DIGESTIVE SYSTEM 749 



carnivores, and a few others. A more complex condition than this is 

 produced by the formation of long, narrow loops along the course of 

 either the ascending or transverse colons, or both, and these loops may 

 remain simple or roll into spirals. Such colon labyrinths are seen in 

 ruminants, in certain rodents as the lemmings and jumping mice, and 

 in a few lemurs. 



"From this brief review of the alimentary canal and its modifications 

 the impression is gained that in this array of enlargements, elongations, 

 diverticula, spiral valves, and other devices, we have to do, not with a 

 consecutive anatomical history, but with numerous special cases of 

 physiological adaptations, developed in response to need ; and that a 

 similarity in one of these particulars implies, not genetic relationship 

 necessarily, but a similar demand responded to in a similar way. The 

 main object to be achieved in all cases is to regulate the amount of diges- 

 tive surface to the demands offered by the various kinds of food, and 

 as there is but a limited number of mechanical or architectural devices 

 possible, the same ones are employed in unrelated groups of animals, 

 having arisen independently in response to a similar physiological need. 

 This phenomenon of parallel development (or 'analogical resemblance,' 

 as Darwin calls it), may appear in any system or part and has been a fre- 

 quent source of error in the estimation of the inter-relationship of ani- 

 mals." 



It has already been shown that the intestinal tract begins as a 

 straight tube, and enlargements take place in various portions of this 

 tube, the most prominent enlargement being the stomach. This enlarge- 

 ment has various paired nerves passing down each side of the digestive 

 canal, prominent among which are the vagus nerves. When the stomach 

 has become sufficiently large and extends some distance ventral, it turns, 

 so that what was the ventral region now points toward the right side 

 of the individual. This means that any nerves or blood vessels which 

 lie along the right side of the embryonic digestive tract will then lie on 

 the dorsal surface of the stomach and, of course, the left nerves and 

 blood vessels then become ventral. It will save considerable confusion 

 of thought if this be remembered. 



THE LIVER (HEPAR) 



The liver, as well as the pancreas (Fig. 293) the two largest diges- 

 tive glands are derived from the mucosa of the intestine. The former 

 grow by a ventral, and the latter by a dorsal, evagination. These 

 organs are in one sense of the word really enlarged intestinal glands 

 which pushed their way through the mucosa, submucosa, and musculosa 

 of the intestine, and then, as they pushed against the serosa (that tissue 

 being held down very loosely), the serosa stretched and grew directly 

 ahead of the two glands. The liver and pancreas are therefore covered 

 by a serous membrane, and both are connected on the side from which 



