1254 THE DIGESTIVE SYSTEM. 



ascending colon, a part of the primitive mesentery (which is similarly continued into 

 the mesentery of the transverse, descending, iliac, and pelvic colons). Subsequently, as 

 shown by the darkly shaded parts in Fig. 974, the back of the mesenteries of the ascend- 

 ing, descending, and iliac portions of the colon adheres to the posterior abdominal wall, 

 and these mesenteries become lost ; whilst the mesenteries of the transverse and pelvic 

 portions of the colon remain free, and persist in the adult. 



At the same time, the mesentery proper (which was at first attached only at its 

 narrow neck, between the duodenum and transverse colon, and below this was continuous 

 on the right with the ascending mesocolon) now acquires a new attachment to the 

 posterior abdominal wall through the absorption of the ascending mesocolon (Fig. 974), 

 and the adult condition is attained. 



DEVELOPMENT OF THE LIVER AND PANCREAS. 



The glandular tissue of the liver and pancreas, and the epithelial linings of the ducts 

 of these organs, including the gall-bladder and cystic duct, are formed from protrusions of 



Bile-ducts 



Veins 



Fia. 976. DIAGRAM illustrating the arrangement of the blood-vessels (on left) and of the hepatic cells and 

 bile-ducts (on right) within a lobule of the liver. The first diagram shows the interlobular veins 

 running around the outside of the lobule, and sending their capillaries into the lobule to join the central 

 vein. In the second diagram the bile capillaries are seen, with the hepatic cells between them, 

 radiating to the periphery of the lobule, where they join the interlobular bile-ducts. 



the endothelial wall of the foregut, below the stomach. The connective tissue framework 

 of the glands is formed from the mesodermic tissue into which the protrusions grow. 



The process of formation is as follows : 



1. Liver. A longitudinal groove appears on the interior of the ventral wall of the fore- 

 gut, close to its union with the midgut, at about the third week. This groove appears on 

 the external surface of the gut as a projection, which rapidly increases in size and grows 

 forwards and upwards towards the lower part of the septum transversum. This septum 

 is a mass of mesodermic tissue which lies in front of the foregut, just below the heart, 

 and which is attached to the anterior and side walls of the trunk. It conveys the 

 umbilical and vitelline veins as they pass to the sinus venosus. 



The liver bud grows into the lower (caudal) portion of the septum transversum, and 

 sends out strands of cells termed trabeculae, which come into contact with the vessels in 

 the septum, and enclose them. 



By the growth of capillary vessels, from the vitelline and umbilical veins, and of the 

 trabeculse, a spongy network is produced, the framework of which is formed by branching 

 and anastomosing trabeculse, while the spaces of the network represent portions of the 

 lumen of the vessels, and are filled with blood. This form of vascular network is known 

 as a " sinusoidal circulation." 



The trabeculse become hollowed out, and are reduced in size, so that eventually a 

 minute channel is formed in the centre of each of them, surrounded by a single layer of 

 cells. The lumen of the channel forms a bile capillary, and the cells surrounding it form 

 the secreting cells of the liver lobule. 



The bile capillaries of adjacent trabeculse meet and unite, and converging together 



