1374 



HANDBOOK OF PH^SIOLOON' 



NEUROPHYSIOLOGY 11 



FIG. 1. Drawing of a dissected 

 human hippocampus. The hemi- 

 sphere has been partially re- 

 moved, the midbrain is cut across 

 and the third ventricle exposed. 

 The dotted line represents the edge 

 of the tentorium vifhich, as shown 

 in the diagrammatic insert, 

 crosses at right angles to the 

 vessels which supply the hippo- 

 campus. In cases of temporal 

 herniation they may be occluded. 



Choroid br of post cerebal art 

 -Choriod plexus 

 Ant chor art 

 Post cerebral art. 



Edge of -- 

 tentorium 



/ 



Anterior choroidal art 



X 

 Qerebral peduncle 



\.Middle cerebral, ort 

 Momfn^llary body 

 Anterior' commissure 



V 



-Corpus collosum 



Anterior cerebral art. 

 Ltic chiasma 



Posterior cerebral art. 



Phytogeny 



The hippocampal primordium (which becomes 

 Ammon's horn) and the piriform cortex comprise the 

 bulk of the cerebral hemisphere in amphibians and 

 other lower vertebrates. Only in amphioxus is the 

 hippocampal primordium, together with the rest of 

 the cerebral hemisphere, wantina;. Thus, the hippo- 

 campal primordium and piriform cortex are the 

 cerebral hemisphere of primitive vertebrates and per- 

 form whatever cortical functions are within the ca- 

 pabilities of these animals. 



The neocortex develops between the piriform 

 cortex and the hippocampal primordium and, conse- 

 quently, distorts the cerebral hemisphere. This process 

 is exaggerated by the growth of the interhemispheric 

 commissures. Of the four commissures, three are 

 found in the anterior wall of the developing neural 

 tube in front of the interventricular foramen of 

 Monro: the anterior coinmissure, the hippocampal 

 commissure, or psalterium, and the corpus callosum. 

 A fourth, the posterior commissure, lies caudally and 

 dorsally to the foramen of Monro. According to 

 Johnston (57), the anterior commissure lies just below 

 the remnant of the anterior neuropore (fossa tri- 

 angularis) and thus marks the rostral extremity of the 

 primitive neural tube. The anterior and posterior com- 

 missures are not greatly modified by the development 

 of the forebrain, nor do the)" modify the relationships 



ol the hippocampal primordium. The development of 

 the corpus callosum, on the other hand, between the 

 two neocortices, leads to considerable change in rela- 

 tionships. 



Much of our knowledge of the formation of the 

 hippocampus, as it exists in mammals, is due to 

 Elliot Smith (102-105). He regarded the primitive 

 hippocampus as being entirely supracallosal in po- 

 sition and accounted for the large infracallosal hippo- 

 campus (the dorsal hippocampus of our terminology) 

 as the result of the formation of the 'hippocampal 

 flexure,' caudal to the splcnium of the corpus cal- 

 losum. Such a concept leads to some difficulties in 

 interpreting the way in which the hippocampal com- 

 missure comes to lie infracallosally and yet remains in 

 contact with the liippocainpus proper which in 

 primates is pushed backward because of the growth of 

 the corpus callosum and eventually comes to lie almost 

 entirely in the temporal lobe. The difficulty is largely 

 resolved by Johnston's (56-58) concept that the sep- 

 tum, fornix and hippocampal commissure represent 

 part of the paraterminal body, a forward and anterior 

 extension of the hippocampal primordium lying in 

 front of the lamina terminalis. The paraterminal body 

 may be considered to be functionally related to the 

 hippocampal primordium, regardless of where the 

 exact boundary between the two exists. Such a con- 

 cept makes the various relationships of the hippo- 



