NERVOUS CENTRES. (HUMAN ANATOMY. THE MENINGES.) 



653 



the middle line, and that to this cause the in- 

 distinctness of the posterior fissure may be due. 

 And this anatomical fact may be quoted as, in 

 some degree, adverse to the theory which re- 

 gards these columns as sensitive : for were they 

 columns of sensation, it is probable that the 

 preservation of their distinctness would have 

 been more fully provided for. 



The anterior and posterior fissures, as Cru- 

 veilhier remarks, leaving on each side a per- 

 fectly symmetrical organ, serve to demonstrate 

 the existence of two spinal cords, one for eacli 

 side of the body, and both presenting a perfect 

 resemblance of form and structure. 



There are no other fissures in the cord 

 besides those just described. Several anato- 

 mists regard the lines of origin of the anterior 

 and posterior roots of the nerves as constituting 

 distinct fissures. But a little careful examina- 

 tion will readily convince any one that there is 

 no real separation of the nervous substance of 

 the cord corresponding to these lines, and that 

 there is no anatomical indication of a sub- 

 division into columns or segments in connexion 

 with them. When the roots of the nerves have 

 been removed on each side, nothing is seen but 

 a series of foramina or depressions correspond- 

 ing to the points of emergence of the nerve- 

 fibres, of which the roots are composed. 



The most natural subdivision of the spinal 

 cord is that which is obviously indicated by its 

 internal structure. In examining a transverse 

 section (fig. 376), we observe that the interior 



Fig. 376. 



of each lateral portion is occupied by grey 

 matter, disposed somewhat in a crescentic form. 

 The concavity of the crescent is directed out- 

 wards ; its anterior extremity is thick, and is 

 separated from the surface of the cord by a con- 

 siderable layer of white nervous substance. The 

 grey matter is prolonged backwards and out- 

 wards in the form of a narrow horn, which 

 reaches quite to the surface of the cord, and near 

 the surface experiences a slight enlargement. 

 This posterior horn constitutes, on each side, a 

 natural boundary between the two columns of 

 which each lateral half of the cord consists. All 

 that is situate in front of the posterior horns is 

 called the antero-hderal column, and this com- 

 prehends the white matter forming the sides and 

 front of the semi-cord, limited anteriorly by the 

 anterior fissure and posteriorly by the posterior 

 roots of the nerves. The posterior column is 

 situate behind the posterior horn of grey matter, 

 and is separated from its fellow of the opposite 

 side by the posterior fissure. 



According to this view, then, the spinal cord 

 will be found to consist of four columns, 

 between which an obvious line of demarca- 

 tion exists throughout the whole length of the 

 organ. These are two untero-lateral columns 



and two posterior columns. The former con- 

 stitute by far the largest proportion of the white 

 substance of the cord, and they envelope the 

 anterior obtuse portion or horn of the grey 

 matter. The white commissure at the bottom 

 of the anterior fissure unites them. The anterior 

 roots of the nerves are connected with them, 

 and the posterior roots adhere to them when 

 the cord is split up along the plane of the pos- 

 terior horn. The posterior columns are small, 

 in section triangular, placed in apposition with 

 each other by their inner surfaces. Their apices 

 are directed forwards, and their bases, which 

 are slightly curvilinear, backwards. No distinct 

 commissure of white fibres can be detected 

 uniting these columns, save, perhaps, in the 

 lumbar region. The connexion of the posterior 

 roots of the nerves with them must necessarily 

 be very slight, as they invariably separate from 

 them in the longitudinal splitting of the cord. 



The arrangement of the grey matter in the 

 cord, as already partly explained, is as follows : 

 In each lateral half there is a portion of grey 

 matter, which is crescentic in form, having its 

 concavity directed outwards and its convexity 

 inwards towards its fellow of the opposite side. 

 The anterior extremity or horn of the crescent 

 is thick and roundish, and its margin has a 

 jagged or serrated appearance, which is more 

 conspicuous in some situations than in others. 

 The posterior horn is directed backwards and a 

 little outwards : it reaches the surface of the 

 cord, and near its posterior extremity it presents 

 a swollen or enlarged portion, which differs in 

 colour and consistence from the rest of the 

 crescent, being somewhat paler and softer. 

 This portion of grey matter has been called by 

 Rolando substantia cinerea getatinosa. It is 

 that part of the grey matter which appears to 

 be more immediately connected with the pos- 

 terior roots of the nerves. 



There is an exact symmetry between the 

 grey crescents of opposite sides, and they are 

 united by means of the grey commissure, a 

 layer which extends between the two crescentic 

 portions, being attached very nearly to the 

 central point of each. This commissure, then, 

 when examined in its length, forms a vertical 

 plane of grey matter, extending throughout the 

 whole of the cord. The lateral portions are 

 solid masses of grey matter, with which the 

 nerve-tubes of the white substance freely inter- 

 mingle, and in which, as in the grey matter 

 elsewhere, very numerous bloodvessels ramify. 

 There seem to be no good grounds for the opi- 

 nion advanced by Mayo that these crescentic 

 portions are hollow capsules. It was supposed 

 by this anatomist that each crescent resembled 

 the dentated body in the cerebellum or that 

 in the corpus olivare ; but careful examination 

 must convince any one who takes the trouble 

 of it that such is not the fact. It is true that 

 the grey matter contains white fibres, but they 

 mingle with its elements and are not enclosed 

 within a layer of it, as described and delineated 

 by Mayo. 



When sections of the spinal cord in different 

 regions are examined, they are found to exhibit 

 differences of dimensions affecting both the white 



