76 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



branch is distributed to the wall of the pelvic cavity. The strength of 

 the whole nerve before branching varies from i to f that of nerve xx, 

 the average being between ^ and ^ ; i. e. it is usually weaker than nerve 

 XIX, but may (four cases in twenty) equal that nerve in strength. 



To gain evidence on the relation of the plexus to the skeleton, I have 

 in each specimen determined the position of the first vertebra which 

 bears a haemal arch, for this is often assumed as a fixed point in discuss- 

 ino' variations in the vertebral column. By reference to the Table on 

 page 81, (see also Parker, '96, p. 712, and Bumpus, '97, p. 457,) it is seen 

 that the position of the first haemal arch is variable ; but, as the fol- 

 lowing tabulations show, there is no correlation between these variations, 

 and those of the " strength-centre" of the plexus as represented by the 

 a and ft types. 



a type, with first haemal arch on vertebra 22 = 4 cases. 

 a " " " " " 23 = 6 " 



Q u ii 'I ii " 22 = 6 " 



Q ii a " « " 23 = 4 " 



Group B includes the individuals having the sacrum borne on the 

 20th vertebra, and of these there were seven specimens. The nerves 

 involved in the plexus (Plate 1, Fig. 4) are xix to xxii inclusive. Nerve 

 xviii in no case entered the plexus. In three cases a delicate branch 

 from XIX was contributed to the formation of X. cruralis. Xerve xix 

 occupies the same relative position as did nerve xviii in group A, but 

 shows more of a tendency to enter the plexus (three cases in seven, 

 as against three cases in twenty in group A) than did the element 

 in the same position in gi-oup A. Moreover, it is distinctly (a half) 

 stront^er than are the ordinary spinal nerves. For these reasons it 

 must be considered as belonging to the plexus. 



Xerve xx corresponds in position and distribution to xix in group 

 A. In every case it branches into three parts, the anterior forming 

 X. ileohypogastricus, the middle X. cruralis, and the posterior entering 

 X. ischiadicus; i. e. the ft type of group A only is represented, but with 

 the modification that at first sight the whole plexus is apparently one 

 segment posterior to the position in the ft type specimens of group A. 



The whole of nerve xxi in each case enters into the formation of N. 

 ischiadicus, for it sends no important branches cephalad or caudad. Its 

 strength averages six times that of an ordinary spinal nerve, i. e. it is a 

 fourth weaker than the nerve (xx) occupying the corresponding position 

 in group A. Its average strength is about 1^ times that of the next 

 anterior nerve, while in group A the ratio is If to 1. 



