646 NERVOUS SYSTEM. (NERVOUS CENTRES. GREY NERVOUS MATTER.) 



Fig. 370. 



ri'irC 

 'Hffe , 31 



Dura mater of part of the spinal cord laid open to 

 show the ligamciitum deiitatum. 



d d d d, dentated processes. On the right the 

 roots of the nerves and the ganglia of the posterior 

 roots are retained. 



all the characters of white fibrous tissue, of 

 which it is chiefly composed. In its dentated 

 processes, however, a considerable quantity of 

 yellow fibrous tissue may be found. The simi- 

 larity of its constitution with that of the pia 

 mater evidently justifies its being regarded as a 



process of that membrane, and not, as some 

 anatomists thought, of the dura mater, with 

 which it has a much less intimate and extensive 

 connexion. Its anterior and posterior surfaces 

 are uncovered by any membrane ; they are 

 smooth, and have the glistening silvery appear- 

 ance of white fibrous membrane. It is evident 

 that during life these surfaces must be bathed 

 by the subarachnoid fluid. 



The office of this remarkable structure seems 

 evidently to be mechanical ; to preserve the 

 spinal cord in a state of equilibrium ; and to 

 prevent lateral movement of it, whilst at the 

 same time it forms a partition between the 

 roots of the nerves. 



General remarks on the structure of the 

 nervous centres. It has already been shewn in 

 a former part of this article that the nerves 

 properly so called are composed exclusively 

 of one kind of nervous substance, namely, 

 the fibrous nervous matter, which is disposed in 

 bundles of peculiar fibres. It is only in the 

 nervous centres or in continuations of them 

 that we find an union of the white and the grey 

 nervous matter; and, indeed, it may be stated 

 in general, that the peculiar and distinctive 

 anatomical character of a nervous centre con- 

 sists in this combination of the two kinds of 

 nervous matter. 



In the nervous centres the white matter exhi- 

 bits, for the most part, the same essential cha- 

 racters of structure as in the nerves ; that is to 

 say, it is disposed in tubes containing a certain 

 pulpy matter in them. It has been found, how- 

 ever, that these tubes are much more prone to 

 become varicose under the influence of pressure 

 or of any other disturbing cause. They are 

 not, as in the nerves, bound together by areolar 

 tissue, but are disposed in bundles and on 

 different planes, with their nutrient bloodvessels 

 ramifying among them, and in some situations 

 the elements of the grey matter are interposed 

 between them. Certain parts of the nervous 

 centres are composed exclusively of white 

 matter, as a portion of the hemispheres of the 

 brain, and of the cerebellum, and the superficial 

 parts of the spinal cord. 



The white fibres which are found in the ner- 

 vous centres may be distinguished according 

 to their physiological office into four different 

 kinds. Two of these are continuations of the 

 fibres of the nerves, and serve to connect the 

 nervous centres with other organs or textures, 

 either by conveying the influence of the centres 

 to them, or by propagating impressions from 

 them to the centres. The former are called 

 efferent, the latter afferent fibres. In addition 

 to these, we find a third and large series of fibres, 

 which serve to establish a connection between 

 different centres, or between different portions of 

 the same centre. These are called comnmsural 

 fibres ; they form a large portion of the mass of 

 the brain and spinal cord. And Henle suggests 

 that the brain contains a fourth series of fibres, 

 associated with the operations of thought. 



We remark in the nervous centres, especially 

 in the brain and spinal cord, a greater difference 

 as regards size between the different nerve tubes, 

 than may be observed elsewhere, and it seems 

 to be a constant character that they diminish 



