156 



APPLIED PHYSIOLOGY 



from the capillaries of the brain so often occurs in cases 

 of high arterial pressure, for the cerebral arteries being 

 unprovided with vasomotor nerves, there is no method 

 of shielding the intracranial capillaries from the effects 

 of the general rise. 



The action of gravity may increase the capillary 

 pressure very markedly. Thus the pressure in the 

 finger is more than twice as high when the hand is 

 hanging by the side than when it is raised to the level 

 of the top of the head. It is for this reason that raising 

 the hand relieves the pain and throbbing in an inflamed 

 finger so noticeably. 



The most important function of the capillaries is to 

 permit of an interchange between the blood and the 

 tissues, and two opposing views have long been held by 

 physiologists as to how this takes place. The upholders 

 of the mechanical theory, first propounded by Ludwig, 

 maintain that the physical processes of diffusion, osmosis, 

 and dialysis (which may be spoken of collectively as 

 * transfusion ') are sufficient to explain all the facts ; on 

 the other hand, Heidenhain and the * vital' school 

 invoke a special secretory power on the part of the cells 

 forming the capillary wall, and look upon lymph as a 

 secretion of the capillaries, just as milk is a secretion of 

 the mammae or urine of the kidneys. The problem is 

 one which is, perhaps, more of abstract physiological 

 interest than of direct concern to the physician, but it 

 may fairly be said that at present the views of the 

 mechanical school tend to hold the field. 



On neither hypothesis has a complete explanation yet 

 been given of the chief anomaly of lymph production 



