OF THE BLOOD AXD CIBCULATIOX. 



209 



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

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

 -tomoses 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- 

 1 with these by small 



<. In the larger ves- 



:he 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, 

 which has the character ap- 

 propriate to the venous cur- 

 rent. In all the vessels, even 

 in the very finest, a distinct 

 boundary, formed by a sim- 

 ple dark line, is perceptible ; 

 the surrounding paren- 



Fig. 226. A portion of the web of a 

 frog's foot, exhibiting the included 

 network of vessels, magnified45 times. 

 The angular unnucleated cells c c, 

 of the parenchyma, lying between 

 the different vessels, are beautifully 

 shown ; a is a deeper-lying venous 

 trunk, with which two smaller capil- 

 The 



lary veins, b b, communicate, 

 rhvma, now 'distinctly eel- superficial net-work of capillaries is 



luhr (fio- 9VW now rather seen admittin g but a sin g le series of 

 *&' **")' r blood-globules. All the vessels here 



figured are furnished with distinct 

 parietes. 



granular and fused, though 



still including individual 



ramified pigmentary cells 



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



as simple channels pierced through its substance and without 



distin, -t parietes. Larger vessels (figs. 227 and 228) are ob- 



nough furnished with darker parietes, composed of 



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



. tor no more than a single row of blood-corpuscles, and 



these can only pass by their long diameters through the 



