670 



CIRCULATION. 



motions of the red globules principally, for it 

 is very rarely indeed that the current of fluid 

 which carries the globules along can be recog- 

 nized in the ordinary modes of observation. 



The capillary circulation is most easily seen 

 in cold-blooded and in young animals, both 

 on account of the large size of the red glo- 

 bules and the small number of the vessels. 

 Since the first discovery of the capillary circu- 

 lation by Malpighi, the transparent web be- 

 tween the toes of the hind feet of the frog has 

 been universally adopted as the most con- 

 venient situation for observing this beautiful 

 spectacle with transmitted light. The fins and 

 tail of fishes, the tail of the larva of the Frog 

 and Newt, the external gills of the same ani- 

 mals as well as of cartilaginous fishes, the 

 mesentery of the Frog or of small warm- 

 blooded animals, the wing of the Bat, the 

 lungs and urinary bladder of Reptiles, the 

 liver of the Frog and Newt, the membranes 

 of the incubated egg, the yolk of the Skate's 

 egg, are all situations favourable for the ob- 

 servation of the capillary circulation. The 

 capillary circulation has been viewed in only 

 a small number of warm-blooded animals, and 

 in very few of their textures ; but the minute 

 injection with coloured fluids of all parts of 

 the bodies of Quadrupeds and of Man leaves 

 little doubt that in them also, whatever vari- 

 eties there may be in the size, number, and 

 distribution of the small vessels, the blood 

 passes in every organ from the small arteries 

 into the returning veins by minute continuous 

 tubes of the same nature as those more easily 

 observed in the situations above-mentioned. 



Some are inclined to consider the minutest 

 or proper capillary vessels as destitute of vas- 

 cular parietes, and consisting of mere passages 

 through the texture of the organ in which they 

 exist without any lining membrane. This 

 opinion is founded on the impossibility of 

 seeing the coats of the vessels, the rapidity 

 with which new capillaries may be developed, 

 and some other circumstances. The extreme 

 degree of minuteness of the smallest capil- 

 lary vessels must render futile any attempts 

 to decide this question by direct observa- 

 tion. Besides the general analogy between 

 the larger and smaller vessels, there are 

 several circumstances known which seem to be 

 strongly in favour of the view that the capil- 

 laries do not differ in this respect from other 

 vessels. 1st, It is allowable to suppose that 

 the active properties of the capillary vessels 

 belong to parieties as in the larger vessels. 

 2d, In many transparent parts of animals in 

 which the terminal arteries and veins do not 

 diminish to a very small size, the coats of the 

 vessel may be seen with the microscope, as in 

 the external gills of the Amphibia, and in the 

 vascular rete of the ear of birds and reptiles, 

 in which the capillary vessels may, after 

 having been injected, be separated from the 

 neighbouring soft texture. 3d, The conver- 

 sion of small into larger vessels with visible 

 coats in those instances in which the course 

 of blood through the vessels of a part has un- 

 dergone an alteration, is in favour of the pre- 



vious existence of parietes in the smaller 

 vessels. And 4th, The constant and regular 

 distribution of the minutest vessels in many 

 parts of animals appears to support the same 

 view. The argument in favour of the non- 

 existence of capillary parietes deduced from 

 the alleged facility with which the blood occa- 

 sionally passes out of the regular vessels and 

 takes an irregular and indeterminate course 

 through the non-vascular parenchyma of an 

 organ, we believe to be founded, in some in- 

 stances, in peculiarities belonging to a few parts 

 only, and in others in inaccurate observation; 

 for in almost all those situations in which the 

 capillary circulation may be seen with ease 

 and distinctness, the constancy of the minute 

 passages which the blood permeates is un- 

 doubted. 



From the more accurate means of making 

 minute anatomical researches that have been 

 introduced in modern times, the existence of 

 serous, exhalent, and white vessels has become 

 a matter of great doubt, for vessels of this 

 description which do not admit the red glo- 

 bules and liquor sanguinis together cannot be 

 made obvious to the senses by the most de- 

 licate injections or dissections ; and the ob- 

 servation of the capillary circulation in the 

 transparent parts of animals affords the most 

 convincing proof that the smaller arteries 

 have no visible terminations excepting in the 

 capillaries and small veins. In observing 

 attentively the web of the frog's foot and other 



Fig. 331. 



Capillaries in the web of the Frog's foot magnified. 



transparent parts in which the motion of the 

 blood is easily seen, we occasionally see glo- 

 bules of blood run into passages of the tissue 

 which we did not perceive before ; but a suf- 

 ficient acquaintance with the structure and dis- 

 tribution of the smallest of the capillaries in 

 these situations will soon convince the careful 

 observer that the vessels into which the blood 

 was seen to pass, apparently for the first time, 

 existed fully formed before, that the fluid part 

 of the blood passed in part through them, and 

 that the stoppage of the red particles was to 



