254 GENERAL ANATOMY. 



The larger capillary vessels experience several successive 

 divisions before they acquire a size capable of admitting a sin- 

 gle red globule of blood. 



These capillaries communicate together by an infinity of 

 anastomoses so as to form a net-work. They constitute the 

 largest portion of the circulating circle, the capacity of the ar- 

 terial system continually increasing from its origin at the heart 

 to the capillary vessels, and that of the venous system decreas- 

 ing from the capillary vessels to the heart. 



The circulating circle being double in man, there are two 

 capillary systems: the one general, between the terminations 

 of the aortic arteries, and the origin of the veins of the body; 

 and the other pulmonary* at the extremities of the vessels 

 which bear this name. It has been advanced, but without any 

 positive proof to support the assertion, that the pulmonary ca- 

 pillary system is as capacious and contains as much blood as 

 the general capillary system. 



There are in the abdomen two other small capillary systems; 

 one between the mesenteric arteries and veins, the other be- 

 tween the hepatic extremities of the vena porta and the origin 

 of the hepatic veins. 



379. The texture of the capillary vessels can not be ob- 

 served with the naked eye. These vessels have very thin and 

 soft transparent parietes, invisible to the naked eye, and slight- 

 ly visible with the microscope, very little different from the 

 substance of organs, and also from the humours they convey. 

 They seem rather formed out of the substance of the organs 

 than provided with its own parietes. It is, however, very 

 probable that the internal membrane of the vessels, at least, is 

 continuous without any interruption, from the arteries to the 

 veins. 



In the living body they are only distinguished by the colour 

 and the direction of the flow of the blood which they contain, 

 and after death by the colour of the matter with which they 

 are injected. They are distinguished from the spongy areolae, 

 and the accidental cavities of the cellular tissue, by their con- 

 stant, continuous, and regular course. 



380. Although the parietes of all the vessels are permea- 



