530 THE SAL J 1 AK I '' 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. 
