528 THE NERVOUS SYSTEM. 
in the form of a delicate band, which crosses the surface of the recurved part of the 
uncus in a transverse direction, thereby constituting the frenulum Giacomini. 
The dentate fissure is a complete fissure, and the elevation on the ventricular 
wall which corresponds to it is called the hippocampus major. It begins behind 
the splenium of the corpus callosum, where it is continuous with a shallow part of 
the callosal fissure, and it proceeds forwards between the gyrus dentatus and the 
hippocampal convolution. Its anterior end is enclosed within the uncus. 
When coronal sections are made through the callosal part of the limbic lobe and the subjacent 
corpus callosum, the cortical gray matter is seen to be reflected from the bottom of the callosal 
fissure in the form of an exceedingly fine layer, which forms a thin coating for the upper surface 
of the corpus callosum. In the midst of this certain delicate strands of longitudinal fibres, the 
strie longitudinales, are embedded, and, with the gray matter associated with them, they re- 
present an aborted or vestigial convolution, termed the gyrus supracallosus. This gyrus is con- 
tinuous with the gyrus dentatus round the hinder margin of the corpus callosum. 
Otractory Lope.—The olfactory lobe is small and rudimentary in the human 
brain. It is described by His as consisting of a posterior lobule, which may be said 
to correspond with that part of the hemisphere which is named the locus perforatus 
anticus, and an anterior lobule, composed of: (1) the olfactory bulb and tract, with 
the two roots of the latter; (2) the trigonum olfactorium ; and (3) the area of Broca. 
The olfactory tract is a narrow, white, prismatic band, which expands anteriorly 
into a swollen bulbous extremity, termed the olfactory bulb. Both the tract and 
the bulb lie upon the olfactory sulcus on the orbital surface of the frontal lobe, 
whilst the inferior surface of the bulb rests on the cribriform plate of the ethmoid 
bone, and receives the numerous olfactory nerves which reach it through the foramina 
in that part of the cranial floor, 
Posteriorly, the olfactory tract divides into two diverging roots. The mesial 
root curves abruptly inwards behind the area of Broca, into which some of its 
fibres pass, and is continued into the extremity of the callosal gyrus. The lateral 
root runs backwards and outwards over the outer part of the locus perforatus anticus, 
and gradually disappears from view. In animals, in which the olfactory apparatus 
is better developed than in man, it may be traced into the uncinate extremity of 
the hippocampal convolution (Fig. 325, p. 439). 
The trigonum olfactorium or olfactory tubercle is the small triangular field of 
gray matter which occupies the interval between the roots of the olfactory tract 
at the point where they begin to diverge. Some fibres from the posterior end of 
the olfactory tract enter the trigonum, and, in certain cases, these constitute a 
more or less distinct middle root. 
The area of Broca lies in front of the curved mesial root of the olfactory tract, 
and is continuous with the commencement of the callosal gyrus. 
Rhinencephalon and Pallium.—The cerebral hemisphere is composed of two naturally dis- 
tinct parts—the rhinencephalon and the pallium—in addition to the corpus striatum. The 
rhinencephalon consists of the olfactory bulb and its peduncle, together with the anterior perforated 
space and anterior part of the uncinate gyrus, the gyrus subeallosus, septum lucidum, and hippo- 
campus with its adnexa. The pallium includes the rest of the hemisphere, exclusive of the corpus 
striatum (Elliot Smith). : 
There can be little doubt that the gyrus supracallosus represents a wasted portion of the 
hippocampus. In monotremes and marsupials the hippocampus occupies a corresponding position, 
but, with the greater development of the corpus callosum in higher mammals, atrophy and stretch- 
ing occur, and the structure is reduced to a vestigial condition (Elliot Smith). 
In their phylogenetic evolution the rhinencephalon and the pallium appear to develop more 
or less independently of each other. In certain cases the former atrophies, whilst the pallium 
attains a high degree of development (e.g. man, monkey, whale, etc.); in others the reverse 
development occurs (e.g. the hedgehog and many other animals), in which the rhinencephalon 
forms a large part of the hemisphere and the pallium is relatively small. 
CorPuUS CALLOSUM, SEPTUM LUCIDUM, AND ForRNIX. 
Corpus Callosum.— This is the great transverse commissure which passes 
between the two cerebral hemispheres. It is placed nearer the anterior than the 
posterior aspect of the brain, and it unites the inner surfaces of the hemispheres 
throughout very nearly a halt of their antero-posterior length. The corpus callosum 
