BLOOD FLOW AS SEEN WITH THE MICROSCOPE. 22? 



parent parts of various animals, as the web of a frog's foot, 

 a mouse's ear, or the tail of a small fish. In consequence 

 of the steadiness with which the capillaries supply the veins 

 the flow in these is also unaffected, directly, by each beat o* 

 the heart; if a vein be cut the blood wells out uniformly, 

 while a cut artery spurts out not only with much more 

 force, but in jets which are much more powerful at regu- 

 lar intervals corresponding with the systoles of the ven- 

 tricles. 



The Circulation of the Blood as Seen in the Frog's Web. 

 There is no more fascinating or instructive phenomenon 

 than the circulation of the blood as seen with the micro- 

 scope in the thin membrane between the toes of a frog's 

 hind limb. Upon focusing beneath the epidermis a net- 

 work of minute arteries, veins and capillaries, with the blood 

 flowing through them, comes into view (Fig. 82*). _ The 

 arteries, a, are readily recognized by the fact that the flow in 

 them is i'ascest and irom larger to smaller branches^ "Th*e 

 latter are seen ending- in capillaries, which form networks, 

 the channels of wTi in n n.re a 1 1 r ft*"'! y equal in size. While 

 in the veins arising from the capillary the flow is from 

 smaller_tq larger -toaaks. and slower than in the arteries 

 but faster than in the capillaries. 



The reason of the slower flow of the capillaries is that 

 their united area is considerably greater than thai, of the 

 arteries supplying them, so that the same quantity of blood 

 flowing Jjirongh them in a given time, has a wider channel 

 to flow in and moves more slowly. The^jirca of the veins 

 is smaller than^thtit of the capillaries but ffreater"~trian 

 that of thejirteries, and hence the rate of niovementTrTEKcm 

 ia^jal^Zmtermediate. Almost always when an artery 

 divides, the area of its branches is greater than that of the 

 main trunk, and so the arterial current becomes slower and 

 slower from the heart onwards. In the veins on the other 

 hand, the area of a trunk formed by the union of two or 

 more branches is less than that of the branches together, 

 and the flow becomes quicker and quicker towards the 

 heart. But even at the heart the united cross-sections 

 of the veins entering the auricles are greater than those of 



*P. 212. 



