504 NERVOUS SYSTEM. 



columns are included together, under the name of the antero-lateral column, 

 which forms rather more than two-thirds of the entire circumference of the 

 cord. 



The posterior column is situated between the posterior median and posterior 

 lateral fissures. It is continuous with the restiform body of the medulla. 



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 its chief portion. The gray substance 

 occupies its centre, and is so arranged as to present on the surface of the section 

 two crescentic 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 at the posterior lateral fissure, near which it presents a 

 slight enlargement. The anterior horn is short and thick, and does not quite 

 reach the surface, but extends towards the point of attachment of the anterior 

 roots of the nerves. Its margin presents a dentate or stellate appearance. Owing 

 to this peculiar arrangement of the gray matter, the anterior and posterior horns 

 projecting* towards the surface, 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 gray commissure, which connects the two crescentic masses of gray matter, 

 is separated from the bottom of the anterior median fissure by the anterior white 

 commissure ; and, from the bottom of the posterior fissure by the posterior white 

 commissure. The gray commissure consists of a transverse band of gray matter, 

 and of white fibres, derived from the opposite half of the cord and the posterior 



roots of the nerves. The anterior commissure is 



Fig. 256. Transverse Sections formed, partly of fibres from the anterior column, 



and partly from the fibrils of the anterior roots of 

 the spinal nerves, which decussate as they pass across 

 from one to the other side. 



^_^ The mode of arrangement of the gray matter, and 



Middle of cervical rcgz its amount in proportion to the white, vary in different 

 parts of the cord. Thus, the posterior horns are 

 long and narrow, in the cervical region; short and 

 narrower, in the dorsal; short, but wider, in the 

 lumbar region. In the cervical region, the crescentic 

 portions are small, the white matter more abundant 



iu Middle, of Dortai reg*. than in any other region of the cord. In the dorsal 

 region, the gray matter is least developed, the white 

 matter being also small in quantity. In the lumbar 

 region, the gray matter is more abundant than in 

 any other region of the cord. To\\ r ards the lower 

 end of the cord, the white matter gradually ceases. 

 The crescentic portions of the gray matter soon blend 

 into a single mass, which forms the only constituent of its extreme point. 



The white substance of the cord consists of transverse, oblique, and longitudinal fibres, with 

 wloodvessels and connective tissue. 



The transvi-rse fibres proceed from the gray substance, and form with each other a kind of 

 plexus between the bundles of longitudinal fibres with which many are continuous ; while others 

 reach the surface of the cord through fissures containing connective tissue. Within the gray 

 substance they are continuous with the roots of the nerves, the processes of the nerve cells, and 

 with the anterior and posterior commissures. The oblique fibres proceed from the gray substance 

 both upwards and downwards : they form the deep strata of the white columns, and, after running 



