xm 



PHYLUM CHORDATA 



561 



The hippocampi extend along the whole length of the lateral ventricles. 

 The layer of nerve-cells in e'ach hippocampus gives origin, as in 

 Eutheria, to numerous fibres, which form a layer on the surface, 

 the alveus, and become arranged in a band the tcenia hippocampi. 

 In the Eutheria, as we have seen in the case of the Rabbit, the 

 tseniae unite mesially to form the body of the fornix (see p. 456). 

 In the Monotremes and Marsupials, on the other hand, there is 

 no such union ; the fibres of the taenia run towards the foramen of 

 Monro, where they become divided into several sets. Of these one 

 set, constituting the great majority of the fibres, pass into the 

 hippocampus of the opposite side, giving rise to a hippocampal 

 commissure (hip. com., cf. Figs. 946 and 982), the great development 

 of which readily leads to its being mistaken for a corpus callosum. 

 The fibres entering into the formation of this commissure corre- 

 spond, however, not to the fibres 

 of the corpus callosum, which 



cbl 



FIG. 1217. Brain of Ornithorhynchus 

 anatinus, dorsal view (natural size), cbl. 

 cerebellum ; olf. olfactory bulbs. 



FIG. 1218. Brain of Echidna aculeata, 

 dorsal view (natural size). 



is the commissure of the neo-pallium, but, as proved by their 

 mode of origin, to the fibres of the fornix, and they connect together 

 only the hippocampi, the fascia dentatce, or specialised lower borders 

 of the hippocampi, and an area of the hemisphere in front of the 

 anterior commissure (pre-commissural area) : they thus constitute 

 an olfactory or archipallial commissure, since all these parts belong 

 to the olfactory region or archipallium of the hemispheres. In 

 the Monotremes (Fig. 1215) the hippocampal commissure is only 

 very slightly bent downwards at its posterior extremity. In most 

 Marsupials (Fig. 1216) it bends sharply round posteriorly and runs 

 forward again, becoming thus folded into two layers, dorsal and 

 ventral, continuous with one another at a posterior bend or splenium, 

 similar to the splenium of the corpus callosum. The dorsal layer 

 of the hippocampal commissure becomes almost completely replaced 



