94 



THE STRUCTURE OF VEINS. 



works, whose shai^e and arrangement differ considerably in different tissues. The diameter of 

 the capillaries varies considerably, but as a general rule it is such as to admit freely a single row 

 of blood -corpuscles. In the retina and the muscles the diameter is 5-6 /*, and in bone-marrow, 

 liver, and choroid 10-20 /*. The tubes consist of a single layer of transparent, excessively thin, 

 nucleated, endothelial cells joined to each other by their margins. [The nuclei contain a well- 

 marked intra-nuclear plexus of fibrils, like other nuclei.] The cells are more fusiform in the 

 smaller capillaries and more polygonal in the larger. The body of the cells presents the 

 characters of very faintly refractive protoplasm, but it is doubtful whether the body of the 

 cell is endowed with the property of contractility (p. 96). 



If a dilute solution (J per cent) of silver nitrate be injected into the blood-vessels, the 

 cement substance of the endothelium, [and of the muscular fibres as well], is revealed by the 

 presence of the black "silver lines." The blackened cement substance shows little specks and 

 large black slits at different points. It is not certain whether these are actual holes through 

 winch colourless corpuscles may pass out of the vessels, or are merely larger accumulations of 

 the cement substance. [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.] 



[Arnold called these small areas in the black silver lines when they are large stomata, and 



when small stigmata. They an 



most numerous after venous congestion, and after the dis- 

 turbances which follow inflammation of a part. They 

 are not always present. The existence of cement sub- 

 stance between the cells may also be inferred from the 

 fact that indigo-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-medul- 

 lated nerves terminate in small end-buds in relation 

 with the capillary wall ; ganglia in connection with 

 the nerves of capillaries occur only in the region of the 

 sympathetic. 



The small vessels next in size to the capillaries, and 

 continuous with them, have a completely structureless 

 covering in addition to the endothelium. 



III. The veins are generally distinguished 

 from the arteries by their lumen being wider 

 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 longitudinally 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. 



Structure. (1) The tunica intima consists of a layer 

 of shorter and broader endothelial cells, under which in 

 the smallest veins there is a structureless elastic mem- 

 brane, 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 double 

 in some parts of the crural and iliac veins. Isolated 

 muscular fibres exist in the intima of the femoral and 

 popliteal veins. 



(2) The t. media of the larger veins consists of 

 divided transversely ; d, longitudinal alternate layers of elastic and muscular tissue united 

 muscular fibres in the adventitia. to each other by a considerable amount of connective- 



tissue, but this coat is always thinner than in the 

 corresponding arteries. This coat diminishes in the following order in the following vessels : 

 popliteal, veins of the lower extremity, veins of the upper extremity, superior mesenteric, other 

 abdominal veins, hepatic, pulmonary, and coronary veins. The following veins contain no 

 muscle : veins of bone, central nervous system ana its membranes, retina, the superior cava, 



Fig. 70. 



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 



