CH. XIX.] 



LYMPHATIC VESSELS 



223 



roundish or polygonal form is the most common, and prevails in 

 those parts in which the capillary network is most dense, such as 

 the lungs (fig. 205), most glands and mucous membranes, and the 

 cutis. The capillary network with elongated meshes is observed in 

 parts in which the vessels are arranged among bundles of fine tubes 

 or fibres, as in muscles and nerves. In such parts, the meshes form 

 parallelograms (fig. 206), the short sides of which may be from three 



FIG. 205. Network of capillary 

 vessels of the air-cells of the 

 horse's lung magnified, a, a, 

 Capillaries proceeding from b, 

 &, terminal branches of the 

 pulmonary artery. (Frey.) 



FIG. 206. Injected capil- 

 lary vessels of muscle 

 seen with a low mag- 

 nifying power. 



(Sharpey.) 



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 

 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 



