CHAP, iv.] THE VASCULAR MECHANISM. 199 



presently, the arteries shrink in calibre, the circularly disposed 

 muscular fibres contracting and so, in proportion 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. We have further seen that, while the relative pro- 

 portion of elastic and muscular elements differs in different 

 arteries, as a general rule the elastic elements predominate in 

 the larger arteries and the muscular elements in the smaller 

 arteries, so that the larger arteries may be spoken of as emi- 

 nently 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 

 sectional area of the arterial bed through which the blood flows 

 goes on increasing from the aorta to the capillaries. If all the 

 arterial branches were thrown together into one channel, this 

 would form a hollow cone with its apex at the aorta and its base 

 at the capillaries. The united sectional area of the capillaries 

 may be taken as several hundred times that of the sectional area 

 of the aorta, so greatly does the arterial bed widen out. 



The capillaries are channels of variable but exceedingly small 

 size. The thin sheet of cemented epithelioid plates which forms 

 the only wall of a capillary is elastic, permitting the channel offered 

 by the same capillary to differ much in width at different times, 

 to widen when blood and blood corpuscles are being pressed 

 through it and to narrow again when the pressure is lessened or 

 cut off. The same thin sheet permits water and substances, 

 including gases, in solution to pass through itself from the blood 

 to the tissue outside the capillary and from the tissue to the 

 blood, and thus carries on the interchange of material between the 



