96 



THE PHILOSOPHY OF BIOLOGY 



Very soon, however, the process of osmosis becomes 

 succeeded by one of diffusion, and the salt molecules 

 pass through the capillary wall into the lymph and 

 are excreted. 



Undoubtedly the purely physical processes of 

 diffusion and osmosis occur all over the animal body 

 and are the means whereby food-materials, secretory, 

 and excretory substances are transported from blood 

 to lymph, or vice versa, from lymph to cell substance 

 or to glandular cavities, and so on. But it is also the 

 case that in very many processes the activity of the cells 



YemU 



FIG 9. 



themselves plays an important part. It may even be 

 the case that a particular process, after all physical 

 agencies are taken into account, reduces down to this 

 action 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-maxillary 

 salivary gland of the mammal. 



What, then, is this mechanism and how does it act ? 

 The gland is a compound tubular one, its internal 

 cavity being prolonged into the duct which opens into 

 the mouth. The saliva prepared in the gland issues 

 from this duct. Blood is carried to the gland by twigs 

 of the facial artery, and, after circulating through it,. 



