THE DIGESTIVE ORGANS. 139 



this sulcus arc peculiar groups of cells, called taste-buds, which will 

 be described in a subsequent chapter. At the junction of the 



middle and posterior thirds of the upper surface of the tongue 

 there are several of these circum vallate papillae which are of 

 unusual size. 



Within the subepithelial areolar tissue, and often extending for 

 some distance between the muscles, there are, here and there, small 

 racemose glands, which secrete a serous or mucous fluid (Figs. 116, a 

 and 117). They are most abundant on the back and sides of the pos- 

 terior part of the tongue, and their ducts frequently open into the sulci 

 of the circumvallate papillae. Within the subepithelial areolar tissue 

 small collections of lymphadenoid tissue (lymph-follicles) are also of 

 not infrequent occurrence. The papilla? covering these are low and 

 inconspicuous, so that the surface of the tongue appears unusually 

 smooth at those points. 



2. The salivary glands belong to the racemose variety of secreting 

 glands. The secretions which they furnish are of two kinds: 1, a 

 thin, serous fluid, containing albuminoid materials, among which are 

 the specific ferments elaborated by the gland ; and, 2, a viscid fluid 

 containing mucin. These two secretions are furnished by acini lined 

 with different varieties of epithelium. The parotid gland secretes 

 only the serous fluid, and is composed of serous alveoli. The sub- 

 lingual gland secretes only the mucous fluid ; but the submaxillary 

 gland secretes both, and, therefore, contains both serous- and 

 mucous-secreting cells. 



The cells which line the mucous acini have clear bodies, as the 

 result of a storage of transparent globules of mucin or naucigen 

 within the cytoplasm. Where these globules are abundant the 

 nuclei of the cells are crowded toward the attached ends of the 

 cells. When the mucin is discharged from the cells they become 

 smaller, less clear, and more granular in appearance. 



At the periphery of the acini, and especially well marked at or 

 near their blind extremities, are, here and there, crescentic, granular 

 epithelial cells, which may reach the lumen of the acinus or be 

 crowded back by the enlarged cells adjoining them. These cells 

 form the " crescents of Gianuzzi." In the submaxillary gland, at 

 least, many of these crescents secrete the serous or albuminoid fluid 

 mentioned above. This secretion reaches the lumen of the gland 

 through minute intracellular channels (Fig. 118). 



The serous alveoli of the salivary glands are lined with cells that, 



