THE BRAIN OR ENCEPHALON.

The brain or encephalon comprises the medulla oblongata and pons Varolii, the cerebellum, the mid-brain, and the cerebrum. The medulla oblongata (fig. 28, D) is the part continuous with the spinal cord; it is the lowest part of the encephalon, and rests against the basilar process of the occipital bone. The pons Varolii (c) is a direct continuation upwards and forwards of the medulla oblongata, but the name also includes the transversely coursing fibres of the middle cerebellar peduncle. The cerebellum (B) occupies the posterior fossa of the cranium. Its central part forms the dorsal boundary of a space, which is bounded ventrally by the dorsal surface of the medulla oblongata and pons Varolii, and which is named the fourth ventricle of the

1

Fig. 28.-PLAN IN OUTLINE OF THE EN-
CEPHALON, AS SEEN FROM THE RIGHT
SIDE.

The parts are represented as separated from one another somewhat more than natural so as to show their connections. A, cerebrum; e, fissure of Sylvius; B, cerebellum; C, pons Varolii; D, medulla oblongata; a, peduncles of the cerebrum ; b, c, d, superior, middle, and inferior peduncles of the cerebellum; the parts marked a, b, form the isthmus encephali.

brain. The cerebellum is connected by peduncles below with the medulla oblongata, above with the cerebrum, and in the middle with the pons Varolii (c). The mid-brain is continued from the pons Varolii and comprises the two crura of the cerebrum (a), which are surmounted in man and mammals by two pairs of tubercles named the corpora quadrigemina (b), but in oviparous vertebrates by one pair only, then named the corpora bigemina or optic lobes. The cerebrum (A) includes all the remaining and by far the largest part of the encephalon. It is united with the parts below by the mid-brain, which forms a comparatively narrow and constricted portion or isthmus. The cerebrum is mainly composed of the large convoluted cerebral hemispheres, and within it are the third and two lateral ventricles. It occupies the vault of the cranium, the anterior and middle cranial fossæ, and the superior fosse of the occipital bone. The cerebral hemispheres are united together by a large commissure termed the corpus callosum, and by smaller commissures (anterior, middle, and posterior).

THE MEDULLA OBLONGATA AND PONS VAROLII.

EXTERNAL CHARACTERS.

The medulla oblongata or spinal bulb (bulbus rhachiticus, Meckel) is continuous below with the spinal cord, on a level with the lower margin of the foramen magnum. Its upper limit is marked off on the ventral aspect from the pons Varolii, into which it is continued above, by the lower margin of the mass of transverse fibres which unite the two hemispheres of the cerebellum; its ventral surface rests in the basilar groove

of the occipital bone, its dorsal or posterior surface is directly continued into that of the pons, and lies in the fossa named the vallecula between the hemispheres of

Fig. 29.-RIGHT HALF OF THE BRAIN DIVIDED BY A VERTICAL ANTERO-POSTERIOR SECTION (from various sources and from nature). (Allen Thomson.)

1, 2, 3, 3a, 3b, are placed on convolutions of the cerebrum; 4, the fifth ventricle, and above it the divided corpus callosum; 5, the third ventricle; 5', pituitary body; 6, corpora quadrigemina and pineal gland; +, the fourth ventricle; 7, pons Varolii; 8, medulla oblongata; 9, cerebellum; 1, the olfactory bulb; II, the right optic nerve; III, right third nerve.

the cerebellum. From its front and sides the sixth to the twelfth cranial nerves issue, while the fifth nerve emerges from between the commissural fibres of the pons.

The term medulla oblongata, as employed by Willis and Vieussens, and by those who directly followed them, included the crura cerebri and pons Varolii, as well as that part to which by Haller first, and by most subsequent writers, this term has been restricted.

Fig. 30.-VIEW OF THE ANTERIOR SURFACE OF THE PONS VAROLII

AND MEDULLA OBLONGATA, WITH A SMALL PART OF THE SPINAL
CORD ATTACHED.

a, a, pyramids; b, their decussation; c, c, olives; d, d, restiform bodies; e, external arciform fibres, curving round the lower end of the olive; f, fibres described by Solly as passing from the anterior column of the cord to the cerebellum; g, anterior column of the spinal cord; h, lateral column; p, pons Varolii; i, its upper fibres; v, v, roots of the fifth pair of

nerves.

The combined mass of the medulla oblongata and pons has an oblong or rhomboidal form, being expanded in the middle, where it is continuous with the crura cerebelli, and tapering towards the spinal cord below and the mid-brain above. The transversely coursing fibres of the cerebellum produce a considerable prominence (tuber annulare) on the ventral surface of the upper or pontine part of the mass; these fibres are arched upwards so as largely

to conceal the crura cerebri from the ventral aspect. The length of the medulla oblongata from the lower part of the decussation of the pyramids to the pons is

nearly an inch (20 to 24 mm.), its greater breadth is about three-fourths of an inch (17 to 18 mm.); its thickness, from before backwards, is rather less (15 mm.). In the lower part where it joins the spinal cord, its diameters differ but little from those of the cord. The pons Varolii is about an inch long and rather more than an inch broad; it is considerably thicker than the medulla oblongata.

The anterior and posterior median fissures which partly divide the spinal cord are continued into similar fissures in the medulla oblongata. The anterior fissure terminates immediately below the pons in a recess, the foramen cæcum of Vicq d'Azyr; it is partly interrupted below by the decussating bundles of the pyramids; the posterior fissure is continued upwards to about the middle of the bulb, it ceases at the lower end of the fourth ventricle.

The structure of the medulla oblongata and pons will be most easily made clear by tracing the several parts of the spinal cord upwards.

In doing this it will be found that the relative position and extent of many of the parts are gradually altered, and that other parts which are not, so far as is known, represented in the spinal cord, become interpolated between those which are there met with. It will further be found that the change of relative position of the parts is largely owing to two causes. In the first place the sudden passage of large bundles of medullated fibres from the posterior part of the lateral column of the cord to the opposite side of the anterior median fissure appears to break up to a great extent the grey matter of the anterior horn, which is traversed by the bundles. In the second place the opening up of the central canal and separation of the lips of the posterior median fissure bring the grey matter to the surface in the fourth ventricle, whilst the posterior horn is coincidently shifted to the side, much in the same way as it would be if a median incision were made from the posterior surface of the spinal cord into the central canal, and the two lateral halves were then turned outwards so that the sides of the posterior median fissure became the posterior surface of the cord.

The lines along which the cranial nerve-roots issue from the sides of the bulb mark the surface of the latter off into three chief columns on either side, which may

SPINAL CORD AT THE JUNCTION (After Lockhart Clarke.)

f, anterior; fp, posterior fissure; p, end of decussation of pyramids; Cla, CIp, anterior and posterior roots of first cervical nerve; XI, root of spinal accessory nerve; c, central canal.

be termed dorsal, lateral, and ventral respectively. Thus the spinal accessory or eleventh cranial nerve (figs. 32, 34, 35, XI) begins to take origin as far down as the lower end of the cervical region of the cord where its roots issue from the side of the cord, and lateral to the posterior roots of the cervical nerves (fig. 31). At the upper end of the cervical region, however, they are approached by the line of the posterior roots, and some of their bundles arise in conjunction with the posterior roots of the first and sometimes of the second cervical nerve. They are succeeded by the bundles of the vagus root, and these again by those of the glossopharyngeal. At the junction of the pons with the medulla oblongata the seventh nerve also is seen issuing along the same line, and still higher the large root of the fifth is given off from the lateral aspect of the pons. The line of exit of this series of nerve-roots is marked below by a shallow sulcus, but if traced upwards it will be seen that the sulcus is obliterated or nearly so before long, so that the issuing series of nerve-roots alone serves to mark its position. As it passes upwards it becomes gradually diverted outwards; so that opposite the place where the central canal opens into the calamus scriptorius of the fourth ventricle, the line in question has left the posterior surface and in the rest of its course runs along

the lateral surface of the bulb. Towards the upper end it passes near the posterior margin of an oval prominence on the surface of the bulb termed the olwary body or lower olive (o), but is separated from that prominence by a tract of longitudinal fibres. In transverse sections of the lower part of the medulla Fig. 32.-VIEW FROM BEFORE OF THE MEDULLA OBLONGATA, PONS VAROLII, CRURA CEREBRI, AND OTHER CENTRAL PORTIONS OF THE ENCEPHALON. (Allen Thomson.) NATURAL SIZE.

On the right side the convolutions of the central lobe or island of Reil have been left, together with a small part of the anterior cerebral convolutions : on the left side these have been removed by an incision carried between the thalamus opticus and the cerebral hemisphere.

1', the olfactory tract cut short and lying in its groove; II, the left optic nerve in front of the commissure; II', the right optic tract; Th, the cut surface of the left thalamus opticus; C, the central lobe or island of Reil; Sy, fissure of Sylvius; x x, anterior perforated space; e, the external, and i, the internal corpus geniculatum; h, the hypophysis cerebri or pituitary body; tc, tuber cinereum with the infundibulum; a, one of the corpora albicantia; P, the cerebral peduncle or crus; III, close to the left oculo-motor nerve; x, the posterior perforated space.

The following letters and numbers refer to parts in connection with the medulla oblongata and pons. PV, pons Varolii ; V, the greater root of the fifth nerve; +, the lesser or motor root; VI. the sixth nerve; VII, the facial; VIII, the auditory nerve; IX, the glossopharyngeal ; X, the pneumogastric nerve; XI, the spinal accessory nerve; XII, the hypoglossal nerve; CI, the suboccipital or first cervical nerve; p a, pyramid; o, olive; d, anterior media fissure of the spinal cord, above which the decussation of the pyramids is represented; c a, anterior column of cord; r, lateral tract of bulb continuous with cl, the lateral column of the spinal cord.

А са

oblongata (fig. 33) it is seen that the bundles of fibres of these nerve-roots traverse the substance of the bulb to reach a group of nerve-cells in the grey matter near the central canal and thus mark off a somewhat oval area on each side at the posterior part of the section. This area is termed by Flechsig the posterior area of the medulla oblongata, and the tracts of white fibres which can be traced, as will be immediately noticed, upon the surface of this part, may therefore be conveniently termed its posterior columns. They correspond in position to the posterior columns of the spinal cord.

The line of origin of the anterior roots of the spinal nerves, although not marked in the spinal cord by a distinct furrow like that whence the posterior roots issue, yet when traced upwards into the bulb, deepens into a well-marked longitudinal groove which is continued almost vertically as far as the lower border of the projecting mass of the pons. In its upper part this groove separates the olivary prominence from the prominence of the pyramid. Just below the olive it is often obliterated for a certain part of its course by a band of transverse fibres. The rootbundles of the hypoglossal nerve (figs. 32, 33, 34, XII) pass out from this furrow.

and those of the sixth nerve emerge in the same line with it but at the lower border of the pons Varolii. In transverse sections of the medulla oblongata and pons these root-bundles may be traced back through the substance of the organ to a group or column of nerve-cells situated in a portion of grey matter close to the antero-lateral side of the central canal in the lower closed part of the bulb (fig. 43), and close to the middle of the fourth ventricle in the upper opened-out part (figs. 44, 45). This

Fig. 33.-SECTION ACROSS THE MEDULLA OBLONGATA A LITTLE BELOW THE POINT OF THE CALAMUS SCRIPTORIUS. (Lockhart Clarke.)

c, central canal; f. anterior median fissure; f.g, funiculus gracilis ; f.c, funiculus cuneatus; t.R., tubercle of Rolando; o, olivary body; a.p., pyramid; XI, XII, spinal accessory and hypoglossal nerves; XI', XII', their nuclei.

Fig. 34.-VIEW OF THE MEDULLA OBLONGATA, PONS VAROLII, CRURA CEREBRI, AND CENTRAL PARTS OF THE ENCEPHALON FROM THE RIGHT SIDE. (Allen Thomson.)

The corpus striatum and thalamus opticus have been preserved in connection with the central lobe and crura cerebri, while the remainder of the cerebrum has been removed.

St, upper surface of the corpus striatum; Th, back part of the thalamus opticus (pulvinar); C. placed on the middle of the five or six convolutions constituting the central lobe or island of Reil, the cerebral substance being removed from its circumference; Sy, fissure of Sylvius, from which these convolutions radiate, and in which are seen the white striæ of the olfactory tract; I, the olfactory tract divided and hanging down from the groove in the convolution which lodges it; II, optic nerves a little way in front of the commissure; a, right corpus albicans with the tuber cinereum and infundibulum in front of it; h, hypophysis or pituitary body; e, external, and i, internal corpus geniculatum at the back part of the optic tract; P, peduncle or crus of the cerebrum; III, right oculo-motor nerve; p, pineal gland; q, corpora quadrigemina; IV, trochlear nerve rising from v, the valve of Vieussens.

The following numbers and letters refer chiefly to parts in connection with the medulla oblongata and pons. V, placed on the pons Varolii above the right nervus trigeminus; s, the superior, m, the middle, and in, the inferior peduncle of the cerebellum cut short; VI, the sixth nerve; VII, facial nerve; VIII, auditory nerve; IX, the glosso-pharyngeal nerve; X, placed opposite to the cut end of the pneumogastric nerve; and XI, the uppermost fibres of the spinal accessory nerve; XII, the hypoglossal nerve; pa, pyramid; o, olive; a r, arciform fibres; r, restiform body; tr, tubercle of Rolando; c a, anterior, cp, posterior, and c l, lateral columns of the spinal cord; CI, Ci, anterior and posterior roots of the first cervical nerve.