PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES. 



81 



blood only during the physiological activity of these parts. In the lungs the 

 meshes are particularly close. In other parts the vessels are not so abun- 

 dant, presenting great variations in different tissues. In the muscles and 

 nerves, in which nutrition is very active, the supply is much more abundant 

 than in other parts, like fibro-serous membranes, tendons etc. In none of 

 the tissues do the capillaries penetrate the anatomical elements of the part, 

 as the ultimate muscular or nervous fibres. Some tissues receive 110 blood, or 

 at least they contain no vessels which are capable of carrying red blood, and 

 are nourished by imbibition of the nutrient plasma of the circulating fluid. 

 Examples of these, which are called extra vascular tissues, are, cartilage, nails 

 and hair. 



The capacity of the capillary system is very great. It is necessary only to 

 consider the great vascularity of the skin, mucous membranes or muscles, to 

 appreciate this fact. In injections of these parts, it seems, on microscopical 

 examination, as though they contained nothing but capillaries ; but in prepa- 

 rations of this kind, the elastic and yielding 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, indi- 

 cating the great richness of the capillary plexus. Estimates of the capacity 

 of the capillary system, as compared with the arterial system, have been 

 made, but they are simply approximative. The various estimates given are 

 founded upon calculations from microscopical examinations of the rapidity 

 of the capillary circulation as compared with the circulation in the arteries. 

 In this way, it 

 has been estima- 

 ted that the ca- 

 pacity of the 

 capillary system 

 is between five 

 hundred and 

 eight hundred 

 times that of the 

 arterial system. 

 These estimates, 

 however, must 

 be regarded as 

 mere supposi- 

 tions based up- 

 on no very ac- 

 curate data. 



Phenomena of the Capillary Circulation. -The most convenient situation 

 for observation of the capillary circulation is the tongue or the web of the 

 frog. Here may be studied, not only the movement of the blood in the true 

 capillaries, but the circulation in the smallest arteries and veins, the variations 

 in caliber of these vessels, especially the arterioles, by the action of their 



FIG. 33. Web of the frog's hind-foot ; magnified (Wagner). 

 o, o, veins ; b, b, 6, arteries. 



