Action of the Heart. 511 



these ready made channels and contributes to their enlargement, and 

 such a soil becomes a network of quite constant veins and capillaries. 



Landois says : < ' Intercellular blood channels of narrow calibre and 

 without walls, occur in the granulation tissue of healing wounds. At 

 first, blood plasma alone is found between the formative cells, but after- 

 ward, the current forces blood corpuscles through the channels. The 

 first blood vessels in the developing chick, are formed in a similar way 

 from the formative cells of the mesoblast. " The arteries which carry 

 the blood to the tissues from the heart, subdivide repeatedly until they 

 become reduced to mere capillary threads. The tissues of the body 

 generally are traversed in all directions by capillary blood vessels which 

 form an intricate netwofk. The veins also are connected with the cap- 

 illaries, and drain the blood from the tissues back to the heart. The 

 capillaries are from -^ to of an inch in diameter. In manv of 



SOOO 15OO J 



them the blood corpuscles which pass through them are obliged to go 

 single file, as there is not room to go abreast. The capillaries are com- 

 posed of protoplasm apparently but little differentiated. One authority 

 describes them as " protoplasm in tubes." This is their condition es- 

 pecially in the young animal. The walls of the capillaries are contrac- 

 tile, especially the nuclei of their cells. The walls are very thin and 

 delicate, and accommodate themselves to the pressure of the contained 

 blood. The larger blood vessels are likewise yielding and readily dis- 

 tended, but persistently regain their accustomed size when the pressure 

 is taken off. The fibrous cells composing the thick walls of the larger 

 vessels, like all others, require to be nourished, and so there are little 

 blood vessels called vasa vasorum, which run through these coats and 

 bathe all the cells. The coats of all the vessels being composed of cells 

 which, in their original character, possess the individuality and contrac- 

 tility of so many amoebae, the general contractility of the organ composed 

 of them is accounted for as the sum of that of the individual cells ex- 

 erted under a simultaneous impulse. The impulses are made common 

 and simultaneous by the nervous system. The foetal heart, like the 

 other blood vessels, is composed of cells not modified into muscle fibres, 

 and it beats for a long time before such modification takes place. There 

 have been cases in embryonic growth, in which no heart was formed, 

 but in which circulation nevertheless took place, producing a normal de- 

 velopment of most of the parts of the body, so that contractility and 

 circulation precede hearts. Furthermore ; < ' It has occasionally been 

 noticed that a gradual degeneration in the structure of the heart has 

 taken place during life to such an extent that scarcely any muscular 

 tissue could at last be detected in it, but without any such interruption 

 to the circulation as must have been anticipated if this organ furnished 

 the sole impelling force. " (Carpenter.) And so we perceive that con- 



