ORIGIN OF THE COEPUS CALLOSUM. 51 



Perameles nasiita at about the junction of body and descending limb of the lateral 

 ventricle. This corresponds to a situation almost immediately behind the commissures of 

 the liemispheres, as we shall see later. 



In this section the 'tween-brain — a somewhat square-shaped mass with rounded corners 

 — will be seen to occuj)y the ventral mid-region. The basal part of the thalamic region 

 lies upon the optic tract {o.tr.), which may be seen extending into the furrow between 

 the hemisphere (tlie region of the nucleus amycjdahv, n.a.) and the 'tween-brain. The 

 two halves of the ojitic thalamus are connected by means of a broad bridge of grey 

 matter — the commissura moUis (cm.) — and above and below the latter portions of the 

 third venti'icle may be observed. The small recess of the third ventricle lying above 

 the soft commissure is roofed by a somewhat dome-shaped epithelial fold, which is 

 attached on either side to a ridge containing a longitudinal band of meduUated fibres — 

 the sti'ia medullarls thalami (s.m.t.). 



Upon its lateral aspect the thalamic region fuses with the corpus striatum of the 

 cerebral hemisphere, and a mass of medulltited fibres, cut either transversely or ol)Iiquely, 

 indicates the place of union. The corpus striatum consists of two parts, which are 

 separated one from another by the internal capsule (ci.). A smaller grey mass, lying 

 above the internal capsule and projecting into the lateral ventricle, is the iiucleus 

 caudatus (n.c). A larger grey mass lying below the internal capsule is the nucleus 

 lentlcularis {u.L). The ventral part of the nucleus lenticularis [n.a.) is the homologue 

 of the grey mass known in human anatomy as the nucleus mnygdalce. It is directly 

 continuous with a cortical area of distinctive structure known as \hQ liyr'iform lobe [i^y.l.), 

 from which the nucleus lenticularis in its upper part is separated by a layer of meduUated 

 fibres (the external capsule, c.e.). 



Along its dorsal border the pyriform lobe is separated from the general cortex (which 

 Turner calls "pallium," p.) by the shallow rhinal fissure {f.r.). Above the corpus 

 striatum, where the lateral wall and roof of the ventricle are formed by this general 

 cortex ov pallium, the inner lining of the latter is formed by a mass of meduUated fibres 

 — the corona radlata {c.r.) of writers. If the cortex be traced in a mesial direction its 

 edge wUl be found to present those peculiarly distinctive features which w^e at once 

 recognize as hippocampus. Thus if the mesial waU of tlie hemisphere be traced down- 

 ward in the section (fig. 1, PL 15) we reach a deep indentation — the hippocamptal fissure , 

 and corresponding to this fissure a marked prominence into the lateral ventricle forming 

 the hippocampus {hip.). Below the hippocampal fissure the characteristic features of 

 the fascia dentata (fd.) (Avhich we shall consider subsequently) are immediately 

 recognizable. And extending into the deep depression between the nucleus caudatus 

 {n.c.) and optic thalamus is the prominent _^mir/ff (fi-), spur-like in transverse section. 

 [In order to avoid an luinecessary confusion in the figure, the choroidal fold of the 

 lateral ventricle which is attached to the fiml)ria has not been represented.] 



Having thus seen how the " hippocampal formation " constitutes the edge of the 

 cortex cerebri, I propose to carefully examine the series of changes whicli this peculiar 

 formation undergoes as we trace it forward in a series of coronal sections of the cerebrimi 

 of the bandicoot (Perameles). 



