PLEXUS AND SACRAL VERTEBRAE OF LIZARDS. 521 



passes through the pubic foramen. Having passed through the foramen, it divides (8), 

 and is distributed to the same muscles as in Iguana. On one side we found that minute 

 branches (16) went to the last-named muscle from the outermost of the commissural 

 fibres above referred to (14 a). The first presacral nerve is like the second presacral of 

 Iguana, bifurcating similarly, and sending one branch to the lumbar and one to the 

 sacral plexus. The former branch (11) is continued on (13) to become the crural nerve 

 (15), receiving on its way the commissural fibres before mentioned (14, 14 a, and 14 b) 

 from the second presacral nerve. Having passed over the brim of the pelvis it divides 

 and supplies the very same muscles as in Iguana. The postaxial bifurcation (12) joins 

 the sacral plexus ; but before doing so gives off a branch (23), going to the obturator 

 interims, the gracilis (24), and the semimembranosus (25) muscles. As in Iguana, the 

 branch to the obturator internus (23) is isolated, while the fibres going to the gracilis 

 and semimembranosus (24 and 25) come off from the branch which also supplies the 

 semitenclinosus (27) and the pyriformis (28). This difference is explicable by the fact 

 that the connexion between the sciatic and first postsacral nerves is less in Monitor than 

 in Iguana. Therefore if the obturator internus, part of the semimembranosus, and the 

 gracilis are supplied by fibres coming from the first presacral, while the semitendinosus 

 and piriformis are supplied with fibres from the first postsacral, the whole set might be 

 supplied by the same main branch in Iguana (on account of the greater fusion above 

 mentioned), while they could not be so in Monitor. The intersacral nerve (29) is like 

 the intersacral nerve of Iguana, save that it is connected with the first postsacral (30) bv 

 only one connecting branch (32) instead of two. It thus appears that in Monitor there 

 is no nerve answering to the first postsacral of Iguana, which seems entirely suppressed. 

 The sciatic nerve (33) was as in Iguana, save that it gave no special branch to the fenioro- 

 caudal muscle and no branch to the gracilis, semimembranosus, semitenclinosus, and 

 pyriformis muscles. The first postsacral nerve (30) is like that of Iguana, save that it is 

 (as has been said) less united with the intersacral nerve and the sciatic plexus, and 

 that it gives off the branch going to the semitenclinosus (27) and pyriformis (28) muscles 

 as well as to part of the semimembranosus (26), which latter comes off from the sciatic 

 nerve in Iguana. This branch, like the branch supplying most of the same muscle in 

 Iguana, curves round the tendon of the femoro-caudal muscle. The same branch which 

 supplies the last-mentioned muscles also sends nerves (36) to the femoro-caudal itself. 

 As in Iguana, it gives off a branch (41) which passes under the tendon going from the 

 caudal muscles to the tuberosity of the ischium, and supplies the transversus perinei and 

 sphincter cloacce muscle. It is also connected by a commissural branch (41 a) with the 

 second postsacral nerve ; but in Monitor it is the commissural branch which is continued 

 (44) into the tail. The second postsacral nerve answers to the second postsacral of Iguana, 

 and gives off branches (42) to the mucous membrane of the cloaca and skin ; but the 

 second branch to the cloaca which exists (43) in Iguana was not found in Monitor. 



Thus the differences between these two very divergent Lacertilian forms may be thus 

 stated : — In Iguana four nerves contribute to form the sacral plexus ; in Monitor only 

 three. In Iguana the second presacral nerve bifurcates and goes to the two plexuses ; 

 in Monitor it is the first presacral that does so. In Iguana the second, third, and fourth 



