670 NERVOUS SYSTEM. 



below to the filum terminate. That portion of the gray commissure situated in front 

 of this canal is sometimes called the anterior gray commissure, the posterior portion 

 being known as the posterior gray commissure. The central canal is immediately sur- 

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

 substance known as the anterior white commissure. 



13 



12 



u 13. n 



FIG. 222. Transverse section of the spinal cord at the origin of the ffth pair of cervical nerves. (Stilling.) 



In this figure, the white substance of tne cord is represented in black, to show more clearly the limits of the grny 

 matter: 1, 1, antero-lateral columns; 2, 2, posterior white columns; 8, anterior median fissure; 4, posterior 

 median fissure; 5, white commissure ; 6, gray commissure: 7, central canal; 8, 9, anterior cornua of gray mat- 

 ter; 10, 10, group of large multipolar cells : 11, 11, 11, anterior roots of the spinal nerves; 12, posterior cornua 

 of gray matter ; 13, posterior roots of the spinal nerves. 



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 great- 

 est quantity in the lumbar enlargement. 



The white substance of the cord is composed of nerve-fibres, connective-tissue ele- 

 ments, and blood-vessels, the latter arranged in a very wide and delicate plexus. The 

 nerve-fibres are variable in their size and are composed of the axis-cylinder surrounded 

 by the medullary substance, without, however, the investing membrane. We shall speak 

 farther on of the direction of the fibres in the cord. 



The anterior cornua of gray matter contain blood-vessels, connective-tissue elements, 

 very fine nerve-fibres, and large multipolar nerve-cells, which are sometimes called motor 

 cells. The posterior cornua are composed of the same elements, the cells being much 

 smaller, and the fibres exceedingly small, presenting very fine plexuses. The cells in this 

 situation are sometimes called sensory cells. Near the posterior portion of each poste- 

 rior cornu, is an enlargement, of a gelatiniform appearance, containing numerous small 

 cells and fibres, called the substantia gelatinosa. 



The foregoing description of the different structures and parts of the cord is neces- 

 sary to a comprehension of the direction of the fibres in the spinal axis and their con- 

 nections with the nerve-cells, which is the anatomical basis of our knowledge of its 

 physiology. The connections between the cells and the fibres have already been de- 

 scribed in the chapter upon the general structure of the nervous system. The multipolar 

 nerve-cells are supposed to present certain prolongations which do not branch and are 

 directly connected with the medullated nerve-fibres. These are called nerve-prolonga- 



