334 CIRCULATION OF NUTRITIVE FLUID. 



restricted sense ; in the former it includes all the minute vessels which 

 pass between the arteries and the veins ; in the latter it is applied only 

 to those which admit no more than a single file of blood-discs at once, 

 and excludes those which admit two, three, or even four rows, even 

 although they establish a direct communication from one side of the 

 network to- the other. The former application of the term is the most 

 convenient, although perhaps not the most strictly accurate ; and it will 

 be therefore here employed in its extended sense. And this is rendered 

 more correct by the fact, that the size of the individual capillaries is 

 by no means permanent ; an enlargement often taking place in one, and 

 a contraction in another, at the same time : so that vessels, which were 

 previously true capillaries, no longer remain such ; and passages, which 

 were previously of far greater calibre, are reduced to the average 

 diameter. 



593. The opinion was long entertained, that there are vessels adapted 

 to the supply of the white or colourless tissues ; carrying from the 

 arteries only the fluid portion of the blood, or liquor sanguinis, and 

 leaving the rest behind. No other such vessels have been really ob- 

 served, however, than the capillaries in a state of unusual contraction, 

 as just now mentioned. And it may be safely affirmed, that the suppo- 

 sition of their existence is not required. For any one who observes 

 the smallest capillary vessels under the microscope, may perceive, that 

 the current of blood which passes through them is almost free from 

 colour, as the red corpuscles themselves appear to be, when spread 

 out in a single layer. Tissues which are rather scantily permeated by 

 such vessels, therefore, may still be white ; and it is only where the 

 network is very close, and the quantity of blood which passes through 

 it is consequently great, that a perceptible colour will be communicated 

 by the red corpuscles. And we have seen, that the idea that Nutrition 

 can only be carried on by direct communication with vessels, is entirely 

 unfounded ; the .tissues into which no blood-vessels can be traced, being 

 adapted to nourish themselves, like cellular Plants, by the imbibition of 

 fluid at their surfaces, on which vessels are (for the most part) copiously 

 distributed. 



594. That the blood can only minister to the operations of Nutrition, 

 Secretion, &c., whilst it is circulating through the Capillaries, is evident 

 from several considerations. The thickness of the walls of the larger 

 vessels interposes an effectual barrier to its transudation ; and so com- 

 pletely is the blood cut off even from penetrating these, that they do 

 not derive their own nourishment from the blood which flows through 

 their tubes, but from a capillary network in their own substance, which 

 is supplied by vessels from collateral branches, these being termed the 

 vasa vasorum. Moreover it is by the inosculation of the capillaries 

 alone, that the minute network is formed, which serves to bring the 

 blood into proximity with the minute parts of the tissues to be nourished ; 

 thus let it be supposed that the minute arteries of Muscle were to ter- 

 minate in veins, without undergoing further subdivision, the islets left 

 between their anastomosing branches would be far too large, and the 

 nutritive materials would consequently not be supplied with sufficient 

 readiness, even supposing that it could freely permeate the walls of 



