458 



ELEMENTARY PHYSIOLOGY 



the crescents of the grey matter approach one another, and 

 are joined by the bridge wliich contains the central canal. 

 The portion of this bridge which lies immediately behind 

 the central canal is known as the posterior grey 

 commissure ; that portion which lies in front of it as 

 the anterior grey commissure. The latter is 

 separated from the inner end of the anterior fissure by a 

 thin bridge of white matter known as the anterior 

 white commissure. (See Figs. 143 and 150. ) 



There is a fundamental difference in structure between 



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FiQ. 144.— The Spinal Cord, 



A. A front view of a portion of the cord. On the right side, the 

 anterior roots, A.R., are entire ; on the left side they are cut, to show the 

 posterior roots, P.li. 



B. A transverse section of the cord. A, the anterior fissure; P, the 

 posterior fissure ; O, the central canal ; C, the grey matter ; W, the white 

 matter : A.R. the anterior root, P.K, the posterior root, Gn. the ganglion, 

 and 7', the trunk, of a spinal nerve. 



the grey and the white matter. The white matter consists 

 almost entirely of nerve-fibres supported in a delicate 

 framework of connective tissue, and accompanied by 

 blood-vessels. Most of these fibres run lengthways in the 

 cord, and consequently, in a transverse section, the white 

 matter is really composed of a multitude of the cut ends 

 of these fibres. 



The grey matter, on the other hand, contains in addi- 

 tion a number of nerve cells, some of them of considerable 

 size. These cells are wholly, or almost wholly, absent in 

 the white matter. 



