550 NERVOUS SYSTEM 



nective-tissue elements, neuroglia, and bloodvessels, the latter arranged 

 in a wide and delicate plexus. The nerve-fibres are variable in size and 

 are composed of the axis-cylinder and the medullary substance, without 

 the tubular membrane. 



The anterior cornua of gray matter contain bloodvessels, connective- 

 tissue elements, neuroglia, 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 posterior cornu, is an enlargement, of a gela- 

 tiniform character, containing small cells and fibres, called the sub- 

 stantia gelatinosa. The relations between the nerve-cells and the 

 nerve-fibres have already been described in connection with the general 

 structure of the nervous system. The multipolar nerve-cells present 

 certain prolongations which do not branch and are directly connected with 

 the medullated nerve-fibres. These are the neurites, or axis-cylinder pro- 

 longations. In addition, fine branching poles are described under the 

 name of dendrites, or protoplasmic prolongations. The neuroglia is 

 particularly abundant in that part of the posteria cornua of gray matter 

 called the substantia gelatinosa (see Plate XII). 



The division of the spinal cord into columns has a physiological 

 as well as an anatomical basis. Anatomists usually recognize, on either 

 side of the cord, an anterior column, bounded by the anterior median 

 fissure and the line of origin of the anterior roots of the spinal nerves, 

 a lateral column, bounded by the lines of origin of the anterior and of 

 the posterior roots of the nerves, and a posterior column, bounded by 

 the line of the posterior roots of the spinal nerves and the posterior 

 median fissure. As the anterior or posterior columns include either 

 white or gray matter, they are called respectively anterior or pos- 

 terior white or gray columns. Physiological and pathological re- 

 searches, however, have shown that the cord may properly be further 

 divided as follows : 



1. Columns of Tiirck. By the sides of the anterior median fissure, 

 are two narrow columns of white matter, one on either side, extending 

 to the white commissure, called the columns of Tiirck, the direct, or the 

 uncrossed pyramidal tracts. The fibres of these columns descend, 

 probably decussate in the cervical region of the cord, and the columns 

 are lost in the lower dorsal region. Destruction of certain motor parts 

 in the brain is followed by descending secondary degenerations in these 

 columns. 



2. Crossed Pyramidal Tracts. These are situated, one on either 



