CIRCULATION IN THE FROG'S FOOT. 63 



Tongue of frog-. 1 . Evert the tongue of the pithed animal out of 

 its mouth and spread it fanwise, avoiding over extension, fixing in 

 position to the cork with short pins. Cover. 



Fig. 14. Frog support for studying the circulation in A the tongue, B the mesentery, 

 and C the web of the foot. 



The foregoing offer different aspects of the subject for study. The 

 web being covered by skin, the pigment and guanin cells, sometimes 

 very numerous, may interfere considerably with a clear view of the 

 vascular structures beneath. Capillaries are numerous. The mesentery 

 being very thin shows the larger vessels well, but there are few capillaries. 

 The tongue exhibits large tortuous (Lingual) arteries in, which the expan- 

 sion at the ventricular systole is particularly well seen. 



Circulation in the web. Search (L) for an artery, the blood 

 stream runs in it from the trunk into the branches; find a vein, 

 in it the streams converge into the trunk. Note the relative 

 difference in the rate of flow and in the diameter of the two 

 vessels. Trace the blood stream from the artery into the vein 

 through the capillaries. (//) Study the flow in the latter. In 

 these the blood cells are individually visible, the red ones bending 

 to the curves around which they travel ; occasionally a red 

 cell is caught on the edge of bifurcation of a vessel it then 

 becomes flexed and responds to each systole, recovering its 



1 In this organ living muscle and nerves can be observed. When thus 

 extended striped muscular fibres may be readily fixed and isolated as follows: 

 Drop absolute alcohol upon the mucosa until bleached, scrape through it in the 

 direction of the bands of muscle until the latter are exposed, and let the alcohol 

 act upon them until they lose their elasticity. Remove portions and immerse them 

 in absolute alcohol for twenty minutes. Separate by teasing and after bathing 

 with water stain them with hrematoxylin and mount in balsam. 



Small nerve bundles accompany the blood vessels, the outlines of the fibres are 

 sharply defined, and the double contour of the medullary sheath and Ranvier's 

 nodes can be recognised. In the thin edges of the preparation search for single 

 fibres of striped muscle, and observe the occasional waves of contraction. 



