THE CAPILLARIES 93 



capillaries as directly or indirectly affect the rest of the 

 circulation. 



Direct observation of the capillaries in the thin tissues 

 of the frog has shown that they are capable of considerable 

 variation in calibre. In a resting muscle they are con- 

 stantly contracting and expanding, the great majority 

 being at any one time contracted to complete obliteration 

 of their lumen. The course of the blood is constantly 

 changing; it flows now through this tube, now through 

 that. The capillaries therefore possess a considerable 

 power of contraction, and experiment shows that this power 

 is independent of nervous influences, being an inherent 

 property of the endothelial cells of which the capillaries are 

 composed. 



When a muscle becomes active there occurs a simul- 

 taneous opening up of all the capillaries, so that the blood 

 supply may be increased several hundred times. The 

 capillaries respond readily to chemical agents. On the 

 direct appHcation of acids they are dilated. It is there- 

 fore probable that the acids produced in activity are the 

 cause of the dilatation. 



It should be reahsed that the degree of t07ius of the capil- 

 laries is not dependent upon the hlood-pressure. The 

 capillaries are not necessarily distended by a rise in the 

 pressure of blood supplying them. Adrenahn, in addition 

 to constricting the arterioles, in weak doses dilates the 

 capillaries. Similarly, histamine, a base derived from the 

 amino-acid histidine (by removal of COg), constricts arterioles 

 and at the same time dilates capillaries. 



Shock 



Confirmatory evidence of the changes in caUbre under- 

 gone by the capillaries is forthcoming from a study of the 

 chnical condition of shock. This is characterised by a 

 great fall of blood-pressure. It is brought on by trauma 

 or haemorrhage, especially under conditions of exposure to 

 cold, excitement and deprivation of food. 



