Chapter 3 The Different Parts of the Nervous System 73 74 MAIN FEATURES OF STRUCTURE AND FUNCTION Figure 3.4 Cross section of the spinal cord at a lumbar level. Note the central H-shaped region of gray matter and the subdivision of the surrounding white matter into funiculi. column growing more rapidly than the spinal cord. In early embryonic life, the neural tube and the primordium of the vertebral column are equally long (Fig. 3.12). The spinal cord is somewhat flattened in the anteroposterior direction and is not equally thick along its length. In general, the thickness decreases caudally, but there are two marked intumescences (Fig. 3.3), the cervical and lumbar enlargements (intumescentia cervicalis et lumbalis). The intumescences supply the extremities with sensory and motor nerves, hence the increased thickness. In the midline along the anterior aspect of the cord, there is a longitudinal furrow or fissure (ventral median fissure, Fig. 3.4). Some of the vessels of the cord enter through this fissure and penetrate deeply into the substance of the cord. At the posterior aspect of the cord there is a corresponding, but more shallow, furrow in the midline (the posterior median sulcus). In addition, on each side there are shallow, longitudinal sulci anteriorly and posteriorly (the anterior and posterior lateral sulci). These laterally placed sulci mark where the spinal nerves connect with the cord. The color of the spinal cord is whitish because the outer part consists of axons, many of which are myelinated. The consistency of the cord, as of the rest of the central nervous system, is soft and jellylike. Spinal Nerves Connect the Spinal Cord with the Body Axons mediating communication between the central nervous system and other parts of the body make up the peripheral nerves. The axons (nerve fibers) leave and enter the cord in small bundles called rootlets (Fig. 3.5). Several adjacent rootlets unite to a thicker strand, called a root or nerve root. In this manner, rows of roots are formed along the dorsal and ventral aspects of the cord-the ventral (anterior) roots and the dorsal (posterior) roots, respectively. Each dorsal root has a swelling, the spinal gan- glion, which contains the cell bodies of the sensory axons entering the cord through the dorsal root (Figs. 3.3 and 3.5). A dorsal and a ventral root unite to form a spinal nerve (nervus spinalis). The spinal ganglion => page 75    start page