Herrick, Cerebrum of the Opossum. 1 1 



with the roof of the thalamus. The splenialis fissure grows deeper 

 and curves ventrad, finally curving ventro-mesad (Plate A, Fig. 5.). 

 Meanwhile, the medullary fibres of both the fornicate and uncinate 

 gyrus accumulate to produce the cephalad parts of the fornix. The 

 relations remain nearly the same throughout the posterior parts of the 

 cerebrum though the curvature of the hippocampus about the thalamus 

 causes the transverse sections to fall in other planes. The longitudin- 

 al sections of Plate B show that the relations between the two gyri are 

 maintained to the very subiculum. 



The cellular structure of the hippocampus bears out the distinc- 

 tions already made. The cells in both are densely clustered and form 

 practically but a single series. The peripheral processes are from the 

 apices of the elongate fusiform or spuriously pyramidal cells. The 

 entad extremity is frequently almost equally prolonged. The nuclei 

 are large and clear. The cells of the fornicate gyrus are nearly twice 

 as large as those of the uncinate and the latter also lie in a different 

 plane, a fact due to the faulting or rotation of the gyrus as a whole. 

 The structure of the hippocampus and its great complexity in other 

 mammals is due chiefly to the flexture and caudad thrust of the 

 callosum. 



For a very complete historical account of the hippocampus and 

 related structures see the works of Honegger ^ while the most recent 

 work on the histology is that by Sala.^ 



After the painful attempt of Honegger to bring into harmony the 

 infinitely diverse nomenclature of this subject we can but feel that a 

 strict morphological terminology best meets the case. Embryology 

 and comparative anatomy leave no doubt that we are dealing with two 

 folds of the caudo-mesal cortex which are simply plicated and then 

 curved upon each other. One margin of the structure is connected 

 directly with the occipital cortex, the other with the tela and proplex- 

 us. The mesal convolution is the fascia dentata or gyrus uncinatus 

 produced, the lateral convolution is the gyrus fornicatus. The alveus 

 is the ental fibre zone of the gyrus fornicatus and the fimbria is the 

 transition of the alveus into the fornix and hippocampal commissure. 



1 J. Honegger. Vergleichend-anatomische Untersuchung ueber den Fornix 



und die zu ihm in beziehung gebrachten Gebilde im Gehirn des Men- 

 schen und der Sangethiere. Genf. 1890. 



2 L. Sala. Zur feineren Anatomie des grossen Seepferdefusses. Zeitschrift 



ftuwissenschaftlicheZoologie. LI I. I. 1891. 



