CHAP, iv.] THE VASCULAR MECHANISM. 151 



from the aorta all over the body. The constitution of their walls, 

 especially of the middle coat, gives the arteries two salient proper- 

 ties. In the first place they are very elastic, in the sense that 

 they will stretch readily, both lengthways and crosswise, when 

 pulled, and return readily to their former size and shape when 

 the pull is taken off. If fluid be driven into one end of a piece 

 of artery, the other end of which is tied, the artery will swell out 

 to a very great extent, but return immediately to its former 

 calibre when the fluid is let out. This elasticity is chiefly due to 

 the elastic elements in the coats, elastic membranes and feltworks, 

 but the muscular fibres being themselves also elastic contribute to 

 the result. By reason of their possessing such stout elastic walls 

 the arteries when empty do not collapse but remain as open tubes. 

 In the second place the arteries by virtue of their muscular ele- 

 ments are contractile; when stimulated either directly as by 

 applying an electric or mechanical stimulus to the arterial walls 

 or indirectly by means of the so-called vaso-motor nerves, which 

 we shall have to study presently, the arteries shrink in calibre, 

 the circularly disposed muscular fibres contracting and so, in pro- 

 portion to the amount of their contraction, narrowing the lumen 

 or bore of the vessel. The contraction of these arterial muscular 

 fibres, like that of all plain non-striated muscular fibres, is slow 

 and long continued, with a long latent period, as compared with 

 the contraction of skeletal striated muscular fibres. Owing to 

 this muscular element in the arterial walls, the calibre of an 

 artery may be very narrow, or very wide, or in an intermediate 

 condition between the two, neither very narrow nor very wide, 

 according as the muscular fibres are very much contracted, or not 

 contracted at all, or only moderately contracted. Further, while 

 the relative proportion of elastic and muscular elements differs in 

 different arteries, as a general rule the elastic elements predomi- 

 nate in the larger arteries and the muscular elements in the 

 smaller arteries, so that the larger arteries may be spoken of as 

 eminently elastic, or as especially useful on account of their 

 elastic properties, and the smaller arteries as eminently muscular, 

 or as especially useful on account of their muscular properties. 

 Thus in the minute arteries which are just passing into capillaries 

 the muscular coat, though composed often of a single layer, and 

 that sometimes an imperfect one, of muscular fibres, is a much 

 more conspicuous and important part of the arterial wall than that 

 furnished by the elastic elements. 



The arteries branching out from a single aorta down to multi- 

 tudinous capillaries in nearly every part of the body, diminish in 

 bore as they divide. Where an artery divides into two or gives off 

 a branch, though the bore of each division is less than that of the 

 artery before the division or branching, the two together are 

 greater ; that is to say, the united sectional area of the branches 

 is greater than the sectional area of the. trunk. Hence the 



