SPINAL CORD. 



503 



Fis. 255. Spinal Cord, Side 

 View. Plan of tho Fissures 

 and Columns. 



than the cord ; so that, in the child at birth, it extends as far as the third lumbar 

 vertebra. Its position varies also according to the degree of curvature of the 

 spinal column, being raised somewhat in flexion of the spine. On examining 

 its surface it presents a difference in its diameter in different parts, being marked 

 by two enlargements, an upper or cervical, and a lower or lumbar. The cervical 

 enlargement, which is the larger, extends from the third cervical to the first dorsal 

 vertebra : its greatest diameter is in the transverse direction, and it corresponds 

 with the origin of the nerves which supply the upper extremities. The lower 

 or lumbar enlargement is situated opposite the last dorsal vertebra, its greatest 

 diameter being from before backwards. It corresponds with the origin of the 

 nerves which supply the lower extremities. In form, the spinal cord is a flattened 

 cylinder. Its anterior surface presents, along the middle line, a longitudinal 

 fissure, the anterior median fissure ; and, on its posterior surface, another fissure 

 exists, which also extends along the entire length of the cord, the posterior median 

 fissure. These fissures serve to divide the cord into two symmetrical halves, 

 which are united in the middle line, throughout their entire length, by a transverse 

 band of nervous substance, the commissure. 



The Anterior median fissure is wider, but of less 

 depth, than the posterior, extending into the cord for 

 about one-third of its thickness, and is deepest at 

 the lower part of the cord. It contains a prolonga- 

 tion from the pia mater ; and its floor is formed by 

 the anterior white commissure, which is perforated by 

 numerous bloodvessels, which pass to the centre of 

 the cord. 



The Posterior median fissure is much more deli- 

 cate than the anterior, and most distinct at the upper 

 and lower "parts of the cord. It extends into the 

 cord to about one-half of its depth. It contains a 

 very slender process of the pia mater and numerous 

 bloodvessels, and its floor is formed by a thin layer 

 of white substance, the posterior white commissure. 

 Some anatomists state, that the bottom of this fissure 

 corresponds to the gray matter, except in the cervical region, and at a point 

 corresponding to the enlargement in the lumbar region. 



Lateral Fissures. On either side of the anterior median fissure, a linear series 

 of foramina may be observed, indicating the points where the anterior roots of 

 the spinal nerves emerge from the cord. This is called, by some anatomists, the 

 anterior lateral fissure of the cord, although no actual fissure exists in this 

 situation. On either side of the posterior median fissure, along the line of 

 attachment of the posterior roots of the nerves, a delicate fissure may be seen, 

 leading down to the gray matter which approaches the surface in this situation : 

 this is called the posterior lateral fissure of the spinal cord. On the posterior 

 surface of the spinal cord, on either side of the posterior median fissure, is a slight 

 longitudinal furrow, marking oft' two slender tracts, the posterior median columns. 

 These are most distinct in the cervical region, but are stated by Foville to exist 

 throughout the whole length of the cord. 



Columns of the Cord. The fissures divide each half of the spinal cord into 

 four columns, an anterior column, a lateral column, a posterior column and a 

 posterior median column. 



The anterior column includes all the portion of the cord between the anterior 

 median fissure and the anterior lateral fissure, from which the anterior roots of 

 the nerves arise. It is continuous with the anterior pyramid of the medulla 

 oblongata. 



The lateral column, the largest segment of the cord, includes all the portion 

 between the anterior and posterior lateral fissures. It is continuous with tho 

 lateral column of the medulla. By some anatomists, the anterior and lateral 



