1-8, the eight cervical nerves. _I_-_II_, first two thoracic nerves. _a_, dorsal roots (ventral roots not shown); _b_, spinal ganglia; _c_, dorsal rami; _d_, ventral rami; _e_, posterior median sulcus; _f_, posterior lateral sulcus.] NERVOUS SYSTEM. I. THE CENTRAL NERVOUS SYSTEM. 1. =The Spinal Cord. Medulla spinalis= (Figs. 133-136).--The spinal cord is that portion of the nervous system which occupies the vertebral canal; it is continuous craniad with the brain. It has the form of a somewhat flattened cylinder and extends from the foramen magnum into the caudal region. It diminishes in diameter after entering the sacral region. The cord has a cervical and a lumbar enlargement, the former (Fig. 133) marking the origin of the nerves which pass to the fore limb, and the latter (Fig. 136) the origin of those which pass to the hind limb. The =cervical= enlargement (Fig. 133) lies in that part of the vertebral canal bounded by the fourth to seventh cervical and first thoracic vertebræ. Caudad of the first thoracic vertebra the cord continues of nearly uniform diameter to the lumbar enlargement (Fig. 136) which stretches from the third to the seventh lumbar vertebræ (inclusive). Caudad of the seventh lumbar vertebra it diminishes uniformly in diameter and ends in a slender cord, the =filum terminale= (Fig. 136, _C_), which may be traced into the caudal region. The surface of the cord is marked by a number of longitudinal grooves or =sulci=, and =fissures=. The most prominent of these is the =anterior median fissure= (Fig. 134, _c_), along the ventral median line; into this the pia mater dips. The =posterior median sulcus= (Fig. 133, _e_; Fig. 134, _a_) is a shallow furrow along the dorsal median line. The anterior fissure and the posterior sulcus thus divide the cord into lateral halves. Each half is subdivided by the =anterior= and =posterior lateral sulci=. The =posterior= lateral sulcus (Fig. 133, _f_; Fig. 134, _b_) lies at the side of the posterior median sulcus. It is broad and shallow and has the posterior roots of the spinal nerves emerging from its bottom. The =anterior= lateral sulcus exists only after the forcible pulling out of the anterior (ventral) roots of the spinal nerves. It then marks the line along which they originate from the cord. In cross-section (Fig. 134) the substance of the cord appears to the naked eye as composed of a darker central “=gray matter=” (_g_) and of an outer “=white matter=” (_f_). The gray matter has in section the form of the letter H. The cord is really tubular, having a =central canal= (_h_) which appears in section in the middle of the cross-bar of the H, while the two ends of each vertical bar extend toward the anterior and posterior lateral fissures. The white matter is divided into funiculi (or columns) by the fissures and sulci, so that there is in each half of the cord an =anterior=, a =posterior=, and a =lateral= white funiculus.