Part 76

The central canal of that part of the medulla which is overhung by the cerebellum is greatly widened (Fig. 141, h). The cavity thus formed is flattened and triangular, with the apex of the triangle caudad, and is the caudal part of the =fourth ventricle= (Fig. 141, h). The roof is very thin (Fig. 143, n) and is intimately connected with the pia mater, so that in removing the latter the thin roof of the fourth ventricle is removed with it and the cavity of the fourth ventricle is left exposed. The thin roof of the fourth ventricle is known as the =velum medullare posterius= (Fig. 143, n).

The ventral (anterior) fissure (Fig. 138, p) of the cord passes onto the medulla oblongata and ends at the pons (i), its end being slightly deeper and forming what is known as the =foramen cæcum= (q). The dorsal (posterior) sulcus (Fig. 141, a) is well marked; it ends at the caudal boundary of the fourth ventricle (h).

The anterior lateral sulcus (Fig. 138, r) may be traced craniad from the origin of the ventral roots of the first cervical nerves (s), along the lateral border of the area elliptica (n), then curving mediad to the lateral border of the pyramis (o), and finally reaching the pons (i). Its position is marked by the origin of the roots of the hypoglossal nerve (XII).

The posterior lateral sulcus (Fig. 141, b), marked on the spinal cord by the origin of the dorsal nerve-roots, curves laterad at the sides of the fourth ventricle (h) owing to the increasing width of the latter, and ends at an elevated area of oblique fibres, the =area ovalis= (f).

The columns or funiculi bounded by the longitudinal fissures present the following peculiarities:

The anterior white funiculus of the cord is replaced in the medulla by the =pyramidal tracts= (=pyramides=) (Fig. 138, o). The pyramidal tracts are formed by fibres which emerge from beneath the pons and pass caudad to disappear just craniad of the level of the first cervical nerve (s). The pyramidal tracts are bounded medially by the anterior median fissure (p), but laterally each is separated from the anterior lateral sulcus (r) over its caudal part by an elongated =area elliptica= (n), the human homologue of which is uncertain. It perhaps represents the =oliva=.

Laterad of the cranial portion of the pyramids is an irregular area known as the =trapezium= (l) which abuts caudad on the area elliptica (n) and the area ovalis (m). The =area ovalis= (Fig. 138, m; Fig. 141, f) (or zonula Arnoldi) is abroad band of oblique fibres which passes from the lateral side of the area elliptica craniodorsad to disappear under the cerebellum.

The posterior white funiculus was divided in the cervical region into two, the =fasciculus gracilis= (column of Goll) and the =fasciculus cuneatus= (column of Burdach). The fasciculus gracilis (Fig. 141, c) extends to the caudal end of the fourth ventricle (h) and ends there in an expansion, the =clava= (d), which forms the posterior boundary of the fourth ventricle. The fasciculus cuneatus (e) passes laterad on account of the width of the fourth ventricle and appears to end at the area ovalis (f), but it may be seen passing beneath the area ovalis, emerging at its cranial border and turning dorsad to enter the cerebellum. It forms the side walls of a part of the shallow fourth ventricle.

The lateral funiculi are divided by longitudinal furrows into three divisions. The dorsal one of these is the =fasciculus cuneatus lateralis= or column of Rolando (Fig. 141, g). It accompanies the fasciculus cuneatus into the cerebellum.

The entire mass formed by the fasciculus gracilis, the fasciculus cuneatus medialis, and fasciculus cuneatus lateralis is known as the =corpus restiforme=, and since its fibres pass into the cerebellum it is sometimes known as the =pedunculus cerebelli= (or crus cerebelli ad medullam).

The following cranial nerves arise from the medulla oblongata (Fig. 138).

The twelfth nerve (XII) (=N. hypoglossus=) arises by ten or fifteen rootlets from the ventral surface of the medulla oblongata in the anterior lateral fissure (r) opposite the caudal portion of the area elliptica (n).

The eleventh cranial nerve (XI) (=N. accessorius=) arises by numerous rootlets from the lateral surface of the medulla oblongata and of the spinal cord as far caudad as the sixth or seventh cervical nerve. These rootlets join to form a nerve which enters the cranium through the foramen magnum and is closely associated at its point of exit with the glossopharyngeal (IX) and vagus (X) nerves. The line of origin on the medulla passes between the dorsal and ventral roots of the cervical nerves and is along the middle of the lateral white funiculi.

The tenth cranial nerve (X) (=N. vagus=) arises by about eighteen very delicate rootlets from the surface of the area ovalis. The rootlets are divided into a dorsal and a ventral series. The dorsal series (about twelve) arise in the groove which separates the fasciculus cuneatus medialis from the fasciculus cuneatus lateralis. The ventral series arise somewhat ventrad of this groove. These rootlets are to be distinguished from those of the ninth nerve by their smaller size.

The ninth cranial nerve (IX) (=N. glossopharyngeus=) arises from the area ovalis from a line craniad of the dorsal line of origin of the vagus roots (X) and between these and those of the auditory (VIII). It arises by a number of rootlets which are larger than those of the vagus (X), with which this nerve is closely associated.

The eighth cranial nerve (VIII) (=N. acusticus=) appears at the lateral end of the trapezium (l). It arises from an elevation (Fig. 141, i) which is continued dorsomediad along the cranial border of the area ovalis.

The seventh cranial nerve (VII) (=N. facialis=) leaves the lateral border of the trapezium (l) near its cranial edge, between the fifth and eighth nerves. It is much smaller than the eighth nerve.

The sixth cranial nerve (VI) (=N. abducens=) arises by about six bundles from the groove between the pyramids and the trapezii and passes craniad.

2. Metencephalon.--The metencephalon includes the =pons= and the =cerebellum=.

The =pons= (Fig. 138, i) is a mass of transverse fibres which forms the ventral and cranial part of the primitive hindbrain. It is a modification of the latter brought about by the development of the cerebellum, and the degree of its development is in direct ratio to that of the cerebellar and cerebral hemispheres. The pons forms a projecting mass of fibres which is marked by a median longitudinal groove, the =sulcus basilaris= (j), which indicates the course of the basilar artery (Fig. 121, c). Laterad the fibres of the pons converge somewhat and turning dorsad disappear in the cerebellum, forming the =brachia pontis= (Fig. 141, l).

The fifth cranial nerve (Fig. 138, V) (=N. trigeminus=) arises by two roots from the caudal border of the pons, near the lateral end. The ventral root (4) is small; the dorsal one is much larger and soon forms the large semilunar ganglion (k) from which three branches (1, 2, and 3) diverge. The ventral root (4) joins one of these branches (1).

The =cerebellum= (Fig. 137, III) is formed by an increase in size of the cranial portion of the primitive hindbrain. This increase has affected principally the surface of the roof, so that as the cerebellum has grown it has been thrown into many folds, the exact form of which varies in different specimens. The cerebellum has at the same time increased in size and has thus extended laterad as well as caudad and craniad. It thus touches the cerebrum in front (separated from it by the tentorium) and aids it in concealing the midbrain and ’tween-brain in dorsal view, while caudad in the same view it conceals the greater part of the medulla. The connections of the cerebellum with adjacent parts of the brain are also overhung and concealed.

The whole surface of the cerebellum is thrown up into numerous folds or =gyri=, separated from one another by deep fissures or =sulci=, which appear at first to render the surface wholly irregular. The entire mass is, however, divisible into a central portion, which from its resemblance to a segmented worm is called the =vermis= (j) (its cranial part is the =superior vermis=, and its caudal part the =inferior vermis=), and into lateral portions, the =hemispheres= (k). The vermis (j) occupies a median longitudinal position, and its gyri and sulci are in the main transverse. It is not directly connected with adjacent parts, and its ventral surface extends farther caudad and craniad than that of the hemispheres. The ventral part of the superior vermis is fitted against the posterior corpora quadrigemina.

The =hemispheres= (k) may again be subdivided into groups of gyri which have received special names. One of these, the so-called appendicular lobe, fits into the appendicular fossa of the petrous bone.

The cerebellum is connected to the adjacent parts by three tracts of fibres, sometimes known as crura cerebelli. The tract connecting it with the medulla oblongata is the =corpus restiforme=; that connecting it with the pons is the =brachium pontis= (Fig. 141, l); these have been described. A third tract passes craniad to the corpora quadrigemina (Fig. 141, p and q); this is the =brachium conjunctivum= (Fig. 141, k).

The cerebellum is composed of white and gray matter, the latter on the surface (Fig. 143, III). The folds of its surface present thus a contrivance for increasing the amount of gray matter. The white matter forms a central mass from which tracts extend into the folds. The whole mass of white matter has thus in section (more particularly in a longitudinal section of the vermis) the appearance of a tree, whence the name =arbor vitæ= (Fig. 143).

The =fourth ventricle= (Fig. 141, h; Fig. 143, m) is the cavity of the original hindbrain. It begins caudad at the clava (Fig. 141, d) as a widening and continuation of the central canal of the spinal cord and extends craniad, becoming wider and passing ventrad of the cerebellum. It becomes narrower craniad and ends at the posterior corpora quadrigemina (p), where it becomes continuous with the aqueductus cerebri (Fig. 141, o; Fig. 143, j). The cavity is shallow and is encroached upon dorsally by the vermis of the cerebellum (Fig. 143).

The floor of the cavity is known as the =fossa rhomboidea= (Fig. 141, h). It is formed by the continuation of the gray matter which surrounds the central canal of the cord. It is marked by a median longitudinal groove. At its widest part are seen two considerable tracts of white fibres (=striæ medullares=) which pass from near the median line laterad and extend into the auditory (eighth) nerves. The floor caudad and craniad of these striæ is marked by a number of elevations and depressions. Similarly situated elevations in the human brain differ from one another slightly in color, are made up of gray matter, and are the centres of origin of most of the cranial nerves. Their homologues in the cat appear not to have been determined.