THE SPINAL CORD. 697 



tains a prolongation from the pia mater, and its floor is formed by the anterior 

 white commi**ni-i\ which is perforated by numerous blood-vessels passing to the 

 centre of the cord. 



The Posterior Median Fissure is not an actual fissure, as the space between 

 the lateral halves of the posterior part of the cord is crossed by connective tissue 

 and numerous blood-vessels, so that no actual hiatus exists, and there is conse- 

 quently no prolongation of the pia mater into it. It extends into the cord to 

 about one half its depth, and its floor is formed by the posterior gray commissure. 



Lateral Fissures. On each 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 

 antero-lateral fissure of the cord, although no actual fissure exists in this situation. 

 And on each 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 

 postero-lateral fissure of the spinal cord. On the posterior surface of the spinal 

 cord, between the posterior median and the postero-lateral fissure on each side, is 

 a slight longitudinal furrow (posterior intermediate furrow} marking off two 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. Each half of the spinal cord is thus divided into three 

 main columns : an antero-lateral column, a postero-lateral column, and a postero- 

 median column. 



The antero-lateral column, which forms rather more than two-thirds of the 

 entire circumference of the cord, includes all the portion of the cord between the 

 anterior median fissure and the postero-lateral fissure. 



By some anatomists the antero-lateral column is subdivided into an anterior 

 column, which includes all the portion of the cord between the anterior median 

 fissure and the line from which the anterior roots of the nerves arise; and a lat- 

 eral column, which includes all the portion between the line of origin of the ante- 

 rior roots of the spinal nerves and the postero-lateral fissure. 



The postero-lateral column is situated between the postero-lateral fissure and 

 the posterior intermediate furrow. 



The posterior median column is that narrow segment of the cord which is seen 

 on each side of the posterior median fissure, usually included with the preceding 

 as the posterior column. 



Structure of the Cord. If a transverse section of the spinal cord be made, it 

 will be seen to consist of white and gray nervous substance. The white matter is 

 situated externally, and constitutes the greater part. The gray substance occupies 

 the centre, and is so arranged as to present on the surface of the section two cres- 

 centic masses, placed one in each lateral half of the cord, united together by a 

 transverse band of gray matter, the gray commissure. Each crescentic mass has 

 an anterior and posterior horn. The posterior horn is long and narrow, and 

 approaches the surface of the postero-lateral fissure, near which it presents a slight 

 enlargement, the caput cornu : from this it tapers to form the apex cornu, which at 

 the surface of the cord becomes continuous with the fibres of the posterior roots of 

 the spinal nerves. The anterior horn is short and thick, and does not quite reach 

 the surface, but extends toward the point of attachment of the anterior roots of the 

 nerves. Its margin presents a dentate or stellate appearance. Owing to the pro- 

 jections toward the surface of the anterior and posterior horns of the gray matter, 

 each half of the cord is divided, more or less completely, into three columns, 

 anterior, middle, and posterior, the anterior and middle being joined to form the 

 antero-lateral column, as the anterior horn does not quite reach the surface. 



The commissure of the spinal cord is composed of white and gray fibres, hence 

 called the white and gray commissures. The -white commissure is formed of fibres 

 which, for the most part, pass horizontally between the gray matter of the ante- 

 rior horn of one side and the anterior white column of the opposite side. 



