Or THE BLOOD AKD CIBCITLA.TION. 



207 



all reduced to a single fundamental type, a type which is most 

 readily observed in the vascular distribution of the intestinal 

 villi (fig. 224) : the terminal 

 twig of an artery (b, b) bends 

 round into the terminal twig of 

 a vein (a, a), and the two are 

 repeatedly connected by means 

 of delicate loop-like twigs, these 

 in their turn being formed into 

 meshes by cross or intermedi- 

 ate branches. The fundamental 

 type of the peripheral vascular 

 system is therefore an arterial 

 and a venous branchlet pro- 

 per capillary vessels, and an in- 

 terposed net-work of fine vas- 

 cular canals vasa intermedia. 

 A distinct separation between 

 capillaries, and intermediate 

 vessels, as this is perceived in 

 the intestinal villi more especi- 

 ally, is not generally to be ob- 

 served, the two blend or are 

 lost insensibly in one another. 

 The parenchyma, or organic 

 substance lying between the 

 finest vascular subdivisions, Fig. 224. Vessels of one of the 

 forms islets of very various size intestinal villi of the hare; after 

 j ,, 7 1 . ,1 an extremely beautiful dry prepa- 



and figure, according as the ratio n by Doellinger. The villul is 

 meshes of the mtercurrerit ves- magnified about 45 times. The vein 

 sels are open or closer, and as a, a, is injected with white ; the 

 they are rounded or angular, artery, b b, with red; between the 



The intimate structure of every f w a most beautiful rete of c ^' 



,, IP- JP lanes is apparent. 



organ, the mode or union and or 



the grouping of its elementary parts, and the diameter of the 



blotting-paper, nearly to the end of the tail, and so laid upon a plate of 

 glass of sufficient size, and placed under the microscope, the wrapper of 

 bibulous paper being kept constantly moist by a few drops of water 'let fall 

 on it from time to time. In this way the circulation may be watched for 

 hours, and the tadpole set free at the end of the observation is nothing the 

 worse. Young and still transparent fishes may also be treated in the same 



