portion of grey matter is continuous below with part of the anterior horn, and the roots of the hypoglossal and sixth nerve appear to correspond generally with the anterior roots of the spinal nerves. In traversing the substance of the bulb they mark off an anterior area, wedge-shaped in transverse section, which is placed between them and the anterior median fissure. This area is on the surface marked by the prominence known as the pyramid or anterior pyramid, which corresponds in position (but only to a small extent in the fibres of which it is composed) with the anterior column of the spinal cord. The remainder of the transverse section of the bulb, after the posterior and anterior areas are deducted, lies between the line of nerve-root bundles of the hypoglossus and sixth on the one side, and that of the successive bundles of the spinal accessory, vagus, glosso-pharyngeal, and seventh on the other. This is termed by Flechsig the lateral area, and on the surface it is marked by a continuation of part of the lateral column of the cord, and by the prominence of the lower olive already alluded to.

We may now proceed to describe in detail the several parts which appear upon the surface of the bulb in the three regions thus marked off by the two sets of nerve-roots, commencing with the posterior area.

Posterior area. It will be remembered that in the upper region of the spinal cord a small portion of the whole posterior column is marked off from the rest by a well-developed pia-matral septum, and is indicated on the surface by a distinct Fig. 35.-POSTERIOR AND LATERAL VIEW OF THE

MEDULLA 03LONGATA, 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.

obl

p.n., line of the posterior roots of the spinal nerves; p.m.f., posterior median fissure; f.g., tuniculus gracilis; c., its clava; f.c., funiculus cuneatus; f.R., funiculus of Rolando; r.b., restiform body; c.8., lower end of the fourth ventricle (calamus scriptorius); l, section of the lingula or tænia; part of the choroid plexus is seen beneath it; 7.r., lateral recess of the ventricle; str, striæ acustica; i.f., inferior (posterior) fovea; s.f., superior (anterior) nd 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. longitudinal prominence bounded laterally by a shallow groove. The portion thus marked off is the postero-mesial column (tract of Goll), and the prominence, which is continued up into the medulla oblongata, becomes there still better marked, and is known as the funiculus gracilis (fig. 35, f.g). This, as it is traced upwards, especially as the fourth ventricle is approached, broadens out into an expansion termed the clava (cl), and as the ventricle opens out the clave of opposite sides diverge and form the lateral boundary to the

ventricle in its lower part. Above, the clavæ are tapered off and soon become no longer traceable.

The funiculi graciles with their clave are sometimes described as the posterior pyramids.

Between the postero-mesial column and the postero-lateral groove from which the posterior roots of the cervical nerves pass out there is found in the upper part of the cord a single distinct column, viz., the postero-lateral column (tract of Burdach). This is also prolonged into the medulla, and also like the funiculus gracilis expands as it is traced upwards: it is here known as the funiculus cuneatus. Outside this funiculus cuneatus and between it and the line of roots of the spinal accessory another longitudinal prominence is caused by the fact that the substantia gelatinosa of Rolando begins near the lower end of the bulb to project towards the surface as a distinct funiculus, narrow below but broadening as it is traced upwards, where it forms a considerable eminence known as the tubercle of Rolando. The longitudinal prominence which passes up into it, is termed therefore by Schwalbe, the funiculus of Rolando (fig. 35, f.R.).

The funiculus of Rolando is termed by Henle the lateral cuneate funiculus.

On a level with the adjoining clava of the funiculus gracilis, the enlarged part of the cuneate funiculus also, like that, exhibits a slight eminence, which is best marked in children, and has been termed the cuneate tubercle (Schwalbe).

In the upper part of the medulla oblongata, the cuneate funiculus is concealed by a set of fibres (external arched or arcuate fibres) which issue from the anterior median

2

pc

Fig. 36. DISSECTION OF THE MEDULLA OBLONGATA AND PONS

SHOWING THE COURSE OF THE PYRAMIDAL TRACTS IN THEM.

(Allen Thomson.)

P, pons Varolii; p, the pyramids, the upper part of the right one has been cut away; p', the fibres of the left pyramid, as they ascend through the pons, exposed by the removal of the superficial transverse fibres; p" is placed on some deeper transverse fibres of the pons on the right side, below the divided fibres of the right pyramid; a, left anterior column of the cord, its median part passes upwards into the outer part of the pyramid, the remainder dips beneath the pyramid and olivary body; o, olivary body; o', the continuation of part of the lateral column ascending through the pons and exposed by the removal of a small portion of the deeper transverse fibres; o", some of the same fibres divided by a deeper incision on the right side; l, l', the lateral columns of the cord; x, their deeper parts passing by decussation into the pyramids; r, direct cerebellar tract passing from the lateral column into the inferior peduncle of the cerebellum, or restiform body; r', fasciculus passing from the anterior column to the same; ft, deep longitudinal fibres derived from the anterior and lateral columns of the cord.

B, explanatory outline of the section of the spinal cord, a, anterior columns; p, posterior; 1, lateral.

fissure and passing laterally over the surface of the pyramid and olive, turn upwards to join the restiform body. There is also a narrow strand of fibres from the lateral column of the cord, marked by its white appearance, which joins this tract of oblique fibres just above the level of the tubercle of Rolando. This is the dorso-lateral (direct) cerebellar tract which has been already noticed in the description of the spinal cord. These obliquely crossing fibres turn upwards as they cross the funiculus of Rolando, and appear to blend with that column and with the cuneate funiculus. They do not, however, actually blend with them, but are reinforced by a large number of fibres coming from the opposite side of the bulb (see p. 52, fig. 44), and the whole mass of fibres thus produced forms a rounded, prominent cord, the corpus restiforme, or rope-like body (fig. 36, r), which passes directly into the corresponding hemisphere of the cerebellum, constituting its inferior peduncle.

Lateral area. The lateral column of the spinal cord appears, on the surface, to be directly continued upwards into the lateral area of the medulla oblongata. In reality, however, a considerable tract of the white fibres-that which in the spinal cord we have noticed as the lateral pyramidal tract-is found, at the upper end of the spinal cord, to cross obliquely in stout bundles through the grey matter of the anterior horn, and across the anterior median fissure to the other side of the bulb, where it forms the mesial and larger part of the pyramid (figs. 36, 7). The rest of the lateral column can be traced vertically upwards (with the exception of the dorsolateral cerebellar tract which passes backwards into the restiform body) as far as the lower end of the olive where its longitudinally coursing fibres become concealed by this prominence and by the arched fibres already noticed.

The olivary body or lower olive (figs. 32, 34, 36, o) is, as its name implies, an olive-shaped prominence, which lies in the upper part of the bulb, immediately above the apparent termination of the lateral column, and extending nearly as far as the lower border of the pons, being only separated from this by a deep groove in which is sometimes a small band of arched fibres. The line of exit of the hypoglossal nerve-roots lies on its inner or mesial border, that of the accessory, vagus and glossopharyngeal roots along its outer side, but the latter are separated from it by a groove in which longitudinal fibres prolonged from the ventro-lateral cerebellar tract can be seen. Sometimes there is a small longitudinal tract running along its inner border also, and in such case, with the arched fibres above and below, the olive appears to be entirely enclosed by a fibrous strand, which has sometimes been described as its capsule (siliqua oliva). This name is, however, more applicable to a layer of fibres which immediately surrounds the dentate nucleus of the olive (see note on p. 56, and fig. 44, s.o.). The longitudinal tracts on either side of the olive are often concealed in great measure by the arched fibres, which may form a complete superficial layer over the olive, and indeed over the whole anterior and lateral surface of this upper part of the medulla oblongata.

Anterior area. The anterior columns of the cord, although on superficial inspection they appear to be prolonged into the pyramids of the bulb, are so

Fig. 37.-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; p. c., continuation of posterior cornu of grey matter; f.g., funiculus gracilis.

only to a small extent. For the lateral pyramidal bundles, crossing the anterior median fissure from the lateral column, are continued upwards close to that fissure, and unite with the comparatively small anterior pyramidal bundle to constitute the prominence known. as the pyramid (anterior pyramid). The prolongation upwards of the rest of the anterior column of the cord lies deeply, being altogether concealed from view by the pyramids.

The pyramids (figs. 30, a, 32, pa), are more prominent above than below. They are bounded mesially by the anterior median fissure, and laterally by the olivary bodies, being separated from these by the groove before mentioned, from which the roots of the hypoglossal nerve issue. At their upper end they are constricted, and become more completely separated from the rest of the medulla oblongata. They are here very frequently crossed by a band of arched fibres, the

ponticulus of Arnold. Each pyramid then enters the substance of the pons in one large bundle, which soon breaks up into several bundles, and these may be traced through the pons into the corresponding peduncle of the cerebrum.

The decussation of the pyramids is the name given to the obliquely crossing bundles of the lateral pyramidal tract which are seen in the anterior median fissure at the lower part of the medulla oblongata. The extent to which the decussation is visible varies considerably in different individuals; for in some the bundles take a deeper, in others a more superficial course. Further, in some cases a larger share than usual of the longitudinal fibres of the pyramids passes down in the anterior pyramidal tract and a correspondingly smaller share in the lateral pyramidal tract. And since the anterior tract, which in the pyramid is external to the lateral tract, does not cross in the medulla oblongata but merely passes obliquely at its lower end to attain the side of the anterior median fissure, the decussation in these cases is of less extent. On the other hand, in rare cases, the whole of the fibres of the pyramid may cross over at the lower part of the bulb and become lateral pyramidal tract, in which case the anterior or uncrossed tract is wanting in the cord, and the bulbar decussation is very well marked. All transitions are found between these two conditions (see p. 24).

The pons Varolii forms a prominence marked by transverse fibres above and in front of the medulla oblongata, and between the lateral hemispheres of the

Fig. 38.-TRANSVERSE SECTION THROUGH LOWER PART OF PONS VAROLII. (After Stilling and Schwalbe.) The description of this figure is given on p. 61.

cerebellum (fig. 38); at the sides its transverse fibres are gathered together into a compact mass, which passes into the cerebellum, and is named the middle crus or middle peduncle of the cerebellum. On its ventral surface the pons has a shallow median groove along which the basilar artery runs, and it is perforated by small branches of the artery. The groove is in some measure due to the circumstance that the pyramids of the medulla oblongata are continued up through the pons with a slightly divergent course, and thus produce a prominence on either side of the middle line, covered, however, by the superficial transverse fibres.

The pons consists ventrally of transverse cerebellar fibres, between which the longitudinal fibres prolonged upwards from the medulla oblongata pass; together with a large intermixture of grey matter. The superficial fibres on the ventral surface (fig. 30, p) are transverse in their general direction, but while the middle fibres are exactly transverse, the lower set ascend slightly, and the superior fibres (fig. 30, i), which are the most curved, descend obliquely to reach the crus cerebelli on each side; some of the upper fibres cross obliquely the middle and lower ones, so as to conceal them at the sides. When the superficial transverse fibres are removed, the prolonged fibres of the pyramids come into view (fig. 36); these, as they ascend through the pons, are separated into smaller bundles, intersected by other transverse white fibres, which, with those upon the surface, are all continued into the middle peduncle of the cerebellum.

At the lower part of the pons, dorsal to the fibres from the pyramids, is a special set of transverse fibres (fig. 38, t), named the trapezium (corpus trapezoides)-so called because in most mammals, in which the inferiorly situated fibres of the pons are less developed and the pyramids are small, these transverse fibres partially appear on the surface in an area of a somewhat four-sided shape. Laterally they curve round a collection of grey matter, called the superior olivary nucleus (fig. 38, o.s.), and probably many of them are connected with its cells. They then course lateralwards, across the bundles of the facial nerve-roots (VII), and ventral to the upward prolongation of the substantia gelatinosa of the tubercle of Rolando and the bundles of fibres belonging to the ascending root of the fifth nerve (a.V), and appear to become connected with the accessory auditory nucleus (p. 55) and with the ventral root of the auditory nerve, of which, according to Flechsig, the trapezium forms a cerebral commissure.

The fourth ventricle.-The external characters of the medulla oblongata and pons may be completed by a description of those parts which enter into the boundaries of the fourth ventricle. This is the space into which the central canal of the cord, after becoming somewhat enlarged and cleft-like, opens out superiorly (fig. 39). The opening-out seems as if effected by the divergence of the funiculi graciles et cuneati on either side at an acute angle. These funiculi, which form the lateral boundaries of the lower part of the ventricle, bend round laterally at about the middle (in length) of the medulla oblongata, so that the ventricle, which is at first narrow, rapidly broadens. Opposite the middle peduncle of the cerebellum it has attained its greatest width. From this point its upper part again narrows, converging gradually above to be continued into the comparatively narrow Sylvian aqueduct. The ventricle is therefore irregularly lozenge- or diamond-shaped, and is sometimes named fossa rhomboidalis. The pointed lower end of the ventricle has the shape of a writing pen, and is termed the calamus scriptorius (fig. 39, c.s.). At its widest part the fourth ventricle is continued for a short distance on either side between the cerebellum and bulb where these come in contact, in the form of the pointed lateral recess (l.r.). The lateral boundaries of the ventricle are, in its lower or medullary part, the clave of the funiculi graciles, the funiculi cuneati, and the restiform bodies; in its upper half the superior peduncles of the cerebellum (fig. 39, s.c.p.). These pass gradually to the roof of the ventricle as they extend upwards. They are at first separated from one another by a tolerably wide interval which, however, gradually narrows near the end of the ventricle, the two crura of opposite sides there approaching one another, and their margins coming in contact. Roof of the ventricle. The triangular interval between the two crura is bridged over by a lamina of white matter marked across with grey streaks. This is the superior medullary velum or valve of Vieussens (fig. 39, s.m.v.), and, with the crura, forms the roof or dorsal boundary of the upper part of the fourth ventricle. The white substance of which it is mainly composed