ANATOMY OF THE SPINAL COED. 267 



The white substance of the cord is composed of nerve- 

 fibres, connective-tissue elements, and blood-vessels, the lat- 

 ter 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, with- 

 out, however, the investing membrane. AYe will speak far- 

 ther 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 ele- 

 ments, the cells being much smaller, and the fibres exceed- 

 ingly small, presenting very fine plexuses. The cells in this 

 situation are sometimes called sensory cells. I^ear the pos- 

 terior portion of each posterior 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 necessary to a comprehension of the di- 

 rection of the fibres in the spinal axis and their connections 

 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 described in the chap- 

 ter on the general structure of the nervous system. 1 The 

 multipolar nerve-cells are supposed to present certain pro- 

 longations which do not branch and are directly connected 

 with the medullated nerve-fibres. These are called nerve- 

 prolongations. In addition, fine, branching poles are de- 

 scribed under the name of protoplasmic prolongations. 



The direction of the fibres in the cord is one of the most 

 difiicult and complicated questions in physiological anatomy ; 

 and, especially as regards the posterior roots of the nerves, 

 is one which cannot as yet be elucidated by purely anatomi- 

 cal investigations, but requires the aid of experimental and 

 pathological observations. 



1 See page &0. 



