OF THE BLOOD AtfU CIECULATION. 



209 



low magnifying power, the directions of the arterial and ve- 

 nous currents are readily discovered (fig. 225, a a, b b). The 

 anastomoses of both orders of vessels are seen distinctly. 

 Under a higher power (figs. 226 and 227) a net-work of very 

 fine vessels is perceived lying 

 now over, now under the 

 larger branches, and con- 

 nected with these by small 





twigs. 



In the larger ves- 



sels the arterial and venous 

 currents are distinguished, 

 not merely by their opposite 

 directions, but also by the 

 kind of motion appropriate 

 to each : that of the arteries 

 is distinctly jerking or pul- 

 satory, but it gets ever less 

 and less, so as the minuter 

 subdivisions are attained, 

 and in the intermediate and 

 finest vessels of all it be- 

 comes a continuous stream, Fig. 226. A portion of the web of a 

 which has the character ap- frog's foot, exhibiting the included 

 propriate to the venous cur- network of vessels, magnified45 times, 

 rent. In all the vessels, even The angular unnucleated cells c c, 



in the very finest, a distinct f ^ P*^ ^8 Between 



, J ' . the different vessels, are beautifully 



boundary, formed by a sim- shown; a is a deeper-lying venous 



pie dark line, is perceptible ; trunk, with which two smaller capil- 

 the surrounding paren- fciry veins, b b, communicate. The 

 chyma, now distinctly eel- superficial net-work of capillaries is 



lular (fig. 226), now rather ^Y^^ ^Af^ 8 f e i f 

 ; & , '' blood-globules. All the vessels here 



granular and fused, though figure d are furnished with distinct 

 still including individual parietes. 

 ramified pigmentary cells 



within it (fig. 227), is sharply limited ; the vessels never appear 

 as simple channels pierced through its substance and without 

 distinct parietes. Larger vessels (figs. 227 and 228) are ob- 

 viously enough furnished with darker parietes, composed of 

 various layers of fibres. In the most minute vessels there is 

 room for no more than a single row of blood-corpuscles, and 

 even these can only pass by their long diameters through the 



