CHAP. I.] STRUCTURE OF THE SALIVARY GLANDS. 11 



GENERAL OBSERVATIONS UPON THE STRUCTURE OF THE 

 SALIVARY ELANDS WHEN AT REST. 



As has been already said, the saliva is secreted by several glands 

 of which the ducts pour their secretion into the cavity of the mouth, 

 where it is mingled and constitutes the 'mixed saliva.' The chief 

 of these glands are the parotid, submaxillary, and sublingual glands, 

 though their secretion is mixed with that of small glands (mucous 

 and serous) scattered through the mucous membrane of the mouth 

 and tongue, and which are included under the term of ' buccal ' 

 glands. Many animals possess also a fairly large orbital gland, the 

 duct of which opens into the mouth. 



structure of The salivary glands all belong to the group of 

 the salivary ' acinous ' or ' compound racemose ' glands, although 

 glands. the terminal alveoli are in reality more tubular than 



spherical. According to the researches of Heidenhain they may, 

 however, be divided into two groups, which he has denominated 

 serous, or albuminous, and mucous glands, according to the structure 

 of the cells of their acini, their chemical characters, and the nature 

 of the secretion which they elaborate. 



The parotid gland is, in most, if not in all, mammals, an albu- 

 minous gland, although a few mucous cells may be present in it. 

 The submaxillary gland is in some animals albuminous, as in the 

 rabbit : in others mucous, as in the dog : in others, again, part 

 albuminous and part mucous, as in man. The sub-lingual gland 

 consists in part of tubes with mucous cells and in part of tubes with 

 albuminous cells ; on account of the general preponderance of the 

 mucous element it is classed with mucous glands. The orbital gland 

 is as a rule mucous or serous according as the submaxillary gland is 

 mucous or serous. Glands belonging to the former of these classes 

 secrete a fluid containing Some, though it may be only a small, 

 quantity of a proteid coagulable by heat, and resembling, if not 

 identical with, serum-albumin ; the mucous glands, on the other 

 hand, as their name implies, secrete a liquid relatively free from 

 albumin ; but containing mucin as its characteristic constituent. 



In the serous glands hardened in alcohol the epithelium lining 

 the acini is composed of comparatively small, polygonal or rounded 

 cells, of which the outlines are not very distinct until acted upon by 

 certain reagents ; the protoplasm, which is but slowly coloured by 

 carmine, presents many dark granules, and the normally spherical 

 nucleus is often shrunken by the reagent. 



In the mucous glands the characteristic (mucous) cells of the 

 alveoli are large and clear, very faintly granular, with a rounded or 

 oval nucleus near their periphery surrounded by a little protoplasm. 

 Here too the nucleus may be much shrunken. The part of the cell 

 near the nucleus is usually prolonged into a process which overlaps 

 the neighbouring cell. 



