362 OF THE CIRCULATION OF BLOOD. 



Tadpole or other animal must be subjected, in order to allow microscopic 

 observations to be made upon its circulation. Under such circumstances, the 

 varieties in the capillary circulation, induced by causes purely local, become 

 very conspicuous ; for when the whole current has nearly stagnated, and a 

 fresh impulse from the heart renews it, the movement is not by any means 

 uniform (as it might have been expected to be) through the whole plexus 

 supplied by one arterial trunk, but is much greater in some of the tubes than 

 it is in others ; the variation being in no degree connected with their size, and 

 being very different at short intervals. 



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

 the supply of the white or colourless tissues ; which carry from the arteries 

 the liquor sanguinis, or fluid portion of the blood, leaving the globules behind. 

 Many objections might be raised against such a supposition ; one of the most 

 obvious of which is the mechanical obstacle that would be created at the 

 entrances to such a system of tubes, by the retention of the globules in the 

 larger vessels from which they diverged. No such vessels have ever been 

 observed ; and it may be safely affirmed, that the supposition of their existence 

 is not required. For any one who observes the smaller capillary vessels may 

 perceive, that the current of blood which passes through them is entirely free 

 from colour, as the 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. On the 

 other hand, the supposition that Nutrition can only be carried on by means of 

 Capillary vessels, is entirely gratuitous, as will be hereafter shown (CHAP. 

 xi.) ; and it would appear from the late researches of Mr. Toynbee,* that car- 

 tilages in general, the true cornea, crystalline lens, and vitreous humour, 

 together with the epidermic appendages, are entirely destitute of them. He 

 has demonstrated, by means of injections, that the arteries, which previous 

 anatomists had supposed to penetrate into their substance, either as serous 

 vessels, or as red vessels too minute for injection, actually terminate in veins 

 before reaching them ; he also shows that, around these non-vascular tissues, 

 there are numerous vascular convolutions, large dilatations, and intricate 

 plexuses of blood-vessels (Fig. 112) : the object of which he believes to be to 

 arrest the progress of the blood, so that its nutrient portion may penetrate into 

 and be diffused through them. There is, as will hereafter appear, no essential 

 difference between the nutrition of the non-vascular tissues, and that of the 

 islets in the midst of the network of capillary vessels which traverses the most 

 vascular (Fig. 89). In both cases the nutrient materials conveyed by the 

 blood are absorbed by the cells of the tissue immediately adjoining 'the vessels, 

 and are imparted by them to others which are further removed ; and the only 

 variation that exists, is in the amount of the portion of tissue,' which has to be 

 thus traversed. There is great variety in this respect, among the tissues that 

 are traversed by vessels ; and we are only required to extend our ideas from 

 the largest of the islets which we find in these, to the still more isolated struc- 

 tures, of which the non-vascular tissues are composed. In the Vegetable 

 Kingdom, as among the lowest Animals, there are entire organisms of con- 

 siderable size, throughout which nutriment is conveyed by mere transudation 

 from cell to cell ; and this seems to be the case, in "those parts of the highest 

 Animals in which the vital changes are least active. 



* Philosophical Transactions, 1841. * 



