THE CIRCULATION 153 



into account as a factor in the morbid processes, as of 

 the state of the nervous system and of the body generally 

 which has determined its rate and character.' 



The Capillary Circulation. 



As the whole aim and end of the circulation is to 

 convey material to and from the tissues, the passage of 

 blood through the capillaries is, in a sense, the most im- 

 portant part of the circulatory process. Unfortunately, 

 however, the attention of physiologists has been until 

 lately so much concentrated upon the arteries that we 

 are still very far from being conversant with all the facts 

 relating to the capillary circulation. 



In what follows an attempt will be made to indicate 

 briefly the present state of our knowledge on the subject, 

 and its application to clinical medicine. 



The capillaries are only about J to 1 millimetre in 

 length, but they form a network so dense that it would 

 hardly be possible to stick a pin into any part of the 

 body without injuring one of them. Notwithstanding 

 this, the capillaries are by no means always full of blood, 

 as is shown, for instance, by the great increase in redness 

 of the skin when one applies a blister to it. They con- 

 sist, anatomically, of a single layer of endothelial cells, 

 which, although they appear to be of the simplest 

 structure, are possibly endowed with important selective 

 and other properties. In health this layer is so thin 

 that it forms a mere hair line, but in disease it may 

 become considerably thickened. 



The appearance presented by the blood as it circulates 

 through the capillaries must be so vividly present to the 



