The philosophy of biology . uppose thata strong solution of common salt in water is injectedinto the blood stream : what happens is that osmosistakes place, the water in the surrounding lymph spacespassing into the blood stream because the concen-tration of salt there is greater than it is in the this is happening, the capillary walls are actingas semi-permeable membranes, allowing the moleculesof water to pass through but not the molecules of salt. 96 THE PHILOSOPHY OF BIOLOGY Very soon, however, the process of osmosis becomessucceeded by one of diffusion, and the salt moleculespa
The philosophy of biology . uppose thata strong solution of common salt in water is injectedinto the blood stream : what happens is that osmosistakes place, the water in the surrounding lymph spacespassing into the blood stream because the concen-tration of salt there is greater than it is in the this is happening, the capillary walls are actingas semi-permeable membranes, allowing the moleculesof water to pass through but not the molecules of salt. 96 THE PHILOSOPHY OF BIOLOGY Very soon, however, the process of osmosis becomessucceeded by one of diffusion, and the salt moleculespass through the capillary wall into the lymph andare excreted. Undoubtedly the purely physical processes ofdiffusion and osmosis occur all over the animal bodyand are the means whereby food-materials, secretory,and excretory substances are transported from bloodto lymph, or vice versa, from lymph to cell substanceor to glandular cavities, and so on. But it is also thecase that in very many processes the activity of the cells. YemU FIG 9. themselves plays an important part. It may even bethe case that a particular process, after all physicalagencies are taken into account, reduces down to thisaction of the cells. To understand this we must con-sider the mode of working of some well-known organ,and the best possible example of such an organ, con-sidered as a mechanism, is that of the sub-maxillarysalivary gland of the mammal. What, then, is this mechanism and how does it act ?The gland is a compound tubular one, its internalcavity being prolonged into the duct which opens intothe mouth. The saliva prepared in the gland issuesfrom this duct. Blood is carried to the gland by twigsof the facial artery, and, after circulating through it,. THE ACTIVITIES OF THE ORGANISM 97 is carried away by factors of the jugular vein. Twonerves supply the gland : one is the chorda tympani,a branch of a cranial nerve, and the other is a sym-pathetic nerve. Lymph also leaves the gland by alittle vessel
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