The posterior median fissure (fissura mediana posterior) is narrow and deep (Fig. 139). It extends, longitudinally, down the posterior median line of the cord from the middle of the posterior surface of the medulla. It divides the cord, dorso ventrally, beyond its middle. The floor of the fissure is formed by the posterior commissure, which, with the gray and white anterior commissures, separates the posterior from the anterior median fissure. The posterior median fissure is not an open fissure; it is occupied by a lamina of connective tissue, the posterior septum, which is attached to the deep layer of the pia mater. In the posterior septum ramify branches of the two posterior spinal arteries and tributaries of the external spinal veins.
Posterior Lateral Sulcus (s. lakralis lateral half of the spinal cord is partially divided near the junction of the posterior fourth with the anterior three-fourths of its semi circumference by the posterior lateral sulcus (Fig. 139). The sulcus is situated opposite the posterior columna of gray matter, to which it transmits the posterior roots of the spinal nerves. It is continuous above with the posterior lateral sulcus of the medulla. It separates the posterior surface and the antero lateral surface from each other.
Anterior Root-line (s. lateralis anterior).—As a landmark, it is convenient to call the longitudinal line through which issue the most lateral fibers of the anterior roots of the spinal nerves, the anterior root-line of the spinal cord (Fig. 139). There is no
groove on the surface of the cord along this line and it is mis leading to call it a sulcus, as has been the custom. It is situated opposite the anterior columna of gray matter and in line with the anterior-lateral groove of the medulla oblongata. Through it and through the surface, just medial to it, emerge the anterior roots of the spinal nerves. It subdivides the antero-lateral surface into anterior and lateral surfaces.
The posterior intermediate sulcus (s. intermedius posterior) is a slight longitudinal groove in the cord which subdivides the upper three-fourths of the posterior surface into postero-medial surface and postero-lateral surface (Fig. 139). From it a con nective-tissue septum extends into the cord and separates the fasciculus gracilis and fasciculus cuneatus from each other. The posterior intermediate furrow is found only in the medulla and in the cervical and upper eight thoracic segments of the cord.
Sulcus Intermedius Anterior.—Rarely an anterior inter mediate sulcus is found along the lateral border of the anterior pyramidal tract. It was described by Rauber.