542 



THE NERVOUS SYSTEM 



the whole length of the cord, opening above into the space at the back of 

 the medulla oblongata and pons Varolii called the fourth ventricle. The 

 canal is lined by a layer of columnar ciliated epithelium. 



The spinal cord consists of exactly symmetrical halves, separated an- 

 teriorly and posteriorly by vertical -fissures (the posterior fissure being deeper 

 but less wide and distinct than the anterior), and united in the middle by 

 nervous matter which forms the commissures. The central part, which 

 contains the central canal, is known as the gray commissure, and is bounded 



15 15 



i-i 



FIG. 360. Horizontal Section of the Cord and its Envelopes, at the Middle of a 

 Vertebral Body (Schematic), i, Spinal cord with 2, its anterior median fissure; 3, its 

 posterior median fissure; 4, anterior roots; 5, posterior roots; 6, pia mater (in red); 7, 

 ligamentum dentatum; 8, connecting fibers passing from the pia to dura mater; 9, visceral 

 layer, and 9', parietal layer of the arachnoid (in blue); 10, subarachnoid space; n, arach- 

 noid cavity; 12, dura mater (in yellow); 13, periosteum; 13', external periosteum; 14, 

 cellular tissue situated between the dura mater and the w r all of the vertebral canal; 15, 

 common posterior vertebral ligament; 16, intraspirial veins; 17, vertebra in section. 

 (Testut.) 



by the anterior white commissure in front. Each half of the spinal cord is 

 marked on the sides (obscurely at the lower part, but distinctly above) by 

 two longitudinal furrows, which divide it into three portions, funiculi, or tracts 

 an anterior, lateral, and posterior. From the groove between the anterior 

 and lateral funiculi spring the anterior roots of the spinal nerves; and just 

 in front of the groove between the lateral and posterior funiculi arise the 

 posterior roots of the same; a pair of roots on each side corresponding to 

 each segment of the cord. 



