THE SMALL INTESTINE. 34 1 



have yet to distinguish differentiate later in a very simple 

 way; these are the gall-intestine {duodenum), which is 

 next to the stomach, the long empty intestine (jejunum) 

 which succeeds, and the last section of the small intestine, 

 the crooked intestine (ileum). '■ 



The two large glands which we have already named, the 

 liver and the ventral salivary gland, grow out, as protuber- 

 ances, from the gall-intestine, or duodenum. The liver first 

 appears in the form of two small sacs, situated right and left 

 just behind the stomach (Figs. 284,/, 285, c). In many low 

 Vertebrates the two livers remain quite separate for a long 

 time (in the Myxinoides for life), and coalesce only imper- 

 fectly. In higher Vertebrates, on the other hand, the two 

 livers coalesce more or less completely at an early period, 

 and constitute one large organ. The intestinal-glandular 

 layer, which lines the hollow, pouch-like rudiment of the 

 liver, sends a number of branched processes into the investing 

 intestinal-fibrous layer ; as these solid processes (rows of 

 gland-cells) again branch out, and as their branches coalesce, 

 the peculiar netted structure of the developed liver is 

 produced. The liver-cells, as the secreting organs which 

 form the bile, all originate from the intestinal-glandular 

 layer. The fibrous mass of connective tissue, which joins 

 this great cellular network into a large compact organ, and 

 which invests the whole, comes, on the other hand, from the 

 intestinal-fibrous layer. From the latter originate also the 

 great blood-vessels which traverse the entire liver, and 

 the innumerable netted branches of whiuh are interlaced 

 with the network of the liver-cells. The gall-ducts, which 

 traverse the entire liver, coUectin^^ the bile and discharoinor 

 it into the intestine, originate as intercellular passages along 



