NERVOUS SYSTEM. VERTEBRATA. 151 



Upon the upper surface of the dorsal commissure there 

 is a narrow band of cortical substance, which is obviously 

 the cephalic prolongation of the fascia dentata. A study 

 of the mesial surface of the hemisphere shows this at a 

 glance ; and an examination o the coronal section confirms 

 the opinion that the structure in question is really the 

 fascia dentata. 



The caudal portion of this structure may be seen (behind 

 the rough area on the corpus striatum) occupying the 

 position in which we are accustomed to look for it in 

 the Eutherian brain. It appears to be depressed below the 

 level of the rest of the surface, because the neopallium 

 tends to bulge over the marginal hippocampal region. 

 The clearly-defined hippocampal fissure separates the fascia 

 dentata from the neopallium. But, unlike the arrangement 

 found in the higher mammals, the fascia dentata (with the 

 hippocampal fissure bounding it on its dorsal or peripheral 

 side) extends far forward above the dorsal commissure. 

 This explains the presence o the hippocampal formation in 

 the peculiar position in which it is seen in the coronal 

 section. There (fig. 42) we see the fibres of the small 

 dorsal commissure spreading out to form the ventricular 

 covering (or alveus) of the hippocampus, from which they 

 are wholly derived. By means of this section, it is easy to 

 correlate the appearance of the ventricular surface of the 

 hippocampus (seen in the last specimen, fig. 40) with the 

 mesial distribution of the fascia denteta (which is merely a 

 specialised marginal fringe of the hippocampal formation) 

 in the hemisphere of this specimen. It is now also possible 

 to understand how it is that the dorsal commissure is 

 derived from the hippocampus. (Elliot Smith, Journ. 

 Anat. & Phys. vol. xxxii. 1897, p. 32.) 



The hippocampal fissure extends only a short distance 

 beyond (i. e. in front of) the dorsal commissure, so that the 

 fascia dentata (of which it forms the dorsal boundary) 

 appears [in a spirit- preserved specimen] to blend with the 

 neopallium above it, and also with the precommissural area 

 below it. In a fresh brain the fascia dentata may, however, 

 be readily distinguished by its colour, and may be traced 

 forward into close proximity to the olfactory peduncle. 



