220 



THE CIRCULATORY SYSTEM 



[CH. XVI II. 



in the different tissues of the bod} 7 : the varieties consist principally 

 of modifications of two chief kinds of mesh, the rounded and the 

 elongated. That kind in which the meshes or interspaces have a 



FIG. 229. Diagram showing valves of veins. A, part of a- vein laid open and spread out, with two pairs 

 of valves. B, longitudinal section of a vein, showing the apposition of the edges of the valves in 

 their closed state, c, portion of a distended vein, exhibiting a swelling in the situation of a pair 

 of valves. 



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. 233), most glands and mucous (membranes, and the 



Tin. 230. A, vein with valves open. B, with valves closed ; stream of blood passing oil by lateral 



channel. (Dalton.) 



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. 234), the short sides of which may be from three 



