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HANDBOOK OF l'HVSIf)LOC;"l' 



NElIRGPHVSIOLOm' 11 



(54) was led to conclude that the hippocampal 

 primordium served to correlate afferent impulses from 

 a variety of brain-stem visceral centers and to relay 

 them secondarily to the neocortex. The commissural 

 systems, developing late, distort the secondary con- 

 nections less than they distort the primary, and the 

 connections we see in mammals of this type are chiefly 

 the fibers of the temporoammonic tract (3, 4, 72, 73, 

 go, 91). The septum lucidum may be considered to be 

 the main remnant of the paraterininal body. It fol- 

 lows, therefore, that the brain-stem connections to and 

 from Amnion's horn enter through the septum 

 lucidum among the fibers of the fornix system. The 

 connections between the mammillary body and 

 anterior thalamus (tract of Vicq d'Azyr) is also 

 phylogenetically new and probai^ly is to be associated 

 with the further development of hippocamponeo- 

 cortical connections. 



The system of fibers which comprise the fornix and 

 psalterium may be considered, perhaps, without im- 

 mediate reference to the exact direction of the axons 

 within them. In an animal like the cat, when the upper 

 part of the cerebral hemisphere and corpus callosum 

 is removed so that the hippocampus may be observed 

 from above (as the floor of the lateral ventricle), two 

 prominent bands of white matter (the fimbriae) are 

 seen converging rostrally to lie above the septum 

 lucidum (see fig. 2). Lateral to these bands lie the 

 choroidal fissure and the caudate nuclei. Medially to 

 them lie the dorsal hippocampi which are separated by 

 a band of white matter composed of dorsally running 

 longitudinal fibers and more ventrally placed trans- 

 verse fibers. The longitudinal fibers which lie between 

 the hippocampi may be seen to consist of two parallel 

 bands which extend forward to blend with the fimbria 

 as it becomes the dorsal fornix above the septum 

 lucidum. These are the colonnes horiznntales. The deeper- 

 lying transv-erse fibers are the fibers of the psalterium. 



The bulk of the fibers of the fimbria do not join the 

 dorsal fornix but instead diverge in two bundles: a) 

 a compact bundle which dives deeply behind the 

 anterior commissure to pass through the hypothalamus 

 and eventually reach the mammillary body, the post- 

 commissural fornix, and h) a looser group of fibers, 

 the precommissural fornix, entering the septum and 

 passing in front of the anterior commissure. The post- 

 commissural fornix is probably almost exclusively 

 efferent (22). The precommissural fornix and dorsal 

 fornix are probably both afferent and efferent, the 

 latter being chiefly related to field hi (30, 52). Ac- 

 cording to Gerebtzoff (39), the colonne horizontale con- 

 tains fibers afferent to the hippocampus from the 



septum. The hippocampal commissure, presumably, 

 contains afferent and efferent fibers between the two 

 adjacent Ammon's horns, but it is possible that its 

 rostral part also contains fibers which cross to the 

 contralateral septum. Grossly, three general areas of 

 the hippocampal commissure may be recognized : a 

 fairly thick caudal bundle which is more or less 

 blended with the splenium of the corpus callosum; 

 a thinner middle segment consisting of string-like 

 fibers passing between the two hippocampi, giving 

 rise to the fanciful name of psalterium (a harp); and, 

 again, a large commissural mass at the rostral end of 

 the hippocampus connecting the two fornices. 



Besides the classic studies of Ramon y Cajal (90, 91 ) 

 and Elliot Smith (102-106), the reader interested in 

 afferent and efferent connections of the fornix and the 

 hippocampal commissure is referred to recent papers 

 by Nauta (82), Blackstad (17), Simpson (loi), 

 Daitz & Powell (30), Powell & Cowan (86-88), 

 Cowan & Powell (28), Gerebtzoff (39), Morin (79), 

 Allen (12), Sprague & Meyer (107), Guillery (52), 

 Fox (34, 35), and the review of Pribram & Kruger 

 (89). Figure 3, constructed on the basis of the electro- 

 physiological studies of Green & Adey (45) shows 

 brain areas affecting and affected by hippocampal 

 activity. 



Projections 



CORTEX. Connections with the cortex, as indicated 

 above, are established through the temporoammonic 

 tracts. They connect the entorhinal cortex to Am- 

 mon's horn, both via the gyrus dentatus and the 

 hippocampus itself. Ramon y Cajal (90) described the 

 spheno-occipital ganglion as the source of origin of the 

 axons of the temporoammonic tract. This is a special- 

 ized part of the entorhinal cortex, particularly promi- 

 nent in smaller mammals, from which these fibers 

 seem to rise but it is not strictly delimited. Although 

 experimental stimulation of the hippocampus (2, 5, 

 45, 51) indicates that evoked potentials can be ob- 

 tained from the entorhinal cortex, so far the anatomi- 

 cal pathways which might subserve these connections 

 have not lieen detected. Functional studies by Adey 

 et al. (5) in the Australian phalanger, a marsupial, 

 showed that stimulation of the entorhinal cortex 

 evoked responses in the hippocampus but only small 

 and irregular responses in the fornix despite the large 

 responses which could be evoked in the hippocampus 

 by stimulating the fornix. [See also the studies of 

 Green & Morin (50) in the guinea pig.] They beliese 

 that the dorsal hippocampus can exert a powerful 



