NEKYOUS SYSTEM AND GENEEAL SENSATION. 43 



tween the tenth dorsal and first lumbar vertebrae ; its inferior 

 termination presents considerable variety ; the spinal cord is 

 divided into two lateral halves by sulci, extending, on its ante- 

 rior and posterior surfaces, throughout its entire length ; it is 

 composed of white and grey substance : the grey occupying the 

 centre, and the white the periphery of the organ. About an 

 inch below the pons the pyramidal bodies of the anterior 

 columns communicate very freely. The white fibrous layer 

 dips into the sulcus, and its fibres interlace along the median 

 line ; those from the right column passing into the left, and 

 vice versa, whilst on the posterior columns no such interchange 

 of fibres is observed : experiments have proved that the an- 

 terior columns are the motory, the posterior columns the sen- 

 sitive centres of the cord. 



[ 90. The spinal cord gives attachment to thirty-one pairs of 

 nerves, which are regular, symmetrical, and double-rooted ; one 

 of the roots of each nerve (fig. 21*, d] is united to the anterior 

 column, the other (6) to the posterior column of the cord ; on 

 the posterior root a ganglion (c) is formed ; the anterior root (d) 

 joins the posterior (A)external to it, and thus forms a nerve (e, f) 

 compound in structure and function. 

 Sir Charles Bell, Mayo, Majendie, and 

 others, have proved by experiments 

 that sensation depends on the posterior 

 root, and the power of voluntary motion 

 on the anterior root. The cord is at- 

 tached, throughout its whole length, 

 to the tube of the dura mater by a 

 thin shining membrane, derived from Y 21*. A segment of 

 the pia mater, which sends out about the sp j nal c ' ord) to how (he 

 twenty dentate processes, to pin it double origin of the spinal 

 to that fibrous sheath ; this ligament nerves : b, the posterior 



is hence called membrana dentata : it root 5 c the ganglion of 



-, c , i P , that root ; d, the anterior 



extends from the foramen magnum to root; g> ' the uud 



the first lumbar vertebra, and forms nerve . 

 a vertical septum, separating the 



anterior from the posterior roots of the nerves. The sheath 

 of the dura mater is not entirely occupied by the spinal cord, 

 but contains a considerable quantity of limpid fluid, in which it 

 is suspended. By this admirable provision this nervous centre 

 is preserved from pressure and commotion, in violent move- 

 ments of the vertebral column. 



