SPINAL CORD. 93 



missure a distinct group of cells (Clarke's posterior vesicular 

 column) will be seen, if the section be low enough in the 

 thoracic region. The cells have an oval outline. 



The grey commissure, containing blue-stained fibres, encloses 

 the central canal of the cord. This is lined by columnar 

 ciliated cells, the attached ends of which are tapered, and 

 branch into the surrounding central gelatinous substance. On 

 each side of this a branch of the anterior spinal artery is 

 frequently seen in cross section. The white substance is com- 

 posed of medullated nerve fibres embedded in a spongework of 

 special connective tissue, the neuroglia. This forms a contin- 

 uous layer upon the surface of the cord, and from it septa pass 

 inwards, giving off-sets in all directions. Nuclei are found in it. 

 The nerves seen in cross section differ much in size. Forming 

 the centre of each fibre observe the axon surrounded by the 

 medullary sheath, which frequently exhibits concentric mark- 

 ings, medullary segmental junctions. 



Fresh cord, for nerve fibres. Place a small fragment of the 

 white substance upon a slide with a little normal saline, cover 

 and compress slightly. (H) Note the medullated fibres, which 

 soon exhibit regular fusiform expansions as the medullary 

 sheath swells. This indicates the absence of the neurilemma. 



Spinal cord of cat. (Injected blue gelatin mass, p. Formol 

 2p- c -'C. G., m. B.) (L) Find the anterior spinal artery entering 

 through the ventral fissure and breaking up into branches at 

 the commissure, from which they radiate into the grey sub- 

 stance. A superficial set of vessels are distributed upon the 

 surface in the pia mater, the branches of which converge 

 through the white substance to the grey, which they do not 

 always reach> The arteries are terminal. (H) Note the rich 

 capillary distribution in the grey matter. 



