520 MESSRS. MIVART AND CLARKE ON THE SACRAL 



postaxial to the cloaca, and, after becoming connected, by tbe short branch (41 a) already 

 mentioned, with the first postsacral, it ends in delicate fibres which supply the sphincter 

 muscle and mucous membrane of the cloaca. 



Monitor arenarhis. (Plate LXVII. fig. 2.) 



The genus Monitor does not appear to have been examined by Professor Gegenbaur ; 

 but Professor Hoffman has described and figured 1 the arrangement of its lumbar and 

 sacral nerves. He represents its sacral plexus as formed from four roots, the most 

 postaxial of which is the first postsacral nerve, while the most preaxial is the lumbo- 

 sacral nerve coming off postaxiad from the second presacral nerve. The root consisting 

 of the first presacral nerve is the thickest, and does not bifurcate, the whole of it going 

 to the sacral plexus. The second presacral nerve is shown dividing at once into three 

 branches, one of which is the lumbo-sacral nerve, another joins a branch of the third 

 presacral to form the crural nerve, while the remaining branch continues on as the 

 obturator nerve. 



We have been fortunate enough to obtain two fine and large specimens of Monitor 

 arenarius, for one of which we are again indebted to the great kindness of Professor 

 Plower. In both these specimens we found on both sides of the body the same arrange- 

 ment of the nerve-trunks, which differed considerably from the couditions described by 

 Professor Hoffman. "We found the sacral plexus to be formed of but three roots, the most 

 preaxial root consisting of the postaxial bifurcation of the first presacral nerve, while the 

 intersacral nerve was slightly the largest, and the first postsacral much the slenderest. 

 The first presacral nerve bifurcated, one branch going to the sacral plexus as already 

 mentioned, the other continuing on, as the crural nerve, over the brim of the pelvis, but 

 being connected on each side with the second presacral nerve by a pair of delicate 

 nervous cords. The second presacral nerve having received (or given off) these nervous 

 cords, received an accession from the postaxial branch of the third presacral nerve, and 

 is then continued on as the obturator nerve. 



We have thought it well to investigate the distribution of the nerves in this species, 

 being a species sufficiently remote from Iguana. We have, however, found a very close 

 similarity in this respect between these two animals. 



The fourth presacral nerve (1) is like the fifth of Iguana ; it goes to the pyramidalis 

 and other abdominal muscles, and takes no part in the formation of the lumbar plexus. 

 The third presacral nerve (2) is like the similarly numbered fourth presacral of Iguana, 

 except that we did not note that its postaxial branch (4) gave off any nerve to the 

 iliacus, although on one side of our specimen it gave off a branch which joined another 

 (10 a) from the second presacral, and thus continued on to the iliacus. The second 

 presacral nerve (9) is like the third of Iguana, except that it is less intimately fused 

 with the nerve postaxiad to it, with which it is only connected by commissural fibres 

 (14, 14 a, and 14 b). Having joined the postaxial branch (4) of the nerve before it, it is 

 continued on as the obturator nerve (5), giving off branches to the iliacus (6) before it 



1 L. c. p. 175, fig. 7. 



