CHAPTER XVI. 



THE CENTRAL NERVOUS SYSTEM. 

 THE SPINAL CORD. 



THE spinal cord contains nerve-cells and nerve- 

 fibres ; the former confined to the central, the latter 

 most abundant in the peripheral, portions. These 

 nerve-elements lie in the meshes of peculiarly 

 arranged connective tissue in which blood-vessels 

 ramify, and the whole is surrounded by a vascular 

 connective-tissue membrane the pia mater spinalis. 

 On its anterior and posterior surfaces, narrow fis- 

 sures, reaching nearly to the centre, divide the cord 

 into lateral halves ; into these fissures, called the 

 anterior and posterior longitudinal fissures, the pia 

 mater sends membranous prolongations. The 

 anterior fissure is complete, the pia being found on 

 both of its sides, which can be easily separated ; 

 while in the posterior fissure the sides are bound 

 together by the pia. 



In transverse sections of the cord a central gray 

 portion is seen which is surrounded by an irregular 

 white zone. The form which the gray matter as- 



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