908 THE EXTERNAL SECRETIONS 



by the goblet cells with which the epithelial lining of the intestine is 

 equipped. Scattered among the ordinary reticular cells are some 

 which undergo constant alterations in their size and shape. Origi- 

 nally columnar in outline, they are slowly elongated, because their 

 cytoplasm gradually increases until their free ends project beyond the 

 general surface of the mucosa. The internal tension having reached 

 its physiological limit, their inner walls rupture, allowing a large part 

 of their contents to escape into the intestinal lumen. Being still in 

 possession of its nucleus, the partially emptied cell forms new material 

 and closes the defect in its wall. Many of these cells, however, go 

 to pieces, their places being taken by cells hitherto dormant. The 

 material which is in this way extruded into the intestinal canal con- 

 tains large amounts of mucin, the purpose of which is to lubricate the 

 mucosa. This is of especial value in the large intestine in which the 

 fecal material becomes partially hardened on account of an absorption 

 of a considerable portion of its water. In this particular segment of 

 the alimentary canal, the ordinary goblet cells are augmented by the 

 modified cells of the crypts of Lieberkiihn. It will be pointed out 

 later on that these crypts possess a true secretory power only in 

 the small intestine and become ordinary mucous glands in the large 

 intestine. 



CHAPTER LXXVIII 



THE DIGESTIVE SECRETIONS 



A. SALIVA 



The Salivary Glands. Heidenhain recognized two types of glands, 

 namely, the mucous and the albuminous or serous. Strictly speaking, 

 this classification is not quite correct, because even the simple mucous 

 glands of the oral mucosa furnish at least some albuminous material, 

 while traces of mucin are also found in the albuminous salivary glands. 

 The first of the digestive secretions is the saliva. It is supplied by the 

 so-called salivary glands of which there are six in all, namely, two 

 parotid, two submaxillary and two sublingual glands. Since these 

 organs are paired, it suffices to state that the first lies above the ramus 

 of the lower maxillary bone, while the last two occupy positions upon 

 the floor of the mouth in close proximity to the angle of the jaw. It is 

 true, however, that the arrangement of these glands differs somewhat 

 in different animals. In the dog and cat, for example, the sublingual 

 is wanting entirely, its function being transferred to the so-called retro- 

 lingual gland which is situated somewhat nearer the angle of the jaw. 

 In the pig, all three basal glands are present, i.e., the submaxillary, 

 retro-lingual and sublingual. Traces of the second are sometimes 

 found in man, in addition to the three just enumerated. 



