304 PRACTICAL ANATOMY. 



brain, is a shut serous sac having a closely attached 

 parietal and a loosely attached visceral layer. 



The pia mater, thick, less vascular than in the 

 brain, sends processes into the principal fissures of 

 the cord and at the sides forms the dentate ligament, 

 a series of processes attached to the dura mater, and 

 which help to sustain the cord in position. The spinal 

 cord is about sixteen inches long. It extends from the 

 atlas to the first lumbar vertebra, weighs about an 

 ounce and a half, and consists of a cylinder of nerve- 

 tissue which terminates below in a leash of nerve-bundles 

 called the cauda equina. The white substance of the 

 cord is on the exterior; the gray matter, arranged in the 



5 



FIG. 135. SECTION OF SPINAL CORD AND MEMBRANES. 



1, dura mater; 2, arachnoid membrane; 3, ganglion on posterior root; 4, anterior root 

 of spinal nerve; 5, subaraclmoid space; 6, posterior branch of spinal nerve; 7, anterior 

 branch of spinal nerve. 



form of the letter H on transverse section, is within. The 

 cord presents two enlargements, the cervical and lum- 

 bar, corresponding to the roots of origin of the brachial 

 and lumbar plexuses, respectively. The cord is divided 

 into two symmetrical halves "By the anterior and posterior 

 median fissures. The anterior median fissure extends 

 the entire length of the cord, and is continuous with the 

 same fissure of the medulla. It is not as deep as the 

 posterior median fissure, which, like the anterior, extends 

 the entire length of the cord and is continuous with the 

 median fissure on the floor of the fourth ventricle. Each 

 half of the cord presents four columns, the anterior, 

 lateral, posterior, and posterior median ; separated by 



