THE CENTRAL NERVOUS SYSTEM. 299 



cular canals may be more clearly demonstrated by forcing a 

 colored gelatine injection at any point under the spinal pia 

 mater care being taken, if the cord is cut, to secure the ends 

 of the sections by ligatures. 



Nerve- elements of the cord. The consideration of the ner- 

 vous elements of the cord will now be taken up in a general 

 way, and the peculiarities of different regions explained later. 



The white substance of the cord contains, besides the blood- 

 vessels and neuroglia already mentioned, myelinic nerve-fibres 

 of different sizes. These fibres pursue a vertical course, with 

 the exception of those forming the root-radicles and commis- 

 sures. 



On examination with a low power, the white substance, in a 

 transverse section stained with carmine, seems to be a collec- 

 tion of minute rings, each with a red dot in the centre. More 

 highly magnified, the transverse section of a nerve-fibre appears 

 as a delicate, rather irregular circle, on the circumference of 

 which, in some cases, are seen nuclei resembling those of the 

 sheath of Schwann, but which are really nuclei of the neurog- 

 lia. Next comes a broad ring of colorless, transparent mate- 

 rial, the medullary or myelinic- sheath, which very often ex- 

 hibits concentric lamination. Lastly, usually in the centre, is 

 seen the solid axis-cylinder. 



When these fibres pursue a more or less horizontal direc- 

 tion they give the appearance of broad, clear bands traversed 

 by longitudinal red fibres (axis-cylinders). The myelinic fibres 

 average about 5 p 1 in diameter. Fibres, when isolated by 

 teasing, present the varicose aspect of myelinic nerve-fibres, 

 which lack the sheath of Schwann. To demonstrate this they 

 are best treated when in the fresh state by osmic acid (see p. 

 114), the result being a black myelinic sheath and brownish 

 axis-cylinder. 



The gray matter is composed of nerve-cells, medullated and 

 non-medullated nerve-fibres, and an amorphous matrix. The 

 most striking elements are the cells of the anterior horns. 

 These, whether teased from specimens fresh or hardened in 

 chromic acid, or whether examined in sections, always present 

 the same general appearance. They are large, multipolar cells, 

 having a slightly granular, protoplasmic body, a large, oval 



ft = micromillimetre = Tihro millimetre. 



