1180 



THE DIGESTIVE SYSTEM. 



testine is examined, it is seen to present a soft, velvety, or fleecy appearance 

 (Fig. 930, B); this is due to the presence of an enormous number of minute 

 processes, known as villi, which cover its surface. 



They are minute cylindrical or finger-like projections of the tunica mucosa 



(Fig. 929) about 

 ^th or ^th of 

 an inch (1/2 to 

 1'6 mm.) in height, 

 and barely visible 

 to the naked eye, 

 which are closely 

 set all over the 

 surface of the 

 lining membrane 

 of the small in- 

 testine. Begin- 

 ning at the edge 

 of the pyloric 

 valve, they are 

 broad but short 

 in the duodenum, 

 and grow nar- 

 rower as they are 

 followed down 

 through the in- 

 testine to the 

 valvula coli, at 

 the edge of which 



FIG. 930. PLIC.E CIROULARES (natural size). 



A, as seen in a portion of jejunum which has been filled with alcohol and hardened 

 B, a portion of fresh intestine spread out under water. 



they cease. They are found, not only on the general surface of the mucous mem- 

 brane, but also upon the plicae circulares, and, while they are not present over 

 the solitary lymph nodules, they are found in the intervals between the individual 

 nodules of the aggregated nodules. 



They play an important part in the absorption of the products of digestion 

 which takes place in the small intestine. 



Plicae Circulares [Kerkringi]. When the intestine is empty and contracted, 

 its mucous membrane is thrown into effaceable folds or rugae, which disappear 

 on distension. But in addition to these, there are found in certain portions of 

 the small intestine a series of large, permanent transverse folds, which are not 

 effaceable ; these are known as plicae circulares (Fig. 930). These are usually more 

 or less crescentic in shape, and resemble a series of closely placed shelves running 

 transversely around the gut. They rarely form more than two-thirds of a circle ; 

 sometimes, however, they present a circular or even a spiral arrangement, the spiral 

 extending little more than once round the tube, as a rule. Occasionally they 

 bifurcate at one or both ends ; sometimes, too, short irregularly directed branches 

 pass off from them. They are usually about 2 to 3 inches (5 to 7*5 cm.) in length, 

 and their breadth, that is their projection into the cavity, may be as much as Jrd 

 of an inch (8 mm.), whilst in thickness, as seen when cut across, they measure about 

 |th inch (3 mm.). 



They are composed of two layers of the tunica mucosa, with a prolongation 

 from the tela submucosa between, to bind the two together. They are covered with 

 villi, and are permeated by intestinal glands. Their use is to increase the amount 

 of surface available for secretion and absorption. 



Plicae circulares are not found in the upper part of the duodenum. They begin 

 at a distance varying' from 1 to 2 inches (2*5 to 5 cm.) from the pylorus. At first 

 they are small, irregular, and scattered ; but they are larger lower down, and at the 

 opening of the bile duct (4 inches from the pylorus) they are distinct and prominent. 

 In the rest of the duodenum, and in the superior half of the jejunum, they are highly 

 developed, being large, broad, and closely set. In the inferior half of the jejunum 

 they become gradually smaller and fewer. Passing down into the ileum, they 



