214 ANATOMY AND PHYSIOLOGY 



invasion, the leucocytes surround and absorb the offending organ- 

 isms, thus protecting the body from the effects of their toxins. 

 This they are doing continually because we are constantly taking 

 bacteria of various sorts into our systems. So long as the number 

 is not too great the phagocytes can take care of them; it is only 

 when there are too many that they cannot be overcome. 



Consisting as it does of a single layer of endothelial cells, the 

 capillary wall also renders possible the interchange of fluids between 

 the blood-vessels and the tissues. This interchange is accom- 

 plished by the physical process of osmosis, which may be defined 

 as the diffusion of two liquids or solutions through an intervening 

 membrane. 



Simple diffusion is the mixing of two liquids when poured together, to 

 form a uniform solution. 



Filtration is the passing of a liquid through a membrane or other substance 

 for the purpose of removing some portion. 



It is probable that all three processes go on in the tissues. 



Clinical note. Two methods of local treatment of inflammation are based 

 upon the above-named conditions: (i) preventive; (2) remedial. By the 

 application of ice to prevent the intensity of hyperemia which leads to 

 destruction of the tissues, or by application of heat to cause dilation of sur- 

 face vessels and relieve it. After tissue destruction has actually occurred ice 

 is no longer useful. 



References have been made to the salinity of the blood and normal saline 

 solution, which is a solution of the same concentration as blood plasma. 

 Several very important things are conditioned by the normal salinity of the 

 blood: (i) The integrity of the corpuscles. They as well as plasma contain 

 substances which give them a certain density. If the plasma were a fluid of 

 lesser density, its watery portion would invade the corpuscle and injure or 

 destroy it. If the plasma were of greater density, the water of the corpuscle 

 would leave it and the cell would shrink. (The " diffusion streams " of osmosis 

 would be set in motion in the effort to equalize the densities of cell and blood, 

 the outer portion of cell protoplasm representing a membrane.) 



The subject of osmosis is considered on page 170. 



Hypodermodysis. This is the injecting of a watery solution 

 of certain salines into the fascia under the skin, for the purpose 

 of adding fluid to the blood, by absorption from the tissue. 

 From the preceding paragraph one understands why the fluid 

 injected must be a normal or physiologic salt solution that 

 is, it must be of the same density as the blood plasma. (The 

 normal salt solution, better physiologic salt solution, must con- 

 tain nine-tenths of i per cent, of salines.) 



