DIGESTION IX THE MOUTH. 301 



circulation it not only loses in oxygen and gains in carbon dioxide, but 

 there is also an actual reduction in the amount of fluid, due to the 

 transudation of the serum of the blood into the lymph-spaces. Such 

 transudation is due solely or mainly to blood-pressure, and does not 

 constitute a permanent loss to the blood ; for the fluids so poured out 

 into the lymph-spaces serve largely to nourish the tissues, and are then 

 pushed on into the lymphatic vessels by fresh quantities coming after 

 them, and finally again reach the veins, and thus re-enter the circula- 

 tion. Such transudations pervade all tissues, but in glandular organs 

 not only is there a constant loss from the nourishment of the tissues 

 forming the glands, but the secretions are produced at the expense of 

 these filtrates from the blood-vessels into the lymph-spaces of the gland- 

 ular tissue. Secretion is, thus, the passage of the substances from lymph- 

 spaces to the exterior of the body, for, as has already been referred to, 

 the alimentary canal may be regarded as such. 



Various views have been proposed to explain the passage of the 

 constituents of the lymph so transuded from the blood-vessels into the 

 excretory ducts of the glands. The blood-pressure is evidently con- 

 cerned in forcing the serum of the blood through the walls of the 

 capillaries into the lymph-spaces, but here the blood-pressure ceases to be 

 of influence. For if a manometer is inserted into the submaxillary duct 

 of a dog and the chorda tympani nerve stimulated, the pressure in the 

 salivary duct will be found to be greater by far — one-third greater, at 

 least — than that of the carotid artery. Where, therefore, the pressure 

 is greater on the side of the excretory duct, blood-pressure of course 

 can be of no avail in causing the passage of the fluids through the 

 glandular tissue into that duct. Osmosis may to a certain extent be 

 concerned in producing the passage of the fluid through the gland- 

 membrane, though there are scarcely any data in favor of this view other 

 than that which is conceded in the fact that the stimulation of the chorda 

 tympani nerve may result electrolytically in the production of certain 

 decomposition products which, having a strong affinity for water, might 

 extract water from the lymph-spaces into the gland-cells. The produc- 

 tion of heat in secretion to a certain extent favors this view, since it 

 has been found that the temperature of the saliva in the salivary duct 

 may be one degree or more higher than that of the blood. Secretion of 

 saliva can, thus, not be a process of mere mechanical filtration; for not 

 only do we find, as already mentioned, the greater pressure on the side 

 of the salivary duct, and an .actual formation of heat in the secretion, 

 hut the secretion may even take place in the absence of the circulation. 

 Thus, if the chorda tympani is isolated in the rabbit, and the animal 

 then rapidly decapitated, the flow of saliva may still take place on 

 stimulation of the chorda, and it may produce in a few moments double 



