THE ALIMENTARY CANAL AND ITS APPENDAGES 171 



least conclude that, as a rule, the shorter appendices show 

 occlusion more frequently than the longer (Eibbert). 



THE LIVER AND THE PANCREAS 



These two organs, which are genetically closely related, ^ 

 occasionally show variations in the manner of their lobation which 

 may amount to constriction, and in the relations of their ducts. 



[Recent investigation at the hands of a number of independent 

 workers has revealed the fact that the pancreas, in all classes of 

 Vertebrates, is a compound organ, derivative of from one to four 

 diverticula of the gut, and in most cases from three, as is said 

 by Felix 1 to be the case in Man himself. One (or more) of 

 these primitive outgrowths gives rise to the chief duct (or ducts) of 

 the adult organ, the rest usually becoming obliterated with advanc- 

 ing development. Pending the working out of further details, 

 considerable interest attaches to the recent discovery by Rolle- 

 ston, 2 that the duodenum of the human adult may sometimes 

 bear a diverticulum (proved to lie distinct from the " ampulla 

 Vateri ") which enters the substance of the pancreas, and which 

 there is reason to suspect may be a persistent vestige of one of 

 the pancreatic outgrowths of the embryo.] 



The average weight of the liver is said to be 1451 grs. in 

 the white races, 1266 grs. in the black. 



THE RESPIRATORY SYSTEM 



The visceral skeletal arches, which lie ventrad of the cranium 

 proper and are intimately related to the cephalic portion of the 

 gut, have been already mentioned in dealing with the head 

 skeleton, and their great phylogenetic importance has been 

 pointed out (cf. ante, pp. 49 and 64, and accompanying Figs.). 

 A few additional remarks, however, are here necessary. 



Whereas certain Fishes (primitive Selachians) have from six 

 to seven pairs of branchial pouches, 3 Vertebrata somewhat higher 

 in the scale (Turtles, Lizards, and Snakes) develop but five pairs, 



1 [Cf. Stbhr, Anat. Anziegcr, Bd. viii. p. 205.] 



2 [Jour. Anat. and Phys., vol. xxviii. p. xii.] 



3 [It is insufficiently recognised that the " Hag Fishes " may bear many more than 

 this, and that in one species of these (Bdellostoma polytrema) from thirteen to four- 

 teen pairs are present (cf. Giinther, Brit. Mus. Cat. of Fishes, vol. viii. p. 512, 

 and Schneider, Archivf. Naturgesch., Bd. xlvi. p. 115.] 



