LIVER 



289 



fibers. It is followed by a tunica muscularis consisting chiefly of circular 

 fibers. These form a sphincter around the common bile duct, at the duo- 

 denal papilla. In the cystic duct there are folds of mucous membrane, con- 

 taining muscle fibers, and forming the "spiral valve." 



The gall bladder is lined with a folded 

 mucous membrane covered with tall epi- 

 thelial cells similar to those of the intes- 

 tine (Fig. 285). They have elongated 

 basal nuclei and secretory granules 

 (mucin) in the outer part of their proto- 

 plasm. The free surface is covered 

 with a distinct cuticular border, and 

 terminal bars have been observed. 



Goblet cells are absent and glands are infrequent. The muscularis con- 

 sists of obliquely circular fibers arranged in a plexiform layer. Among 

 them are groups of sympathetic nerve cells which supply the muscle, and 

 medullated fibers which end in the epithelium. The subserous tissue is 

 highly developed and contains large lymphatic vessels. 



FIG. 284. THE CONNECTION BETWEEN BILB 

 CAPILLARIES AND BILE DUCTS IN A HUMAN 

 EMBRYO OF FOUR AND A HALF MONTHS. 

 (After Toldt and Zuckerkandl.) 



b. c., Bile capillary; h. c., hepatic cell; p. d., 

 periportal duct. 



Epithelium. 



Muscularis. 



Tunica 

 propria 



FIG. 285. FROM A SECTION OF THB GALL BLADDER OF AS ADULT, A. 



X S6o. 



X too. B, the portion x of A 



PANCREAS. 



Development and General Features. Although the pancreas in the adult 

 is a single gland, it arises in the embryo as two entirely distinct entodermal 

 outgrowths, known as the dorsal and ventral pancreases respectively. 

 The dorsal pancreas grows out from the dorsal wall of the intestinal tube, a 

 little below the level of the common bile duct in most mammals, but a little 

 above it in man. The ventral pancreas grows down from the common bile 

 duct at its junction with the intestinal tube. As seen in Fig. 286, A and B, 

 the ventral pancreas may be more or less bi-lobed. Usually it grows to 

 the right of the intestine and there meets the dorsal pancreas, which ap- 

 proaches it in close relation with the portal vein. 

 19 



