530 THE SALIVAR Y GLANDS. 



We may speak of the alveolar cells as forming a membrane, and call the 

 part towards the lymph the outer layer of it, and the part towards the lumina 

 of the gland the inner layer. In the inner layer are spaces containing soluble 

 organic substance. 



The explanation of the above-mentioned facts on the osmotic theory might 

 be as follows : The membrane is impermeable in the unstimulated state ; on 

 stimulation, a rearrangement of its molecules takes place, so that, of immedi- 

 ately adjoining portions, parts are permeable to water and parts are permeable 

 to salts also, whilst parts remain impermeable. On increasing the strength of 

 the stimulation, a larger and larger area of the membrane becomes permeable, 

 and of this a proportionately larger and larger part becomes permeable to 

 salts. 



The increase in the percentage of organic substance in saliva, which accom- 

 panies increased rate of flow, might be due simply to the greater percentage 

 of salts causing an increase in the solvent power of the fluid, or to a larger 

 proportion of the fluid passing into the spaces of the inner layer. 



Proteid molecules do not pass through the gland-cells, but they enter it, 

 and are deposited, forming the outer non-granular zone ; the process is spoken 

 of as the growth of protoplasm. We have reason to believe that the rate of 

 growth of the protoplasm increases more than the rate of flow of fluid as the 

 stimuli pass from weak to strong. 



To account for this on the osmotic theory, it must be supposed that only 

 the outermost portion of the membrane becomes permeable to proteids, so that 

 the proteid molecules are blocked in their passage, and further that the ratio 

 of permeability to proteids and to water is greater with strong than with weak 

 stimuli. The theory becomes further complicated, if we have to apply it also 

 to a taking up of proteid during rest (cf. p. 486), when there is no passage of 

 fluid ; for in this case the inner part of the membrane at least must be im- 

 permeable to water, whilst the outer part is permeable to proteids. 



So far we have assumed that the conditions of the solutions on the two 

 sides of the membrane are such as would lead to an osmotic flow through it, 

 directed from its outer to its inner surface. But this is precisely the point it 

 is difficult to be clear about. There is no obvious reason why the fluid in 

 contact with surfaces of the membrane bounding the spaces should be very 

 different from the fluid issuing from the inner surface of the membrane. But 

 the saliva contains commonly less organic substance and less salts than the 

 lymph. Why then should fluid pass from the lymph to the saliva 1 It can 

 only be said that it is perhaps possible that a passage both of water and salts 

 might take place if the organic substance in the spaces formed some combina- 

 tion with water and salts, of which at present we have not sufficient evidence. 



The hypothesis which I have stated above seems capable of being put 

 to the test of experiment, and of being either proved or disproved. Failing 

 it, we are, I think, driven to suppose apart from the hypothesis of special 

 vital activity that the outer layer of the cell forms a loose chemical combina- 

 tion with various substances of the lymph, and that these are passed on from 

 molecule to molecule and disassociated at the inner surface. A process of this 

 kind forms the basis of the chemical theory of osmosis. And it seems to me 

 not improbable that such a process occurs in gland-cells, but it is extremely 

 difficult to see how to bring any experimental evidence to bear directly on the 

 question. The investigation appears to demand, as a preliminary, an intimate 

 knowledge of the chemical nature of the membrane. The membrane consists 

 of protoplasm. And there are few problems in physiology which appear more 

 remote from solution than that of the chemical nature of living substance. 



