NERVOUS SYSTEM. 



half of the cord as consisting of three columns : The ante- 

 rior column is bounded by the anterior fissure and the ori- 

 gin of the anterior roots of the spinal nerves ; the lateral 

 column is included between the anterior and the posterior 

 roots of the nei'A^es ; the posterior column is bounded by the 

 line of origin of the posterior roots and the posterior fis- 

 sure. Some anatomists include the lateral with the anterior 

 column, under the name of the antero-lateral column, taking 

 in about two-thirds of the cord. Next the posterior median 

 fissure, is a narrow band, marked by a faint line, which is 

 sometimes called the posterior median column. 



The arrangement of the white and the gray matter in 

 the cord is seen in a transverse section. The gray substance 

 is in the form of a letter H, presenting two anterior and two 

 posterior cornua connected by whatsis called the gray com- 

 missure. The anterior cornua are the shorter and broader, 

 and do not reach to the surface of the cord. The posterior 

 cornua are larger and narrow, and extend nearly to the sur- 

 face, at the point of origin of the posterior roots of the spi- 

 nal nerves. In the centre of the gray commissure, is a very 

 narrow canal, lined by cells of ciliated epithelium, called the 

 central canal. This is in communication above with the 

 fourth ventricle, and extends below to the filum terminale. 

 That portion of the gray commissure in front of this canal 

 is sometimes called the anterior gray commissure, the poste- 

 rior portion being known as the posterior gray commissure. 

 The central canal is immediately surrounded by connective 

 tissue. In front of the gray commissure, is a mass of white 

 substance known as the anterior white commissure. 



The proportion of the white to the gray substance is 

 variable in different portions of the cord. In the cervical 

 region, the white substance is most abundant, and, in fact, 

 it progressively increases in quantity from below upward 

 throughout the whole extent of the cord. In the dorsal 

 region, the gray matter is least abundant, and it exists in 

 greatest quantity in the lumbar enlargement. 



