DIGESTION. 227 



secreting cells, of cubical or columnar form, are arranged parallel to one 

 another surrounding a middle central canal. The granular appearance 

 which is frequently seen in the salivary cells is due to the very dense net- 

 work of fibrils which they contain. When isolated, the cells not unfre- 

 quently are found to be branched. Connecting the alveoli into lobules is 

 a considerable amount of fibrous connective tissue, which contains both 

 flattened and granular protoplasmic cells, lymph corpuscles, and in some 

 cases fat cells. The lobules are connected to form larger lobules (lobes), 

 in a similar manner. The alveoli pass into the intralobular ducts by a 

 narrowed portion (intercalary), lined with flattened epithelium with elon- 

 gated nuclei. The intercalary ducts pass into the intralobular ducts by 

 a narrowed neck, lined with cubical cells with small nuclei. The intra- 

 lobular duct is larger in size, and is lined with large columnar nucleated 



- 166. Section of submaxillary gland of dog. Showing gland-cells, &, and a duct, a, in section. 



cells, the parts of which, toward the lumen of the tube, presents a fine 

 longitudinal striation, due to the arrangement of the cell network. It is 

 most marked in the submaxillary gland. The intralobular ducts pass into 

 the larger ducts, and these into the main duct of the gland. As these 

 ducts become larger they acquire an outside coating of connective tissue, 

 and later on some unstriped muscular fibres. The lining of the larger 

 ducts consists of one or more layers of columnar epithelium, containing 

 an intracellular network of fibres arranged longitudinally. 



Varieties. Certain differences in the structure of salivary glands 

 may be observed according as the glands secrete pure saliva, or saliva 

 mixed with mucus, or pure mucus, and therefore the glands have been 

 classified as: (1) True salivary glands (called most unfortunately by some 

 serous glands), e.g., the parotid of man and other animals, and the sub- 

 maxillary of the rabbit and guinea-pig (Fig. 167). In this kind the 

 alveolar lumen is small, and the cells lining the tubule are short, granular 

 columnar cells, with nuclei presenting the intranuclear network. During 

 rest the cells become larger, highly granular, with obscured nuclei, and 

 the lumen becomes smaller. During activity, and after stimulation of 



