CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 387 



secreting elements and the ducts, and when we come to examine 

 them more closely we find that they differ markedly in structure. 

 Moreover, when we examine the three glands, parotid, submaxillary 

 and sublingual, and especially when we employ for the purpose 

 different kinds of animals, we find that, while the ducts have nearly 

 the same structure in all cases, two kinds of alveoli may be 

 distinguished differing from each other in the characters of the 

 cells lining them. In the one case the cells, for reasons which will 

 presently appear, are called mucous cells, in the other serous cells, 

 or perhaps better albuminous cells. In one gland all the alveoli 

 may be lined with mucous cells, in which case it is called a 

 ' mucous gland,' or with albuminous cells, in which case it is called 

 an ' albuminous gland,' or some alveoli may be ' mucous ' and 

 others ' albuminous,' the gland being a mixed one ; and this dis- 

 tinction between mucous and albuminous obtains also in glands 

 of the mucous membrane which are not distinctly salivary, for 

 instance in the small ' buccal ' glands of the mouth, and in the 

 glands of the pulmonary passages and of other structures. 



216. Mucous glands. -The submaxillary gland of the dog is 

 a fairly typical mucous gland. The alveoli of this gland vary 

 a good deal in diameter, but are on an average about 35 /u-. 

 The outline of each alveolus is defined by a distinct basement 

 membrane formed of a number of flattened connective tissue cor- 

 puscles fused together into a sheet ; in a section the long oval 

 nuclei of the constituent cells may be seen here and there imbedded, 

 as it were, in the membrane. Outside the basement membrane 

 lie, as elsewhere in a mucous membrane, the lymph spaces of the 

 fine connective tissue. 



The space defined by the basement membrane is nearly wholly 

 filled, a very small central lumen only being left, by cells arranged 

 for the most part in a single layer. The cells are large relatively 

 to the alveolus, so that in a transverse section of an alveolus, 

 about 5 or 6 cells will be seen. Each cell is more or less spherical 

 or rather conical in form, with its broader base, which is sometimes 

 irregular in outline, resting on the basement membrane and the 

 narrower apex abutting on the lumen. The characters of the cell 

 differ according to the condition of the gland. If the gland has, 

 previous to its preparation for examination, not been actively 

 secreting, the cells have certain characters and may be spoken of 

 as ' loaded ' or ' charged.' If the gland has been actively secreting, 

 these characters are replaced by others, and the cells may be 

 spoken of as ' unloaded,' ' discharged.' In the ' loaded,' or as it is/ 

 often called the ' resting ' phase, the cell, in hardened specimens, is 

 as a whole transparent, and stains very slightly with the ordinary 

 staining reagents. The nucleus, which in hardened specimens 

 appears disc-shaped and sometimes curved or bent, but in the fresh 

 living cell is seen to be spherical, lies at the base of the cell not far 

 from the basement membrane. Around the nucleus is gathered a 



252 



