PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES 63 



From the fact that the arterial system progressively increases in 

 capacity, there should be found a corresponding diminution in the 

 rapidity of the flow of blood. There are, however, many conditions, 

 aside from simple increase in the capacity of the vessels, which modify 

 the blood-current and render inexact calculations made on purely physical 

 principles. These are the tension of the blood and the conditions of 

 contraction or relaxation of the smallest arteries. It is necessary, there- 

 fore, to have recourse to actual experiments to arrive at definite results 

 on this point. Volkmann found a great difference in the rapidity of the 

 current in the carotid and metatarsal arteries, the averages being about 

 10 inches (254 millimeters) per second in the carotid, and about 2.2 

 inches (56 millimeters) in the metatarsal. The same difference, al- 

 though not quite so marked, was found by Chauveau, between the 

 carotid and the facial. As the vessels are farther and farther removed 

 from the heart, the systolic impulse rapidly diminishes, being reduced 

 in one experiment by about two-thirds; the dicrotic impulse becomes 

 feeble or may even disappear ; but the constant flow is much increased 

 in rapidity. The rapidity of the flow in any given artery must be sub- 

 ject to modifications due to the condition of the arterioles which are sup- 

 plied by it. When these little vessels are dilated, the artery, of course, 

 supplies blood with greater facility and the rapidity of the flow is 

 increased. 



CIRCULATION OF BLOOD IN THE CAPILLARIES 



Before beginning the study of the capillary circulation, it should 

 be understood what is meant by capillary vessels as distinguished from 

 the smallest arteries and veins. From a physiological point of view, the 

 capillaries are to be regarded as beginning where the blood is brought 

 near enough to the tissues to enable them to separate the matters neces- 

 sary for their regeneration and to give up the products of their physio- 

 logical wear ; but at present it is impossible to assign any limit where 

 the vessels cease to be simple carriers of blood, and it is not known to 

 what part of the vascular system the processes of nutrition are exclu- 

 sively confined. The divisions of the bloodvessels must be, to a certain 

 extent, arbitrarily defined. The most simple view is to regard as capil- 

 laries vessels that have but a single coat ; for in these, the blood is brought 

 in closest proximity to the tissues. Vessels that are provided, in addi- 

 tion, with a muscular or with muscular and fibrous coats may be con- 

 sidered as either small arteries or veins. 



Physiological Anatomy of the Capillaries. If the arteries are fol- 

 lowed out to their minutest ramifications, they will be found progressively 

 diminishing in size as they branch, and their coats, especially the mus- 



