184 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Glands of the Intestine. At the very beginning of the intestine in the 

 immediate neighborhood of the pylorus is found a small area of mucous mem- 

 brane containing distinct tubular glands known usually as the glands of 

 Brunner. These glands resemble closely in arrangement those of the pyloric 

 end of the stomach, with the exception that the tubular duct is more branched. 

 The secreting cells are similar to those of the pyloric glands of the stomach. 

 Little is known of their secretion. According to some authors it contains 

 pepsin. The amount of secretion furnished by these glands would seem to 

 be too small to be of great importance in digestion. Throughout the length 

 of the small and large intestine the well-known crypts of Lieberkiihn 

 are found. These structures resemble the gastric glands in general appear- 

 ance, but not in the character of the epithelium. The epithelium lining the 

 crypts is of two varieties the goblet cells, whose function is to form mucus, 

 and columnar cells with a characteristic striated border. The changes in the 

 goblet cells during secretion and the probability of a relationship between them 

 and the neighboring epithelial cells has been discussed (see p. 157). 

 Whether or not the crypts form a definite secretion has been much debated. 

 Physiologists are accustomed to speak of an intestinal juice, " succus entericus," 

 as being formed by the glands of Lieberkiihn, but practically nothing is known 

 as to the mechanism of the secretion. The succus entericus itself, however it 

 may be formed, can be collected by isolating small loops of the intestine and 

 bringing the ends to the abdominal wall to form fistulous openings. The 

 secretion thus obtained contains diastatic and also inverting ferments, the action 

 of which is described on p. 247. Histologically, the cells in the bottom of 

 the crypts do not possess the general characteristics of secreting cells. 



D. LIVER ; KIDNEY. 



The liver is a gland belonging to the compound tubular type. The 

 hepatic cells represent the secretory cells and the bile-ducts carry off the 

 external secretion, which is designated as bile. In addition it is known that 

 the liver-cells occasion important changes in the material brought to them 

 in the blood, and that two important compounds, namely, glycogen and urea, 

 are formed under the influence of these cells and afterward are given off to 

 the blood-stream. The liver, then, furnishes a conspicuous example of a 

 gland which forms simultaneously an external and an internal secretion. In 

 this section we have to consider only certain facts in relation to the external 

 secretion, the bile. 



Histological Structure. The general histological relations of the hepatic 

 lobules need not be repeated in detail. It will be remembered that in each 

 lobule the hepatic cells are arranged in columns radiating from the central 

 vein, and that the intralobular capillaries are so arranged with reference to 

 these columns that each cell is practically brought into contact with a mixed 

 blood derived in part from the portal vein and in part from the hepatic 

 artery. 



As a gland making an external secretion, the relations of the liver-cells to 



