to as the indusium griseum 

 (gray matter) and striae longitudinales (white). 



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