292 OF THE VEINS. 



times its diameter. The blood current may, indeed, 

 for a time, be completely stopped in consequence of 

 tbe sudden and violent conti'action of the mnsciilar 

 fibres which encircle the little vessel. See fig. 1, 

 Plnte XV, page 293, fig. 4, Plate XVI, also plates 

 XVII and XX. 



It has been suggested that the encii'chng muscu- 

 lar fibres of the small arteries by their contraction 

 are instrumental in driving onwards the blood to- 

 wards the capillaries. A number of facts, however, 

 mihtate against the reception of such an idea, and 

 are conclusive in proving that the contraction of the 

 muscular fibres exerts only an obstructive influence. 

 The blood current may be diminished or completely 

 checked for a time, but under no circumstances can 

 it be urged on by the contraction of the arterial 

 walls. 



The fibres of unstriped muscle found upon arteries 

 are of two kinds. 1. Ordinary spindle-shaped fibres, 

 which encircle all the small arteries of all vertebrate 

 animals, as has been described and figured by many 

 observers. 2. Fibres have three or four processes 

 similar to those I have described as existing in the 

 bladder of the frog, § 262. These last by their con- 

 traction would tend to shorten the vessel as well as 

 to reduce its calibre, and these are probably instru- 

 m.ental in preventing elongation of the tube which 

 the circular fibres would tend to produce when they 

 contracted strongly. The fibres under consideration 

 are found in the arteries, the coats of which are 

 composed principally of yellow elastic tissue. In tlie 

 coats of the veins these fibres pulling in three direc- 

 tions are also numerous. 



300. Of the veins. — The walls of the larger veins 

 are composed of elastic fibres and muscular fibre cells 

 which are more spi-ead out and arranged with less 

 regularity than those of the arterial coats. The 

 smaller veins are, however, completely destitute of 



