PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES 



Although their arrangement presents certain differences in different 

 organs, the capillary vessels have everywhere the same general charac- 

 ters, the most prominent of which are a nearly uniform diameter and an 

 apparent absence of any definite direction in their branchings. The 

 network thus formed is rich in the parenchyma of the glands and in 

 the organs of absorption. In the walls of the pulmonary alveoli the 

 meshes are particularly close. In other parts the vessels are not 

 so abundant, presenting great variations in different tissues. In the 

 muscles and nerves, in which nutrition is very active, the supply of 

 blood is greater than in other 

 parts, like fibre-serous mem- 

 branes, tendons, etc. In none 

 of the tissues do the capillaries 

 penetrate the true anatomical 

 elements of the parts, as the 

 ultimate muscular or nervous 

 fibres. Some tissues receive no 

 blood at least they contain 

 no vessels that are capable of 

 carrying red blood and are 

 nourished by imbibition of nu- 

 trient matters from the blood- 

 plasma. 



The capacity of the capillary 

 system is very large. It is neces- 

 sary only to consider the great 

 vascularity of the skin, mucous 

 membrane or muscles, to appre- 

 ciate this fact. In injections of 

 these parts, it seems, on microscopical examination, as though they 

 contained nothing but capillaries ; but in preparations of this kind, 

 the coats of the capillaries are distended to their utmost limit. Under 

 some conditions in health, they are largely distended with blood, as in 

 the mucous lining of the alimentary canal during digestion, the whole 

 surface presenting a vivid red color, indicating the richness of the capil- 

 lary plexus. Estimates of the capacity of the capillary system, as com- 

 pared with the arterial system, have been made, but they are simply 

 approximative. The various estimates given are founded on calcula- 

 tions from microscopical examinations of the rapidity of .the capillary 

 circulation as compared with the circulation in the arteries. In this way, 

 it has been calculated that the capacity of the capillary system is be- 

 tween five hundred and eight hundred times that of the arterial system. 



Fig. 25. Small arterv and capillaries from the 

 muscular coat of the urinary bladder of a frog, x 400 

 (from a photograph taken at the United States Army 

 Medical Museum). 



This preparation shows the endothelium of the 

 vessels. It is injected with silver nitrate and mounted 

 in Canada balsam. 



