FEATURES OF VASCULAR APPARATUS. 135 



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 

 elements, 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 vasomotor nerves, which we shall have to study 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 mus- 

 cular 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 proportion 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 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 pass- 

 ing 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 multitudinous 

 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 sec- 

 tional area of the arterial bed through which the blood flows goes on increas- 

 ing 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 sub- 

 stances, 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 blood and the tissue. In 

 certain circumstances, at all events, white or even red corpuscles may also 

 pass through the wall to the tissue outside. 



The minute arteries and veins with which the capillaries are continuous 

 allow of a similar interchange of material, the more so the smaller they are. 



The walls of the veins are thinner, weaker and less elastic than those of 



