74 bulletin: museum of comparative zoology. 



the " strength-centre " ^ of the plexus, such variation usually resulting 

 in displacement anteriorly; but this is not correlated with the variations 

 in the position of the pelvic girdle, — indeed most such cases were found 

 in specimens in which the pelvic girdle was normally placed, — nor have 

 I found any correlation between the displacement of the strength-centre 

 in the brachial and lumbo-sacral plexi respectively. This is not in accord 

 with the conclusions of Adolphi ('96, p. 118), who says for Anura "... 

 die beiden Extremitatenplexus, der Plexus sacralis und der Plexus brachi- 

 alis, ihren Schwerpunkt in der gleichen Richtung verlegen . . ." Such 

 a condition indicates that the variations in the brachial plexus are inde- 

 pendent of variation in the position of the pelvic girdle ; and since there 

 is no posterior displacement of the brachial plexus, nor any change of 

 topography in cases where the pelvic girdle is placed on the 20th ver- 

 tebra, we have evidence that there has been no interpolation of vertebrce 

 in the part of the column anterior to the posterior limit of the brachial 

 plexus. 



The Lumbo-Sacral Plexus. 



The variations in the lumbo-sacral plexus are most conveniently 

 grouped under three heads, viz. : A, those in which the girdle is attached 

 to the 19th vertebra; B, those with the girdle carried by the 20th ver- 

 tebra ; and C, those in which the constituents of the girdle have an 

 unsymmetrical position (seethe Table, page 81). 



Group A is represented by twenty specimens. The nerves here form- 

 ing the plexus (Plate 1, Figs. 2, 3) are xviii to xxi inclusive, neither 

 XVII nor XXII in any case entering it. The plexus is distributed by three 

 trunks. The anterior is N. ileohypogastricus (il-h'ga.), which is the 

 anterior branch of nerve xix, and extends cephalad on the inner body- 

 wall; its strength and relation to other nervous parts seem uniform. 

 The middle trunk (cru.) is N. cruralis (obturator), which passes out 

 between the ilium and pubis, to be distributed to the anterior part of the 

 thigh. The posterior is N. ischiadicus (isch.), which is the main trunk, 

 and is distributed to the posterior part of the thigh and to the leg. 



We may distinguish in the topography of the plexus two types, based 

 on differences in the source of the bi-anches which go to form N. cruralis 

 and N. ischiadicus ; these I shall designate as a type and ft type. 



1 The term " strength-centre " has been adopted to designate that point in the 

 plexus which is central with regard to the combined " strengths " of the component 

 nerves. We may thus conceive the wliole " strength " of the plexus as concen- 

 trated at this point, the " strength-centre." 



