712 THE NERVOUS SYSTEM. 



the so-called anterior and lateral columns cannot be defined on account of the 

 bundles of the anterior roots being spread over a considerable area. It is there- 

 fore customary to divide each half of the spinal cord into two columns, separated 

 by the postero-lateral groove : (1) a small posterior column, which is bounded in- 

 ternally by the posterior median fissure, and externally by the postero-lateral fissure, 

 and (2) a large antero-lateral column, which comprises the rest of the cord. The 

 posterior column is further divided, at all events at its upper part, by the posterior 

 intermediate septum, into a postero-mediaii column and a postero-lateral 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 occu- 

 pies the 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 (ventral) and a posterior (dorsal) 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 some of 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 attach- 

 ment of the anterior roots of the nerves. Its margin presents a dentate or stellate 

 appearance. Owing to the projections toward the surface of the anterior and poste- 

 rior horns of the gray matter, each half of the cord is divided, more or less com- 

 pletely, 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 matter, and 

 is therefore divided into the white and gray commissures. The white commissure 

 is situated at the bottom of the anterior median fissure, and is formed of medullated 

 nerve-fibres, which pass between the gray matter of the anterior horn and the 

 anterior white column of the one side into similar parts on the other. The fibres 

 are oblique in direction ; many which enter at the posterior part of the commissure 

 on the one side leave it at the anterior part of the commissure on the other, and 

 vice versd, a decussation taking place in the middle line. 



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. It consists of transverse medullated nerve-fibres, with a considerable 

 quantity of neuroglia between them. The fibres when they reach the lateral cres- 

 cents diverge : some pass backward to the posterior roots ; others spread out, at 

 various angles, into the crescent. 



Running through the gray commissure of the whole length of the cord is a 

 hninute canal, which is barely visible to the naked eye in the human cord, but is 

 proportionately larger in some of the lower vertebrata. It is called the central 

 canal*}* ^it opens above into the fourth ventricle, and terminates below in a some- 

 what diluted extremity. It is surrounded by an area of neuroglia, which, in the 

 recent state> has a gelatinous appearance, and in which there are no nerve-fibres. 

 This is sometimes called the substantia gelatinosa centralis. When hardened in 

 alcohol or chrWnic salts it has a finely reticulated appearance. The canal is lined 

 in the foetus by? columnar ciliated epithelium, but in the adult the cilia have disap- 

 peared, and the \canal is filled with their remains. 



The mode of\ arrangement of the gray matter, and its amount in proportion to 

 the white, vary i\n 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, and the white matter more abundant than in any other region of the cord. 

 In the dorsal reguon the gray matter is least developed, the white matter being 

 also small in quantity . In the lumbar region the gray matter is more abundant 



