GIL XVIII.] LYMPHATIC VESSELS 221 



to eight or ten times less than the long ones ; the long sides are 

 more or less parallel to the long axis of the fibres. 



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 



FIG. 231. Surface view of an artery from the mesentery of a frog, eiisheathed in a perivascular lym- 

 phatic vessel, a, the artery, with its circular muscular coat (media) indicated by broad transverse 

 markings, with an indication of the adventitia outside. I, lymphatic vessel ; its wall is a simple 

 endothelial membrane. (Klein and Noble Smith.) 



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

 bones, ligaments, and other 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, 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. 



