TRANSPORT OF MATERIALS 141 



their normal heights. Such solutions were found of great service 

 in the treatment of wounded men in the late war. 



We saw previously that fat is absorbed in the intestine by 

 passing into lymphatic channels. 



In part of their course the lymphatic vessels pass through what 

 are called " lymphatic glands. 3 ' These are ductless glands, which 

 supply small leucocytes to the lymph, and thus to the blood. They 

 sometimes make their existence known by becoming inflamed and 

 painful when the tissue from which the lymphatics passing through 

 them arise is in a state of disease and giving off poisonous sub- 

 stances to the lymph. 



In some situations the wall of the blood vessels appears to be 

 more permeable than is the rule. Thus, in the liver, proteins pass 

 through slowly. Such an increased permeability may be conferred 

 on the blood vessels in other parts by the injection of certain 

 substances, such as an extract made from dried mussels. It 

 happens also in the remarkable state called " anaphy lactic shock'" 

 As mentioned in our second chapter, many diseases are due to 

 poisonous substances given off by bacteria to the blood. These 

 are normally counteracted, or made innocuous, by the production 

 of " anti-bodies " in the blood, which act upon the foreign poisons 

 in various ways. A similar reaction occurs when proteins other 

 than those of the animal's blood are introduced. But to produce 

 this " immunity " in a marked degree requires the injection to be 

 repeated several times with certain intervals. If, however, the second 

 injection is delayed for more than ten days or so, it is found that a 

 greatly increased sensibility is produced, so that the injection is 

 followed by serious collapse and fall of blood pressure. One of the 

 symptoms present is frequently a swelling of the subcutaneous 

 tissue due to escape of liquid from the blood. If the animal 

 recovers from this state, it is found to be " desensitised," that is, any 

 further dose is innocuous. Satisfactory explanations of these com- 

 plex phenomena have not yet been given. 



The Proteins of the Plasma. We may now add a little more 

 with respect to the function of these constituents. They do not 

 act as food ; the tissue cells require amino-acids. They are, appar- 

 ently, the source of the anti-bodies spoken of in the preceding 

 paragraph. The two most important properties that they possess 

 are their osmotic pressure, by which the blood is prevented from 

 losing water and the tissues from becoming water- logged, together 

 with that of clotting. This is a process, involving several of the 

 proteins of the plasma, which occurs when the blood comes into 

 contact with most foreign substances, if they are wetted by it. 

 Clotting consists in the separation of a solid, " fibrin," in the form 

 of a network of filaments in which the corpuscles and liquid of the 



