STRUCTURE OF CAPILLARY BLOOD-VESSELS. 



189 



structure to the other. Similar passages exist to a great extent among the In- 

 vertebrata ; the venous circulation in particular being mainly carried on by 

 them. We have an example of them, even in Man, in the Sinuses through 

 which the venous blood is returned from the Brain ; these sinuses being simple 

 passages formed by the folds of the Dura Mater. Tn the higher Plants, however, 

 the circulation of fluid is for the most part carried on through Ducts, having 

 distinct membranous parietes ; and these ducts may be either straight and 

 simple tubes, as are those of the interior of the stem through which the sap 

 ascends ; or they may form a network by their mutual anastomosis, such as 

 that by which the sap descends through the bark and newer wood. Both of 

 these forms of ducts appear to be formed by the coalescence of cells ; the 

 straight cylindrical ducts being formed from cells, arranged in a simple linear 

 manner; and the network of vessels for the descent of the elaborated sap, 

 being produced by the junction, at various points, of cells of less regular form, 

 which stretch out to meet each other. 



219. In all the higher Animals, in their adult condition at least, the 

 Capillary circulation is entirely carried on through tubes having distinct mem- 

 branous parietes. These tubes commonly form a minutely-anastomosing net- 

 work ; into which the blood is brought by the ramifications of the arteries on 

 one side, and from which it is returned by the radicles of the veins on the other. 

 The walls of the tubes are composed of a delicate membrane, in which an 

 appearance of transverse striation (as if produced by minute annular fibres) 

 can sometimes be discerned. The diameter of the Capillaries varies in dif- 

 ferent animals, in accordance with that of their blood-corpuscles; thus the 



Fig. 90. 



Capillary circulation in a portion of the web of a Frog's foot, magnified 110 diameters ; 1, trunk of 



vein ; 2, 2, its branches j 3, 3, pigment cells. 



Capillaries of the Frog are of course much larger than those of Man. The 

 diameter of the latter appears, from the measurements of Weber, Muller, and 

 others, to vary from about the 1 -3700th to the l-2500th of an inch , but as 

 they can only be examined after death, it is probable that these statements are 



