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.