84 HUMAN PHYSIOLOGY. 



largest in the anterior horns of gray matter of the spinal cord. Some of 

 the cell processes become continuous with the fibres of the white matter, 

 while others anastomose with those of adjoining cells and form a plexus. 



Structure of the White Matter. The white matter, found for the 

 most part in the interior of the brain, on the surface of the spinal cord, and 

 in almost all the nerves of the cerebro- spinal and sympathetic systems, 

 consists of minute tubules or fibres, the ultimate nerve filaments, which in 

 the perfectly fresh condition, are apparently structureless and homogeneous ; 

 but when carefully examined after death are seen to consist of three distinct 

 portions, (i) a tubular membrane; (2) the white substance of Schwann; 

 (3) the axis cylinder. 



The Tubular membrane, investing the nerve filament, is thin, homoge- 

 neous, and lined by large, oval nuclei, and presents, in its course, annular 

 constrictions; it serves to keep the internal parts of the fibre in position, 

 and protects them from injury. 



The White substance of Schwann, or the medullary layer, is situated 

 immediately within the tubular membrane, and gives to the nerves their 

 peculiar white and glistening appearance. It is composed of oleaginous 

 matter in a more or less fluid condition; after death it undergoes coagula- 

 tion, giving to the fibre a knotted or varicose appearance. It serves to 

 insulate the axis cylinder, and prevents the diffusion of the nerve force. 



The Axis cylinder occupies the centre of the medullary substance. In 

 the natural condition it is transparent and invisible, but when treated with 

 proper reagents, it presents itself as a pale, granular, flattened band, albu- 

 minous in character, more or less solid, and somewhat elastic. It is com- 

 posed of a number of minute fibrillae united together to form a single 

 bundle. (Schultze.) 



Nerve fibres in which these three structural elements coexist are known 

 as the medtillated nerve fibres. In the sympathetic system, and in the gray 

 substance of the cerebro- spinal system, many nerves are destitute of a me- 

 dullary layer, and are known as the non-medullated nerve fibres. 



Gray or Gelatinous nerve fibres, found principally in the sympathetic 

 system, are gray in color, semi-transparent, flattened, with distinct borders, 

 finely granular, and present oval nuclei. 



The diameter of the gelatinous fibres is about the-g^ 1 ^ of an inch; of 

 the medullated fibres from ^^Q- to T ^Q^ of an inch. 



Ganglia are small bodies, varying considerably in size, situated on the 

 posterior roots of spinal nerves, on the sensory cranial nerves, alongside of 

 the vertebral column, forming a connecting chain, and in the different vis- 



