of the anterior end of the hippocampal fissure. The caput gyri hippocampi does not reach the apex of the temporal lobe, being partly separated from it by the incisura temporalis above and the anterior part of the collateral fissure below. As a rule these two fissures do not meet, and the caput gyri hippocampi is joined to the temporal pole by a bridging convolution (gyrus rhinencephalo-temporalis anterior) of very variable breadth. Occasionally the anterior part of the collateral fissure is separated from the posterior part and joins the incisura temporalis to form the rhinal fissure which is very distinct in many apes. The surface of the caput is marked by a small depression to which Retzius has applied the term sulcus rhinencephali inferior. In addition to the inferior aspect of the caput gyri hippocampi, just described, there is an upper surface which is overlapped by the optic tract, anterior perforated spot, and frontal lobe. Upon this upper concealed surface Retzius has described two constant elevations-a median (gyrus semilunaris) and a lateral (gyrus ambiens), separated by the sulcus semiannularis. The caput gyri hippocampi is the cortical centre for smell and corresponds to the lobus pyriformis of osmatic mammals. The hippocampal gyrus is covered superficially by a well-marked reticular layer of white fibres (substantia reticularis alba, Arnold, fig. 259, s.r.a.). The part of the hippocampal gyrus which is adjacent to the gyrus dentatus and forms the lower lip of the hippocampal sulcus is termed the subiculum.

Uncus. This small process is crossed about its middle by a continuation of the gyrus dentatus, generally known as the band of Giacomini. The portion of the uncus behind this band forms the gyrus intralimbicus of Retzius; that in front is similar in structure to the caput gyri hippocampi (fig. 270).

The dentate gyrus (fascia dentata Tarini, figs. 258, 270) lies above the hippocampal gyrus, from which it is separated by the hippocampal or dentate fissure. It is a narrow convolution having a toothed or notched appearance at its free border: hence the name. Above and overlapping it is the fimbria, from which it is separated superficially by a shallow sulcus (fimbrio-dentate), narrow in front but broadening out behind, where it forms a triangular depression below the splenium of the callosum. The gyrus dentatus begins posteriorly just behind and above the splenium by a fine curved lamina (fasciola cinerea), which is continuous with the longitudinal stria (both lateral and mesial) of the corpus callosum. From this point it bends downwards, lying along the isthmus of the gyrus fornicatus and then along the upper border of the hippocampal gyrus; here the crus of the fornix passes to its upper border in continuity with the fimbria (see p. 314). Anteriorly it is continued into the cleft between the hippocampal gyrus and its uncus, where it becomes lost to view. But if this cleft be opened out the dentate gyrus is seen to exhibit a sharp curve within it, and in continuity with the curved end a greyish band emerges from the cleft and passes transversely over the uncus to disappear on the ventricular surface of that gyrus (Luschka, Giacomini).

The part of the gyrus dentatus which lies below the splenium is sometimes much more developed than usual, and shows strongly marked folds or dentations which recall the broad and secondarily convoluted gyrus dentatus which covers the under surface of the splenium in some mammals (G. Retzius).

The fimbria hippocampi (fig. 270) represents the white matter of the hemisphere, which here comes to the surface along the side of the dentate gyrus. It is continuous with the alveus which covers the hippocampus within the lateral ventricle, and this is continuous with and forms part of the central white matter. The fimbria itself, however, appears to be actually formed of fibres which are

prolonged into it posteriorly from the crus of the fornix; anteriorly it is continued into the white matter of the uncus. In section the fimbria appears as a somewhat club-shaped expansion of the alveus, exhibiting a hook-like lateral prolongation. This is the section of a thin lamina (tania fimbria) continuous with the epithelium covering the choroid plexuses which here invaginate the ventricular epithelium.

The only sulcus within the limbic lobe which remains to be described (the callosal sulcus and the fimbrio-dentate sulcus having been already noticed) is the hippocampal or dentate fissure. This begins as a shallow furrow just below the posterior end of the splenium corporis callosi, between the fasciola cinerea and the end of the gyrus cinguli; it then lies between the remainder of the dentate gyrus above and the isthmus gyri fornicati and hippocampal gyrus below, and becoming deeper as it passes forwards ends in the bend between the hippocampal gyrus and its uncus. This fissure causes the elevation of the hippocampus or cornu Ammonis in the inferior horn of the lateral ventricle (fig. 261).

The fornix (see p. 313) also belongs to the limbic lobe as above defined. It contains a bundle uniting the hippocampus with the olfactory lobe. This bundle, the olfactory bundle of the cornu Ammonis (Zuckerkandl), best developed in osmatic mammals, is contained within the body of the fornix. Traced forwards, it is found to leave the column to pass in front of the anterior commissure, and then to run in the subcallosal gyrus to the vallecula Sylvii. Here it divides into two parts, one (pars olfactoria) passing by the anterior circumference of the lamina perforata anterior to the inner root of the olfactory tract, and the other (pars temporalis) at the hinder border of the anterior perforated lamina to the point of the hippocampal gyrus. This is the band which has been described (by F. Arnold and Broca) as continuing the peduncle of the corpus callosum with the hippocampal gyrus.

In some mammals (e.g. monkeys) there is a band of grey matter running along the upper surface of the fornix, on each side near the middle line, wedged in between it and the corpus callosum. This closely resembles the supracallosal gyrus above the callosum (see below), and appears to be part of another rudimentary gyrus which may be termed gyrus infracallosus, or, including certain patches of grey matter which occur here and there in the substance of the fornix, gyrus fornicis.

The cingulum, which also belongs to the limbic lobe, is a tract of fibres running in the gyrus hippocampi and gyrus cinguli. The bundle has for the most part a longitudinal course within the white matter of these gyri. Beevor thought that its fibres connect the hippocampal and cingulate gyri with the cortex of the outer surface of the hemisphere. According to Flechsig and Cajal, however, the cingulum consists of projection fibres, and is not to be regarded as an association bundle. A bundle of fibres having a somewhat similar course in the anterior descending part of the gyrus fornicatus is usually described as part of this tract; but according to Beevor it is doubtful if these fibres can be considered to belong to the cingulum; at all events, their continuity with the remainder of that tract could not be traced. A fuller description of the origin and course of the fibres of the cingulum, based upon Cajal's observations, will be given subsequently (see p. 400).

Gyrus limbicus.-The longitudinal striæ of the corpus callosum (p. 310) belong to a thin lamina of grey matter which extends over the surface of the corpus callosum from the lower edge of the gyrus fornicatus, and is much better developed in osmatic mammals than in Primates (Valentin, Jastrowitz). The striæ, together with this grey matter, represent a degenerated convolution

(supracallosal gyrus, Zuckerkandl), which is continuous posteriorly with the fasciola cinerea-i.e. with the dentate gyrus. Together with another degenerated gyrus in front, which is represented only by the prolongation of the stria medialis (gyrus geniculi of Zuckerkandl), these form a rudimentary gyrus (bordering gyrus, gyrus marginalis, Germ. Randwindung), which is curved around the brain-stem and the central parts of the hemisphere, lying within and concentric with the larger and well-developed gyrus formed by the cingulate and hippocampal gyri. The lamina of the septum pellucidum, and the so-called peduncle of the corpus callosum (gyrus subcallosus of Zuckerkandl) also belong to this bordering gyrus, but have become separated from the supracallosal part by the development of the corpus callosum; and the gyrus infracallosus (where this exists) and the fornix and fimbria may also be considered to form part of it.

OLFACTORY LOBE.

Under this head are included the olfactory bulb, its peduncle (olfactory tract), the trigonum olfactorium, and the anterior perforated space. The other portions of the brain connected with the olfactory apparatus have already been described in connexion with the limbic lobe.

The olfactory bulb lies on the under surface of the frontal lobe, its anterior extremity being about 1 cm. behind the level of the frontal pole in man, although in many mammals the bulb extends forwards considerably in front. of the other parts of the hemisphere. It (fig. 259) is oval in shape and of a reddish-grey colour when viewed from the ventral aspect. It is nearly a centimetre long and about a third of this in width; from its posterior extremity the olfactory tract emerges. Its dorsal surface, which is in contact with the frontal lobe, is white and is directly prolonged into the tract. This surface is marked by a longitudinal ridge which fits into the anterior end of the sulcus olfactorius of the frontal lobe.

The bulb is rudimentary in man and other Primates, and in the seals, and is lacking in Cetacea, but in macrosmatic mammals (Turner), and in vertebrates generally, it is well developed and forms a distinct portion of the cerebral hemisphere, enclosing an extension of the ventricular cavity. In some mammals (e.g. horse) this extension remains throughout life in free communication with the anterior horn of the lateral ventricle; in others (e.g. dog) the communication is lost. In the human foetus of from two to four months it appears as a hollow projection of the fore-brain; but as the walls of this projection thicken by the development of nervous tissue within them, the cavity becomes gradually obliterated, and is ultimately entirely occupied by neuroglia (central neuroglia of the olfactory tract and bulb).

The olfactory tract (peduncle), sometimes erroneously spoken of as the olfactory nerve, is a band of white matter, flattened on the ventral aspect but ridged along the dorsal aspect (where it fits into the olfactory sulcus), and therefore triangular in section. It measures about 2 cm. in length and 24 mm. in breadth, being broadest anteriorly where it passes out of the bulb, and narrowing posteriorly, owing to lateral compression, until just before its termination, where it broadens to form the roots.

Trigonum olfactorium (fig. 299).-The olfactory peduncle joins the convoluted grey matter of the frontal lobe situated just in front of the anterior perforated spot. If the olfactory tract be thrown backwards it will be seen to pass into a small triangular area bounded at the sides by the bifurcated

end of the sulcus olfactorius, and having its base directed towards the anterior
perforated spot. This is termed the trigonum olfactorium or gyrus tuberis
olfactorii (Retzius). The two roots of the peduncle curve inwards and outwards
respectively into the grey matter which is directly continuous with the sides of
the gyrus tuberis olfactorii. The inner root (gyrus olfactorius medialis) is situated
behind the gyrus rectus, and is sometimes separated from it by a shallow fissure.
This grey matter is often termed the area of Broca. The fibres of this root curve
sharply round the posterior limit of Broca's area, and its fibres pass partly into
this, partly into the callosal gyrus. The outer and larger root (gyrus olfactorius
lateralis) is directed outwards towards the limen of the insula, and often
appears as though it passed on to the island. In reality, however, as is best
shown in the foetus, it turns sharply inwards and backwards towards the gyrus
semilunaris and gyrus ambiens of the caput gyri hippocampi (Retzius). The

roots of the olfactory peduncle are largely composed of grey matter, and hence
they are termed by Retzius gyri; the grey matter is traversed by the white
fibres of the olfactory peduncle, which are more or less distinctly visible
according to their distance from the surface.

The anterior perforated space is a nearly smooth depressed area of grey
matter which owes its name to the fact that it is perforated by numerous small
branches from the commencement of the anterior and middle cerebral arteries.
It is somewhat triangular in shape, and is bounded in front by the fissura prima
(His), which separates it from the trigonum olfactorium and the mesial and
lateral roots of the olfactory peduncle. Behind it is limited internally by the
anterior part of the optic tract and externally by the sulcus between it and the
projecting temporal lobe. On the outer side it is separated from the insula by
the external root of the olfactory nerve, and is concealed by the temporal lobe
which overlaps it.

If the optic commissure be turned inwards and backwards, a smooth whitish band will be seen to pass across the space immediately external to the optic commissure. This, which is known as the diagonal band of Broca, is continuous in front with the gyrus subcallosus, and behind joins the temporal lobe. In macrosmatic mammals there is a considerable swelling of the grey matter at the anterior perforated space, called the tuberculum olfactorium. According to Retzius, there is frequently to be seen in man an oval elevation on the anterior perforated space, which represents this tubercle. The grey matter forming the floor of the space is continuous above with the corpus striatum.

Rhinencephalon and pallium. Phylogenetically the olfactory is the first of the nerves of special sense to acquire a cortical centre, and this lies on the basal aspect of the hemisphere. With the gradual evolution of other cortical centres, the surface of the hemisphere becomes divisible into two main portionsa ventral, the rhinencephalon, and a dorsal, the pallium-separated by a longitudinal cleft, the rhinal fissure. Turner describes the rhinencephalon as consisting of an olfactory bulb, a crus, and a lobus hippocampi (natiform protuberance or pyriform lobe). As there are other parts of the hemisphere which belong to the olfactory apparatus, the rhinencephalon may be regarded as formed by the olfactory bulb and its peduncle, the trigonum olfactorium with the mesial and lateral olfactory roots or gyri, the anterior perforated spot and tuber olfactorium, the caput gyri hippocampi, the hippocampus (cornu Ammonis), gyrus dentatus and supracallosal gyrus, the fornix, and the septum pellucidum. The term 'pallium' includes all the cortex of the hemisphere which does not belong to the rhinencephalon. It is distinguished from the latter by its progressive development in the vertebrates and its tendency to become convoluted in the higher mammalia until in the Primates it attains a remarkable degree of complexity, in striking contrast to the rudimentary convolutions of the rhinencephalon. Elliot Smith suggests that, as part of the rhinencephalon is pallial, all those portions of the cortex which do not belong to the rhinencephalon should be termed the neopallium. T. H. Bryce divides the pallium into the rhinopallium and the neopallium.

1

DEVELOPMENT OF THE CEREBRAL FISSURES AND CONVOLUTIONS.

Soon after their formation from the anterior cerebral vesicle, as described in the volume on 'Embryology,' the cerebral hemispheres present the same general outline as in the adult, the ventral portions having already grown downwards and forwards, giving rise to the temporal lobes and the stems of the Sylvian fissures. The backward extension of the hemispheres by which the occipital lobes are formed is, however, at first faintly marked. At this stage each hemisphere, except at its base, is very thin, and a relatively highly developed choroid plexus protrudes into its cavity and is covered by ependymal cells continuous at the choroidal fissure with the wall of the hemisphere. Until the fourth month the hemisphere-walls remain thin and the ventricles are relatively large, a condition obviously favourable to the formation of folds of the entire thickness of the wall, so that fissures appear on its outer aspect, with corresponding projections towards the ventricular cavity. Such fissures have been described as of normal occurrence by numerous anatomists, and termed transitory or temporary, since they apparently disappeared as the cerebral wall thickened. As already stated, Hochstetter has shown that these so-called temporary fissures are the result of post-mortem changes, or imperfect methods of preservation, and this view has been confirmed by Retzius3 and Mall.'

1901.

'Notes on the Natural Subdivisions of the Cerebral Hemispheres,' Journ. Anat. and Phys. July

2 Bibliotheca Medica, Abtheilung A. Anatomie, Heft 2, 1898.

5 Biol. Untersuch, Bd. x. 1902.

American Journal of Anatomy, vol. ii. 1903.