The fornix is also composed of fibres which arise in the hippocampal region, and after pursuing a more or less antero-posterior course enter the nuclei of the corpus mamillare. In many animals the majority cross the middle line and end in the corpus mamillare of the opposite side. These last three commissural tracts (anterior commissure, psalterium, and fornix) belong mainly to the olfactory region of the hemisphere (rhinencephalon), and will be further noticed in dealing with that part.

The association-fibres which pass between different parts of the same hemisphere are either short or long. The short association-fibres (fibræ propriæ, Meynert; laminae arcuate gyrorum, Arnold) (fig. 322, a, b; fig. 323, s.) serve to connect adjacent convolutions, passing round below the grey matter at the bottom of the fissures. The long association-fibres are mostly collected into

FIG. 323.-DIAGRAM TO SHOW THE GENERAL COURSE OF THE ASSOCIATION-FIBRES OF THE CEREBRAL .HEMISPHERE. (E. A. Schäfer, after Meynert.)

8., short association-fibres, connecting adjacent gyri; f.l.s., fasciculus longitudinalis superior; ci., cingulum; fp., fasciculus perpendicularis; f.l.i., fasciculus longitudinalis inferior; f.u., fasciculus uncinatus; fo., fornix; fi., fimbria; v.d'A., bundle of Vicq d'Azyr.

definite bundles, which can be traced for a considerable distance between the bundles of commissural and projection fibres, or which run free for a certain part of their course. The principal bundles or tracts of long association-fibres are the following:

(a) Superior association-bundle (superior longitudinal fasciculus, fasciculus arcuatus, Burdach).-This consists of sagittal fibres which run below the grey matter of the convex surface of the hemisphere, between the frontal and occipital lobes, and between the external part of the temporal lobe and frontal lobe (fig. 323, f.l.s.).

Mott states that these fibres, without doubt, arise from the large pyramidal cells of the visuo-psychic area of the occipital lobe.

(b) Inferior association-bundle (temporo-occipital bundle, inferior longitudinal fasciculus). This is a bundle of fibres which lies close to the outer wall of the posterior and inferior cornua of the lateral ventricle and is usually stated to connect the temporal and occipital lobes (f.l.i.). But according to Redlich' and La Salle-Archambault 2 its fibres are projection-fibres and not association-fibres. (c) Anterior association-bundle (uncinate fasciculus).-Under this name is described a white bundle, seen on the lower aspect of the hemisphere, passing across the bottom of the Sylvian fissure at the limen insulæ, and connecting the frontal with the temporal lobe (f.u.). The fibres of this bundle expand at each extremity, and the more superficial of them are curved or hooked sharply between the contiguous parts of the two lobes-from which circumstance it has derived the name uncinate. Its fibres appear especially to connect the third frontal gyrus with the temporal lobe and with the anterior part of the limbic lobe.

(d) Cingulum (ci.).-This forms the principal association-bundle of the gyrus fornicatus, its fibres coursing immediately above the transverse fibres of the corpus callosum, from the anterior perforated space in front, round the splenium of the callosum behind, and in the gyrus hippocampi as far as its anterior extremity. Some of the fibres diverge, as they pass backwards, into the white matter of the hemisphere; these may be projection-fibres. The constitution of this bundle and its connexions have already been dealt with and will again be referred to (see p. 398).

(e) The perpendicular fasciculus (Wernicke), which runs vertically immediately in front of the occipital lobe and connects the inferior parietal lobule with the fusiform lobule (f.p.). According to La Salle-Archambault, some of its fibres pass also to the temporal lobe.

In most cases it is not yet possible to say at which end of an associationbundle is to be found the origin and at which the termination of its fibres, and it may be that some of the bundles which have been described as composed of association-fibres really contain projection-fibres, not continuous throughout, but turning towards the basal ganglia (see above, inferior association. bundle ').

FIG.

2

324. SECTIONS

1

OF CEREBRAL

CONVOLUTIONS. (After Baillarger.)

The parts are nearly of the natural size. 1, shows the layers seen in many parts of the cerebral cortex when carefully examined with the naked eye; 2, the appearance of a section of a convolution from the neighbourhood of the calcarine fissure, with the conspicuous white line of Gennari.

STRUCTURE OF THE CORTEX CEREBRI.

The grey matter on the convoluted surface of the cerebrum forms a continuous layer indistinctly divided into strata by interposed thin layers of paler substance.

On examining a section macroscopically (fig. 324, 1; fig. 335) we meet from without inwards -1. A thin coating of white matter, which in section appears as a faint white line, bounding the grey surface externally: it may be indistinct. This superficial white layer is not equally thick over all parts of the cortical substance, but becomes thicker as it approaches the borders of the convoluted surface; it is accordingly less conspicuous, or even indistinguishable, on some parts of the lateral convex aspect of the hemispheres, but is plainly visible on 2 Nouv. Iconogr. d. 1. Salpêtrière, 1906.

1 Arb. a. d. Neur. Instit. Wien, x. 1905.

3 Op. cit.

FIG. 325.-DIAGRAM SHOWING THE MOST COMMON TYPES OF CELLS OF THE CEREBRAL CORTEX. (From Barker's 'Nervous System,' after Starr, Strong, and Leaming.)

a, b, c, cells of Cajal in plexiform layer; d, e, small pyramids; f, medium pyramids; g, g, large pyramids, with axons passing to white matter but sending collaterals towards surface; k, m, n, cells of Golgi's type ii., with short axons; m, a cell sending its axon towards the more superficial layers; 2, 9, polymorph cells with descending axons; r, r, afferent fibres passing into cortex from white matter.

the convolutions situated in the longitudinal fissure which approach the white surface of the corpus callosum, and on those of the under surface of the brain. It is especially well marked on the hippocampal gyrus and subiculum, and it

b

FIG. 326.-A PYRAMIDAL CELL OF THE CORTEX
CEREBRI OF THE MOUSE. (Cajal.)

a, basal dendrons; p, apical dendron ramifying near surface; e, axon; c, its collaterals; b, fibres of white matter of brain.

has been there described under the name of the reticulated white substance. 2. Immediately beneath the white layer just described is found a layer of grey or reddish-grey matter (superficial grey layer), the colour of which, as indeed of the grey substance generally, is deeper or lighter according as its very numerous vessels contain much or little blood. 3. A layer, appearing in section as a thin whitish line (outer line of Baillarger). 4. A second grey stratum (middle grey layer). 5. A second thin whitish layer (inner layer of Baillarger). 6. A yellowish-grey layer which lies next to the central white matter of the convolution (deep grey layer).

In the convolutions bordering on the calcarine fissure the outer line of Baillarger (here known as the line of Gennari) is very distinct, but the inner line of Baillarger is not visible (fig. 324, 2). In certain other situations also only a single white line is visible (see fig. 335).

Medullated fibres radiate from the white centre of each convolution in all directions into the grey cortex, having a course for the most part perpendicular to the free surface. In passing through the grey substance they are arranged in bundles, and separate the nerve-cells into elongated groups, giving the section a streaked appearance (figs. 338, 341). The direction of the fibres varies according to the part of the convolution in which they occur, whether near the summit or the base, and the radiating direction is somewhat lost in the sulci between the convolutions, where arched fibres which connect the adjacent convolutions tend to obscure the radiating bundles.

Cells. The cells of the cortex may be grouped under several heads:

1. Most numerous are the pyramids (figs. 325, 326), which are classified according to relative size as small, medium, large, and 'giant.' They have a conical or pyramidal cell-body, from the base of which a single axon, provided with several collaterals which ramify in the adjacent grey matter, extends towards the white matter. The dendrons are

of two kinds-viz. those which pass off from the base and sides of the pyramid, and the single large apical dendron. The former are relatively simple, ending in the grey matter not far from the cell-body. The apical dendron is large

a

b

d

FIG. 328.-SECTION OF PART OF THE THICKNESS OF THE POST-
CENTRAL GYRUS OF MAN, SHOWING THE LAYER OF GRANULES
WHICH SEPARATES THE LARGE PYRAMIDS INTO TWO ZONES,
SUPERFICIAL AND DEEP. (Cajal.)

a, b, c, d, granules (small nerve-cells) of various size and shape,
mostly stellate, but some (b) pyramidal.

FIG. 327.-SECTION OF POSTCENTRAL GYRUS OF MAN. Stained by Nissl's method. (Cajal.) 1, plexiform layer; 2, small pyramids; 3, medium pyramids; 4, superficial large pyramids; 5, small stellate cells (granules); 6 and 7, deep large and medium pyramids; 8, fusiform cells.

and appears as a peripheral extension of the cell-body. It varies in length with the position of the cell-body in the depth of the grey matter. In all cases it extends nearly straight to the surface, giving off only a few lateral