OF THE CAPILLARY VESSELS. 255 



ble, nevertheless, this property is particularly observable in 

 the smallest vessels. 



They are very extensible and very contractile. Irritability 

 increases while elasticity diminishes in the vessels in the same 

 degree as they approach their termination. The capillaries 

 are the most irritable.* Their contractility is produced either 

 by local and direct agents, or by the nervous system. 



381. It is in this part of the vascular system that the 

 most important phenomena of the organism occur, at least 

 of the vegetative functions. The capillary circulation, i. e. 

 the passage of the blood through the vessels of this name, 

 is, of all the parts of the circulation, that, which without be- 

 ing independent of the action of the heart, is the least under 

 its control. It is the point of the circle at which the move- 

 ment of the blood is slowest; it is that in which the blood, 

 divided in very small streams, has the greatest number of 

 points of contact with the parietes of the vessels, and the 

 most influenced by the nervous action. The blood takes its 

 regular course through the capillary system by going directly 

 from the arteries to the veins; if it meets an obstacle, many 

 anastomosing vessels are opened and permit it to continue 

 its round. But this system may also be the seat of conges- 

 tions, irritations, and constrictions, which change the ordinary 

 course of the liquids. Thus the application of warm fluids, 

 for a few minutes, to the lower extremities of a frog, produce 

 a dilatation of the capillary vessels, a local and partial stoppage 

 of the circulation, a congestion, in a word, it causes the tis- 

 sues which were before white, to become very red. The same 

 thing occurs, from various causes, on the mammiferous ani- 

 mals and on man. The application of cold or of a diluted acid 

 produces entirely opposite effects. Mechanical or chemical 

 irritation produces at first the latter effect, and afterwards, by 

 a kind of attraction, a concentric afflux of the liquids which, in 

 many vessels, are then pursuing a course opposite to that of 

 the blood. 



* Whytt, Physiological Essays, 8tc. Edin. 1761. H. Van den Bosh, iiber 

 das Hfusketwermogen der Haargefasschen. Monast. 1786. 

 34 



