60 DE. G. ELLIOT SMITH ON THE 



As we proceed forward the supracommissural hippocampus becomes more and more 

 rudimentary. Had we not been tracing- these structures in a series of sections, we 

 should never recognize in the few cells situated above the commissures in fig. 16 the 

 representatives of the fascia dentata {f.d.) and the rest of the hippocampus i])^)'.). 



If we were to examine a series of sections further forward we should find, after the 

 anterior extremity of the dorsal commissure had been passed, that at the junction of the 

 precommissural area and general cortex (pallium) a few small cells would be found to 

 represent the hippocampus which is present in the marsupial (fig. 7). 



Now that we have seen how the hippocampus is disposed in transverse sections, it 

 will complete oiu* ideas concerning the commissural region if we study its disposition in 

 sagittal section. The transverse sections wliich I have figured were those of Jliniojpterus. 

 The two series of sections of the brain of Nyctophilus, which would have served our 

 purpose even better, were not exactly transverse, and therefore might be confusing. 

 It may be remarked, however, that the " iion-hippocampal " element in the dorsal 

 commissure is even less developed in Nyctophilus than it is in the single specimen of 

 Minioptcrus which I have examined. I have made several series of sagittal sections 

 of the brain of Nyctophilus, and, for illustration, a section slightly to the side of the 

 middle line has been chosen in order to show the relation of th(; hippocampus 

 to the " splenium " of the dorsal commissure, A section passing through the mesial 

 plane presents no difi'erence in the appearance of the commissures from that presented 

 in fig. 17. 



In this section (fig. 17) we cannot fail to be struck with the resemblance to the 

 Marsupial. The relatively enormous size of the ventral commissure (c.v.) and the shape 

 and proportions of the dorsal commissure (c.D.) are strongly suggestive of the Metatherian 

 structures. Yet in no Marsupial are the two limbs of the commissure so broadly 

 expanded. For the ventral limb (c.D.") is nearly vertical, and the dorsal limb (c.D.') nearly 

 horizontal, so that the angle included between the two limbs is not far short of 00". Of 

 the two lim1)s the ventral is the larger. Both limbs become thicker as they are traced 

 towards their place of union, where they form a massive " splenium." Immediately 

 behind the splenium in this section we see the fascia dentata {f.d.) cut very obliquely, 

 and extending forward from the concavity of the pouch formed by the stratum 

 granulosum the layer of pyramidal cells of the hijjpocampus {hip.) immediately above the 

 dorsal commissure. 



The " commissure-bed " (s) enclosing both commissures fills up the great interval 

 between the two limbs of the commissura dorsalis and tlie commissura ventralis. 



In the dorsal limb of the commissura dorsalis (c.i).') I have represented by means of dots 

 the position which a study of transverse sections leads me to regard as approximately 

 those occupied by the commissural fibres which do not spring from the hippocampal 

 formation, i. e. by the fibres of the corpus callosum proper. From this we see that the 

 anterior extremity of the dorsal limb is almost purely " callosal " — if we may use such a 

 term. The transition from pvirely " callosal " to purely " hi2)i)ocampal " parts of the 

 commissure is a very gradual one, " hippocampal " and " non-hippocampal " fibres being 

 intimately mixed one with another at the place of junction. 



