566 THE DUODENUM. 



covering only at the superior part, the inferior and trans- 

 verse portions simply lying between the laminas of the 

 mesocolon, without any proper serous investment. By this 

 arrangement the superior portion of the duodenum only is 

 movable, the rest being fixed. In the circuit it makes, it 

 encloses the head of the pancreas, and at the posterior part 

 of its second curve, the ductus communis choledochus, or the 

 common biliary and pancreatic duct, is seen to enter. 

 This duct sometimes enters as two separate tubes, and 

 passes in either case very obliquely through the coats of 

 the duodenum. 



The muscular coat has two sets of fibres ; the one longi- 

 tudinal, thinly scattered and superficial, the second inter- 

 nal, with its fibres arranged more closely together, so as to 

 form a more perfect layer. They run circularly, though 

 none surround the tube completely, forming only segments 

 of circles. The muscular coat is rather pale and thin, 

 though its color is deeper than that of any other portion of 

 the small intestine. 



The mucous coat presents on its surface a series of folds 

 or processes termed valvulce conniventes, which are perma- 

 nent elevations of this membrane, and unlike the rugae of 

 the stomach, which are only accidental, are not effaced by 

 distension. At the lower part of the duodenum they are 

 large and numerous, while they are few in number and 

 small at the upper portion. They represent a succession of 

 arches or duplications of the mucous coat, nearly parallel 

 to each other, running round the tube in almost, though 

 not entirely perfect circles, about three lines in breadth, 

 though generally wider in the middle, and having their 

 extremities frequently bifurcated. These valvulse greatly 

 increase the extent of the absorbing surface of the chyle, 

 and serve to retard the food in its downward passage, so 

 that ample time shall be allowed for the extraction of all 

 nutritious matter. 



On the mucous coat are also seen numerous little pro- 

 cesses resembling the down upon the cuticle of an unripe 

 peach, and hence called mill. They are described as being 



