418 THE ACT OF SECRETION. [BOOK IL 



the quantity of water passing into the alveolus, something else de- 

 termines how much of common soluble salts that water contains, and 

 still something else determines to what extent that water is also 

 laden with specific constituents and other organic bodies. The 

 whole action is too complicated to be described as consisting 

 merely of the two processes mentioned above, but the time has 

 not yet come for clear and definite statements. Everything 

 however tends to shew that the cell is the prime agent in the 

 whole business, though we cannot at present define the nature 

 of the several changes in the cell, nor can we say how those 

 changes are exactly related to each other, to changes of the blood- 

 pressure in the blood vessels, or, we may add, to changes taking 

 place in the lymph-spaces which lie between the blood and the 

 cell. 



We may perhaps add that, since in certain cutaneous se- 

 creting glands the alveolus, or what corresponds to the alveolus, 

 is wrapped round with plain muscular fibres, the contraction of 

 which appears to force the secretion outwards, the idea has been 

 suggested that in glands, such as we are now considering, the cell- 

 substance making use of " protoplasmic " contraction instead of 

 actual muscular contraction, may force part of the cell contents 

 into the lumen of the alveolus. Such a mode of secretion would 

 be comparable to the ejection of undigested material, or "excre- 

 tion," by an amoeba. But we have no satisfactory evidence in 

 favour of this view. 



240. Throughout the above we have spoken as if the 

 secretion were furnished exclusively by the cells of the alveoli or 

 secreting portion of the gland, as if the epithelium cells lining the 

 ducts, or conducting portion of the gland, contributed nothing to the 

 act. In the gastric glands the slender cells lining the mouths of the 

 glands (which correspond to ducts) and covering the ridges between, 

 are mucous cells secreting into the stomach generally a small, but 

 under abnormal conditions a large amount, of mucus, which has its 

 uses but is .not an essential part of the gastric juice. In the 

 salivary glands we can hardly suppose that the long stretch of 

 characteristic columnar epithelium which reaches from the alveoli 

 to the mouth of the long main duct serves simply to furnish a 

 smooth lining to the conducting passages; but we have as yet 

 no clear indications of what the function of this epithelium can be. 



241. Before we leave the mechanism of secretion there are 

 one or more accessory points which deserve attention. 



In treating just now of the gastric glands we spoke as if pepsin 

 were the only important constituent of gastric juice, whereas, as we 

 have previously seen, the acid is equally essential. The formation 

 / of the free acid of the gastric juice is very obscure, and many 

 ingenious but unsatisfactory views have been put forward to 

 explain it. It seems natural to suppose that it arises in some way 

 from the decomposition of sodium chloride drawn from the blood ; 



