THE CAPILLARY CIRCULATION. 673 



after long fasting, and is also common at puberty, in old age, and in 

 various diseases. In healthy persons, its duration is somewhat longer 

 than that of a single beat of the heart. 



Motion of the Blood through the Capillaries. 



The tissues in which capillary bloodvessels are found, and those in 

 which they are absent, their number and size in the various tissues 

 and organs, the varieties in the arrangement of the capillary network, 

 and the structure of their delicate walls, are elsewhere detailed (p. 56). 

 The form of the capillary network in different parts, has no relation to 

 the functions of those parts, otherwise than so far as these depend on 

 the forms and disposition of the structural elements, between not into 

 which the vessels penetrate. But the closeness of the network, and 

 the consequent number of the capillary vessels in a given space, are 

 proportional to the activity and importance of those functions. 



The capillaries form the intermediate blood channels between the 

 finest arteries and veins. When examined in the transparent part of 

 a living animal, they are seen to be of different sizes, some conveying 

 two or more rows, and others only a single row of blood corpuscles. , 

 Moreover, when watched sufficiently long, they are observed to undergo 

 slow changes in diameter, so that vessels, at one time capable of con- . 

 veying several rows of blood corpuscles, shrink, and no longer convey 

 more than a single row, or even become temporarily incapable of ad- 

 mitting any corpuscles, so that they merely convey the liquor sanguinis. 

 It was at one time supposed that vessels, named vasa serosa or serous 

 vessels, constantly so small as only to admit the fluid portion of the 

 blood, existed in all or many parts of the body ; but their presence 

 generally, which was purely conjectural, has not been confirmed. By 

 some authorities, however, it is at least suggested that, in the cornea, 

 capillaries may exist, which habitually convey only the liquor sanguinis 

 (Kolliker, Hyrtl). 



In watching the capillary circulation, it is seen that such vessels as 

 have ceased for a time to convey blood particles, again dilate and admit 

 them, and, from this alternate contraction and dilatation, a vital con- 

 tractility has been attributed to the coats of these vessels. The struc- 

 ture of their delicate walls, however, which are composed of homoge- 

 neous membrane containing nuclei but destitute of muscular fibre-cells, 

 negatives the idea of their possessing vital contractility; and, more- 

 over, it has been found that no contraction, or other change of these 

 walls occurs, on the direct application of the electric stimulus to them. 

 Their walls are, however, elastic, and the changes in diameter of the 

 vessels are probably due, either to disturbed conditions in the neigh- 

 boring small arteries, owing to the contraction or relaxation of their 

 muscular coat, or to movements in the tissues in which the capillaries 

 are distributed, and in which organic muscular fibres, fibre-cells, or 

 other contractile elements, such as pigment-cells, are frequently pres- 

 ent. The capillaries do not, therefore, seem to exercise any mechani- 

 cal influence on the circulation of the blood through them, by virtue of 

 an active contractile force, resident in their walls ; but they may adapt 



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