a

d

Other isolated grey nuclei lie in the white matter of the middle

lobe.

The grey matter of the cerebellum consists of two layers (fig. 237). The inner one (that next the white centre) is composed of a large number of very small nerve-cells (granule layer, d). The outer layer

(b) is thicker, and is formed of neuroglia, with rounded and angular small nerve-cells and neuroglia-cells scattered through it. Into its outer part processes of the pia mater conveying blood-vessels pass vertically, and there are also in this part a number of long tapering cells, somewhat like the Müllerian fibres of the retina. Lying

between the two layers of the grey matter is an incomplete stratum of large flask-shaped cells (cells of Purkinje, c). Each of these gives off from its base a fine process, which becomes the axis-cylinder of one of the medullated fibres of the white centre, while from the opposite pole of the cell large ramified processes spread out into the superficial layer of the grey matter, and some of these processes become connected with its small nervecells (fig. 238).

Structure of the cerebrum. The grey matter of the cerebral cortex is described as being composed of a number of layers, but they are not sharply marked off from one another. The following are usually distinguished (fig. 239).

1. Most externally a thin stratum containing a few scattered cells, probably neuroglia-cells. In the most superficial part of this layer immediately under the pia mater, is a very thin stratum of medullated nerve-fibres.

2. A layer of closely set small pyramidal nerve-cells several deep. This layer is also thin.

3. A thick layer containing larger and longer pyramidal cells less closely set. These cells are largest in the deepest part of the layer; and in the region of the motor centres of the brain some of them are of very large size, and are collected into small groups.

4. A thin layer of numerous small irregular angular cells.

5. A rather thicker layer of small scattered cells, many of a fusiform shape. This layer lies next to the white centre. In the island of Reil this stratum is considerably developed, and is somewhat sepa

FASCIA AND FIMBRIA.

Gh, part of the gyrus hippocampi or uncinate convolution; Fd, fascia dentata, or dentate convolution; between them is the dentate fissure; Fi, fimbria, composed of longitudinal fibres here cut across; 1, 2, medullary centre of the hippocampal gyrus prolonged around the hippocampus, H, as the so-called alveus, into the fimbria; 3, layer of large pyramidal cells; 4, their processes (stratum radiatum); 5, reticular neuroglia (stratum laciniosum); 6, superficial medullary lamina, involuted around the dentate fissure; **, termination of this lamina, the fibres here running longitudinally; 7, superficial neuroglia of the fascia dentata; *, ring of small cells within this (stratum granulosum).

rated from the rest of the grey matter by a layer of white substance. It is known as the claustrum.

From the white centre bundles of medullated nerve-fibres pass in vertical streaks through the deeper layers of the grey matter, to lose

themselves amongst the pyramidal cells of the more superficial layers, with processes from which they are probably continuous.

In the hippocampal region a remarkable difference of structure manifests itself (fig. 240). The superficial layer of neuroglia and the white stratum, which overlies it as a thin band in other parts of the cortex, are in this region both very strongly marked (5, 6), the neuroglia layer having a very distinctly reticular aspect, and being in part beset with small cells. All the rest of the thickness of the grey matter appears to be mainly composed of, or at least to contain, long conical cells (3, 4), the distal processes or apices of which are prolonged into fibres which lose themselves in the superficial layer of neuroglia. The pyramidal cells rest upon the white centre, here known as the alveus (1), which is the part of the hippocampus seen within the ventricle, and which is prolonged externally into the fimbria (Fi), where its fibres become longitudinal in direction.

In the dentate gyrus (fascia dentata, fig. 240, Fd) the pyramidal cells are arranged in an irregularly radiating manner, occupying the centre of the convolution, and surrounded by a ring of closely packed small cells (*), external to these being the relatively thick layer of superficial neuroglia (7).

The olfactory tract is an outgrowth of the brain which was originally hollow, and remains so in many animals; but in man the cavity has become obliterated, and the centre is occupied by neuroglia, containing, however, no nerve-cells (fig. 241). Outside the central

neuroglia lies the white or medullary substance, consisting of bundles of longitudinal white fibres. Most externally is a thin superficial layer of neuroglia.

The olfactory bulb (fig. 242) has a more complicated structure. Dorsally there is a flattened ring of longitudinal white bundles enclosing neuroglia (1, 2, 3), as in the olfactory tract, but below this ring a number of layers are superadded as follows:

1. A granule layer (fig. 242, 4), characterised by the presence of a

third and lateral ventricles. The fourth ventricle is roofed over by a thin layer of pia mater, with projecting choroid plexuses, the under surface of which is covered by a thin epithelial layer continuous at the side with the ciliated epithelium of the floor.

A section taken just above the level of the olivary prominence will still show very much the same form and structural arrangements as that just described. The nucleus of the hypoglossal (fig. 230, n. XII.) is still visible in the grey matter of the floor of the ventricle, but the nerve which is now seen arising from the outer part of that grey matter is the eighth or auditory (VIII.), the bundles of which, as they leave the medulla, enbrace the inferior crus of the cerebellum (c.r.), which is now passing into that organ. The reticular formation still

FIG. 230.-TRANSVERSE SECTION OF THE UPPER PART OF THE MEDULLA

OBLONGATA. 1

py, pyramid; o, olivary nucleus; V.a. ascending root of the fifth nerve; VIII. inferior (posterior) root of the auditory nerve, formed of two parts, a, (striæ acustica), and b, which enclose the restiform body, c.r.; n.VIII.p. principal nucleus of the auditory nerve; n.VIII.ac. accessory nucleus; g, ganglion-cells in the root; n.f.t. nucleus of the funiculus teres; n.XII. nucleus of the hypoglossal; r, raphe.

occupies the greater part of each lateral half of the medulla between the grey matter at the floor of the fourth ventricle and the pyramids (py), and a small portion of the olivary nucleus (0) may still be seen, as may also the upward continuation of the grey matter of the tubercle of Rolando; this is intimately connected with some well-marked bundles of nerve-fibres, which are passing up to the pons to join eventually the root of the fifth nerve (Va).

A section through the middle of the pons Varolii (fig. 231) shows very much the same arrangement of grey and white matter as that