222 



THE CIRCULATORY SYSTEM. 



[CH. XVIII. 



The number of the capillaries and the size of the meshes in 

 different parts determine in general the degree of vascularity of 

 those parts. The capillary network is closest in 'the lungs and 

 in the choroid coat of the eye. 



It may be held as a general rule, that the more active the 

 functions of an organ are, the more vascular it is. Hence the 

 narrowness of the interspaces in all glandular organs, in mucous 

 membranes, and in growing parts, and their much greater width 

 in bones, ligaments, and other tough and comparatively inactive 

 tissues. 



Lymphatic Vessels. 



The blood leaves the heart by the arteries ; it returns to the 

 heart by the veins ; but this last statement requires modification, 



Fig. 227.' Superficial lymphatics of right groin and upper part of thigh. J. 

 i. tipper inguinal glands. 2, 2'. lx>wer or inguinal or femoral glands. 3. 3'. 

 Plexus of lymphatics in the course of the long saphenous vein. (Mascagni.) 



for in the capillaries some of the blood-plasma escapes into the 

 cell spaces of the tissues and nourishes the tissue-elements. 

 This fluid, which is called lymph, is gathered up and carried back 

 again into the blood by a system of vessels called lymphatics. 



