THE DIGESTIVE SECRETIONS 909 



These anatomical variations are associated with very striking 

 histological differences as well as with differences in the character of 

 the secretion. Naturally, the saliva obtained from the mouth is 

 a mixed secretion, because it is derived from three sources, namely: 

 (a) from Stenson's duct- which drains the parotid gland and opens upon 

 the inner surface of the cheek opposite the second molar tooth, (6) 

 from Wharton's duct which conveys the submaxillary secretion into 

 the groove next to the frenulum of the tongue, and (c) from the ducts 

 of Rivinus which drain the sublingual gland. The latter are multiple 

 and may form as many as twenty different tubules. In the dog, one 

 of them most generally attains a considerable caliber and pursues a 

 course parallel to Wharton's duct. It is known as the duct of Bartho- 

 lin. While the chemical characteristics of saliva will be dealt with in 

 a later chapter, it may be stated at this time that the parotid secretion 

 is clear serous in character, while that of the sublingual gland is very 

 viscous and stringy^ The submaxillary furnishes a secretion which 

 displays intermediate qualities. 



The Histological Character of the Salivary Glands. ^^ — Each gland 

 is made up of lobes and lobules, and each lobule in turn of numerous 

 groups of tubulo-saccular alveoli or acini. Each acinus consists of 

 a number of large and rather square cells which surround the inner 

 extremity of every small duct. The appearance of these so-called 

 chief cells varies with the character of the secretion. In the fresh 

 state, those forming the mucous glands, such as the sublingual, con- 

 tain large granules of mucinogen which is the precursor of mucin. 

 In the fixed state, on the other hand, these cells appear swollen, the 

 center of their clear cytoplasm being occupied by a well differentiated 

 rounded nucleus. In many of these mucous glands, such as the sub- 

 maxillary of the dog and cat, the different alveoli of chief cells are in- 

 vested by crescentic groups of marginal cells which stain deeply and 

 contain no mucinogen. These formations are the demilune cells or 

 crescents of Gianuzzi. Other glands present a mixed character and 

 embrace acini composed of mucous cells right beside those made up 

 of albuminous cells. This is true not only of the submaxillary gland 

 of man in which the serous cells predominate, but also of the sublingual 

 gland, in which the mucous cells are more numerous. In the rabbit, 

 the submaxillary presents the characteristics of a serous gland and the 

 sublingual those of a mucous gland. It need scarcely be mentioned 

 that these structural peculiarities are associated with corresponding 

 differences in the character of the saliva. 



Such glands as the parotid, and in part also the submaxillary, 

 furnish a watery and non-viscid secretion. Their chief cells are filled 

 with small granules of an albuminous type which constitute the mother- 

 substance of the active principle of the saliva, called ptyalin. Upon 

 it depends the digestive power of this secretion. While resting in 

 the cells these granules are designated as zymogen granules, or ptyaHn- 

 1 R. Metzner, in Nagel's Handb. der Physiol., Braunschweig, 1907. 



