ORIGIN OF PRE-GANGLIONIC FIBRES. 633 



the spinal nerves leave the grey matter in a continuous band. Adjacent 

 portions of this continuous band are gathered up into bundles and form 

 the spinal nerves. In this gathering up into bundles there is a certain 

 amount of individual variation. The fibres, which in one individual 

 are gathered up so as to make the lowermost fibres of a given bundle, 

 may, in another individual, be taken to form the uppermost fibres of 

 the bundle below. The variation rarely involves more than two-thirds 

 of the fibres of a nerve. It may be represented diagrammatically as in 

 Fig. 312, A, M, P. 



The three arrangements may be spoken of as anterior (A), median 

 (M), and posterior (P) arrangement of nerves. This relation of roots 

 to nerves affects a whole series of nerves simultaneously, and to very 

 nearly the same relative extent. Thus, if one lumbar nerve is anterior 

 in arrangement, all the lumbar and sacral nerves will also be anterior in 

 arrangement. On this basis it follows that the relative size of the 

 strands which the several spinal nerves send to the trunks issuing from 

 the lumbo-sacral plexus x will be different with each arrangement, and 

 thus the anatomical characters of the lumbo-sacral plexus form a ready 

 means of determining whether the arrangement of nerves is anterior, 

 middle, or posterior. In consequence, instead of saying that in a given 

 individual the arrangement of nerves is anterior, we may say, simply, 

 that the plexus is anterior. The brachial nerves appear to vary much 

 less frequently than the lumbo-sacral nerves. 



The variation in the two sets of nerves is not necessarily in the same 

 direction ; the brachial plexus may be anterior, and the lumbo-sacral 

 plexus posterior, but the statistics are insufficient to show how frequently 

 this is the case. 



The importance of this, for the subject we are considering, is that, 

 in order to state at all accurately the several sympathetic ganglia to which 

 each lumbar nerve sends nerve fibres in any one species of animal, it is 

 imperative to know whether the arrangement of nerves is anterior, middle, 

 or posterior. At the same time, it must be remembered that these variations 

 are very numerous, and that the grouping of them into three forms is purely 

 arbitrary. 



The connections of the spinal nerves in relation to these variations 

 have been only worked out in detail in the cat. I shall take the 

 connections with an anterior arrangement of nerves as a basis for 

 description; other forms I shall not deal with in detail ; a few illustrations 

 will be sufficient to show the general trend of the variations. 



It need hardly be said that the spinal nerves in one species have 

 only approximate homologues in the spinal nerves of another species. 

 But as the structural outlines not only of species but of all mammals 

 are the same, information as to the connections of the sympathetic 

 fibres known for one animal, can, with proper allowance for anatomical 

 peculiarities, be applied to any other. 



1 Cf. Langley, Journ. Physiol., Cambridge and London, 1891, vol. xii. p. 347 ; 1893, 

 vol. xv. p. 210 ; 1894, vol. xvii. p. 296 ; Langley and Anderson, ibid., 1896, vol. xx. 

 p. 392 ; Sherrington, ibid., 1892, vol. xiii. p. 639. In this paper references will be found 

 to earlier observations bearing on the subject. Sherrington divides the lumbo-sacral 

 plexus into two classes, pre-fixed and post-fixed. Eisler, "Der Plexus lumbo-sacralis des 

 Menschen," Diss., Halle, 1892; cf. also Thane, Quain's "Anatomy," vol. iii. pt. 2, 

 p. 313. 



