is marked dorsally by three or four flat transverse grey laminæ, with intervening sulci, which together constitute the lingula (fig. 39, Ing). This is continued laterally into the grey cortex of the cerebellum, while the subjacent white substance of the velum is in direct continuity with the central white matter of the cerebellum,

s.c.p

THE MEDULLA OBLONGATA, FOURTH VENTRICLE
AND MESENCEPHALON. (E. A. S.) NATURAL
SIZE.

The cerebellum and inferior medullary velum, and the right half of the superior medullary velum, have been cut away, so as to expose the fourth ventricle.

p.n., line of the posterior roots of the spinal nerves; p.m.f., posterior median fissure; f.g., funiculus gracilis; cl., its clava; f.c., funiculus Lg cuneatus; f. R., funiculus of Rolando; r.b, restiform body; c.s., lower end of the fourth ventricle (calamus scriptorius); 7, section of the ligula or tænia; part of the choroid plexus is seen beneath it; l.r., lateral recess of the ventricle; str, striæ acustica; i.f., inferior (posterior) fovea; s.f., superior (anterior) fovea; between it and the median sulcus is the funiculus teres; cbl, cut surface of the left cerebellar hemisphere; n.d., central grey matter (nucleus dentatus) seen as a wavy line; s.m.v., superior (anterior) medullary velum; Ing, lingula; s.c.p., superior cerebellar peduncle cut longitudinally; cr., combined section of the three cerebellar peduncles (the limits of each are not marked); c.q.s., c.q.i., corpora quadrigemina (superior and inferior); fr, frænulum veli; f, fibres of the fillet, seen on the surface of the tegmentum; c, crusta; l.g., lateral groove; c.g.i., corpus geniculatum internum; th, posterior part of thalamus; p, pineal body. The Roman numbers indicate the corresponding cranial nerves.

into which a pointed tent-shaped pro

jection of the roof of the ventricle extends (fig. 59, A, in longitudinal section). This projection is bounded below by the inferior medullary velum, which in like manner is prolonged from the white substance of the central part of the cerebellum. It is less easily displayed than the superior velum, being concealed by

ta

Fig. 40.- ANTERIOR BOUNDARY (FLOOR) OF THE FOURTH VENTRICLE. (E. A.S.)
NATURAL SIZE.

m.s., median sulcus; str, striæ acustica, marking the limit between the pontine part of the ventricle and the medullary part or calamus scriptorius; l.r., lateral recess; i.f., inferior (posterior) fovea; a.c., ala cinerea; t.a., trigonum acustici ; 8.f., superior (anterior) fovea, close to the lateral margin of the superior part of the ventricle.

a part of the cerebellum, which is attached to its under or posterior surface. It will be further noticed in the description of the cerebellum. Below the inferior velum the roof is formed by a simple layer of flattened epithelium covered by pia mater; but it is not quite complete, for there is a hole in it termed the foramen of Magendie a little above the place where the central canal opens out into the ventricle, and there are two other apertures in the epithelial roof in the lateral recesses just mentioned. At the sides and below, this layer of epithelium passes into continuity with the epithelium covering the floor,

but it is somewhat thickened by the addition of white nervous matter before reaching the lateral boundaries of the floor. This thickening is left as a slightly prominent and often ragged membrane when the epithelium of the roof of the

Fig. 41.-DIAGRAMS TO SHOW THE SITUATION OF THE CHIEF NERVE-NUCLEI IN THE MEDULLA oblongata
AND PONS NEAR THE FLOOR OF THE 4TH VENTRICLE. TWICE THE NATURAL SIZE.

A, from behind; B, profile view of the right half, the medulla and pons being supposed to be transparent. The efferent or motor nuclei are shaded with oblique lines, the afferent or sensory nuclei with dots. In A the efferent or motor nuclei are represented on the right side only, the afferent or sensory on the left. III, IV, oculomotor and trochlear nucleus; V.d, descending root of the fifth nerve; V. s, so-called sensory nucleus of the fifth; V.a, ascending root of the fifth; V.m, motor nucleus of fifth nerve; VI, nucleus of abducens; VII, nucleus of facial; n. VII, root of facial curving round abducens nucleus; VIII, inner or dorsal nucleus of auditory; VIII', outer or ventral nucleus of auditory; IX, X, vago-glosso-pharyngeal nucleus; n.a., nucleus ambiguus, accessory or efferent vagoglosso-pharyngeal nucleus; XI, nucleus of spinal accessory; XII, nucleus of hypoglossal; XII, issuing roots of hypoglossal.

ventricle is torn off with the pia mater. It commences at the apex of the clava, and accompanies the lateral boundary for a short distance; then turns over the surface of the restiform body and terminates close to the place whence the roots of the vagus and glosso-pharyngeal nerves issue. It is termed the tania or ligula (fig. 44, t), and its upper transverse part forms the lower boundary of the lateral

VOL. III.

E

recess of the ventricle. Another thickening of the roof is seen at the apex of the ventricle covering the point of the calamus scriptorius: this is named the ober.

Two longitudinal vascular inflexions of the pia mater, known as the choroid plexuses of the 4th ventricle, project from the roof into the cavity, one on either side of the middle line, covered everywhere, however, by the epithelium of the roof. Offsets from these pass also into the lateral recesses, from the apices of which they emerge, encircled by a duplicate of the ligula, which was termed by Bochdalek the cornucopia. The epithelial layer of the roof of the ventricle follows all the convolutions of the choroid plexuses, but is nowhere pierced by them; it is generally described as the epithelium of the plexuses.

The floor or ventral boundary of the 4th ventricle is marked at its widest part, ie., at the level of the lateral recesses, by some transverse white lines, which cross the grey matter of the floor, and are known as the striæ medullares seu acustica (figs. 39, 40, 41, str).

These striæ are caused by bundles of white fibres which emerge from the raphe, within which they can be traced to the ventral surface of the bulb, and curve outwards over the restiform body, where they are usually described as passing into the lateral root of the auditory nerve. It is certain, however, that this is not the destination of most of the fibres of the medullary striæ, which are distinctly to be traced to the flocculus of the cerebellum (fig. 49, p. 59). They vary greatly in development even upon the two sides of the same brain (compare especially Bechterew, Neurol. Centralbl., No. 10, 1892).

One bundle of these stric is sometimes seen, usually on one side only, taking a course obliquely upwards and outwards, passing at the lateral boundary of the ventricle into the middle cerebellar peduncle (aberrant bundle of striæ medullares, Klangstab of Bergmann).

The floor of the ventricle is bisected by a slight median groove. A little on either side of this groove and immediately below the striæ medullares, is a small triangular depression (inferior fovea, fig. 40, i.f.), the apex of which extends only as far as the striæ, but the base is prolonged into two grooves extending one from each angle. The inner of the two grooves passes with a slightly curved course towards the point of the calamus scriptorius, and thus cuts off a pointed triangular area, which is bounded mesially by the median sulcus, and the base of which is turned towards the striæ acustica. This area (trigonum hypoglossi) is slightly prominent, and constitutes the lower end of the fasciculus teres; in it is the prolongation of the tract of nerve-cells from which the roots of the hypoglossal nerve take origin. The outer of the two grooves passes downwards with a slight outward obliquity nearly to the lateral boundary of the ventricle, and marks off externally another triangular area (trigonum acustici, t.a.), the base of which is also directed upwards, where it can be traced into a prominence (best marked in children) over which the striæ acusticæ course. To this prominence the name tuberculum laterale seu acusticum has been applied, since the main part of the auditory nerve arises in connection with it and with the triangular lateral area below it.

Included between the two grooves is a third triangular area (trigonum vagi), the apex of which is at the inferior fovea, while its base looks downwards and outwards. This area has a distinctly darker colour than the rest of the floor of the ventricle, and especially than the trigonum hypoglossi on the inner side, which has a whitish grey appearance, and it has accordingly been named the ala cinerea (a.c). Towards the apex it is somewhat depressed, but below it is elevated into a distinct prominence (eminentia cinerea). It contains the nucleus of the vagus, and superiorly, near the inferior fovea, of the glosso-pharyngeal nerve.

Above the striæ acusticæ the floor of the fourth ventricle is marked in the middle of each lateral half by a distinct somewhat angular depression in a line with the inferior fovea, from which it is separated by the eminence over which the striæ acusticæ pass. This depression is termed the superior fovea (s.f.). Between it and the median sulcus is the prolongation of the funiculus teres, which is pro

minent (eminentia teres) opposite the fovea but becomes gradually less so above and below. Extending from the superior fovea to the upper end of the ventricle, where this narrows to the Sylvian aqueduct, is a shallow depression (locus cæruleus) distinguished in the adult by its dark grey or slaty tint, which is due to a subjacent tract of pigmented nerve-cells (substantia ferruginea). The trophic fibres in the fifth nerve have been supposed to be derived from these cells.

INTERNAL STRUCTURE OF THE MEDULLA OBLONGATA.

The internal structure of the medulla, like the external form, will be best understood by tracing its several parts upwards from the spinal cord; and this can be

Fig. 42.

SECTION OF THE MEDULLA OBLONGATA AT THE MIDDLE OF THE DECUSSATION OF THE PYRAMIDS (Lockhart Clarke).

f, anterior; f.p., posterior fissure; a p., pyramid; a, remains of part of anterior cornu, separated by the crossing bundles from the rest of the grey matter; l, continuation of lateral column of cord; R, continuation of substantia gelatinosa of Rolando; pc., continuation of posterior cornu of grey matter; f.g., funiculus gracilis.

most readily done by a comparison of the appearances of successive transverse sections.

Sp f.g

a.c

Lower or closed part of the medulla oblongata. The first changes are produced, in its internal structure as in its external form, by the passage of the fibre-bundles of the lateral pyramidal tract obliquely through the grey matter of the anterior horn, and across the anterior median fissure to the pyramid of the opposite side (fig. 42). By this abrupt passage of a large number of white fibres through it, the anterior horn is broken up, and one part, the caput cornu (a), is entirely separated from the rest of the grey matter; whilst only

Fig. 43. SECTION OF THE MEDULLA OBLONGATA

IN THE REGION OF THE SUPERIOR PYRAMIDAL

DECUSSATION. (Schwalbe.)

a.m.f., anterior median fissure; f.a., superficial arciform fibres emerging from the fissure; py, pyramid; n.ar., nucleus of the arciform fibres; f.a'., deep arciform fibres becoming superficial; o., lower end of olivary nucleus; o', accessory olivary nucleus; n.l.. nucleus lateralis; f.r., formatio reticularis ; f.a., arciform fibres proceeding from formatio reticularis; g, substantia gelatinosa of Rolando; a. V., ascending root of fifth nerve; n.c., nucleus cuneatus: n.c., external cuneate nucleus; f.c., funiculus cuneatus; n.g., nucleus gracilis ; f.g., funiculus gracilis ; p.m.f., posterior median fissure; c. c., central canal surrounded by grey matter, in which are, n. XI. nucleus of the spinal accessory, and, n. XII., nucleus of the hypoglossal; s.d., decussation of fillet or superior pyramidal decussation.

the base of the horn remains, as a small portion of grey matter close to the anterolateral aspect of the central canal.

n.XII.

The separated portion of the anterior horn becomes pushed over to the side by the development of the pyramid and the interpolation higher up of the olivary body between them, so that it comes to lie close to the separated caput cornu posterioris (see below). The greater part of the grey substance is broken up into a formatio reticularis (fig. 43, f.r.), i.e. a compara

tively coarse network of grey matter containing nerve-cells, intersected by bundles of white fibres; but a small part, probably representing the lateral horn of the cord, and like that containing numerous nerve-cells many of relatively large size, remains for a time in the lateral column, near the surface, and is known as the nucleus lateralis (fig. 43, n.l.).

Meanwhile the posterior horns have become gradually shifted laterally, simultaneously with an increase in size of the posterior columns of the medulla, so that in place of forming an acute angle with the posterior median fissure, they now lie almost at right angles to it (fig. 42). Moreover, the caput cornu enlarges and

Fig. 44. TRANSVERSE SECTION OF THE MEDULLA OBLONGATA SOMEWHAT ABOVE THE MIDDLE OF THE OLIVARY BODY. (E.A.S.) MAGNIFIED 5 DIAMETERS. (From a photograph.)

p.l.b, posterior longitudinal bundle; n.t, nucleus of funiculus teres; n. XII, nucleus hypoglossi ; n.X, nucleus vagi; n'X, nucleus ambiguus; 8, fasciculus solitarius; n.p, nucleus posterior (cuneatus); f.r, formatio reticularis; t, tænia; s. R, substantia Rolandi; a. V, ascending root of fifth; c.r, corpus restiforme; ar.int, internal arcuate fibres; X, issuing root of vagus; n.l, nucleus lateralis; n', groups of large cells, perhaps belonging to nucleus lateralis; a-l.tr, antero-lateral ascending tract; n.d.o, nucleus dentatus oliva; acc.o, accessory olivary nucleus; 8.0, siliqua olive; h.o, hilum olive; P, pyramid; f, fillet; r, raphe; n.ar, nucleus of arcuate fibres; ar.ext, external arcuate fibres. comes close to the surface, where it presently forms a distinct projection, the funiculus of Rolando, which, a little higher up, swells into the tubercle of Rolando (fig. 42, R.). At the same time the cervix cornu diminishes in size and like the anterior cornu is eventually broken up by the passage of transverse and longitudinal bundles of white fibres through it, into a reticular formation, which then separates the caput cornu posterioris (fig. 43, g.) from the rest of the grey matter, and joins the reticular formation derived from the rest of the grey matter. In the tubercle of Rolando the caput cornu is close to the surface, and its grey substance can readily be seen, but above the tubercle it lies deeper, being covered by a well-marked bundle of white fibres, the so-called ascending root of the fifth nerve (a. V.), and by the oblique arched fibres which are passing over it to form the restiform body.

The fibres of the ascending trigeminal root have been supposed to take origin from the cells of the tubercle of Rolando, but this is not the case, for firstly they do not grow from these