STRUCTURE OF VEINS. 



123 



sulphate of soda is deposited in it (Thoma), and particles of cinnabar and China 

 ink are fixed in it, when these substances are injected into the blood (Foa).] 



Fine anastomosing fibrils derived from non-medullated NERVES 

 terminate in small end-buds in relation with the capillary wall; 

 ganglia in connection with capillary nerves occur only in the region 

 of the sympathetic (Bremer and Waldeyer). 



[If a capillary is examined in a perfectly fresh condition (while 

 living) and without the addition of any reagent, it is impossible to 

 make out any line of demarcation between adjacent cells owing to 

 the uniform refractive index of the entire wall of the tube.] 



The small vessels next in size to the capillaries and continuous 

 with them have -a completely 

 structureless covering in addition 

 to the eudothelium. 



III. The Veins are generally 

 distinguished from the arteries by 

 their lumen being under than the 

 lumen of the corresponding 

 arteries; their walls are thinner 

 on account of the smaller amount 

 of non-striped muscle and elastic 

 tissue (the non-striped muscle is 

 not unfrequently arranged longi- 

 tudinally in veins). They are 

 also more extensile (with the same 

 strain). The adventitia is usually 

 the thickest coat. 



The occurrence of valves is 

 limited to the veins of certain 

 areas. (1.) The inttma consists 

 of a layer of shorter and broader 

 endothelial cells, under which in 

 the smallest veins there is a 

 structureless elastic membrane, 

 sub-epithelial layer, which is fibrous 

 in veins somewhat larger in size, 

 but in all cases is thinner than in 

 the arteries. In large veins it 

 may assume the characters of a 

 fenestrated membrane, which is 



Fig. 42. 



Longitudinal section of a vein at the 

 level of a valve a, hyaline layer of 

 the internal coat; b, elastic lamina; 

 c, groups of smooth muscular fibres 

 divided transversely; d, longitudinal 

 muscular fibres in the adventitia. 



double in some parts of the 



crural and iliac veins. Isolated muscular fibres exist in the intima 



of the femoral and popliteal veins. 



