246 CIRCULATION OF THE BLOOD. 



should now be pithed in the same manner as the first. A 

 canula is then introduced into the abdominal vein, with its 

 orifice towards the heart, and connected, by an India-rubber 

 tube guarded by a clip, with a funnel containing three-fourths 

 per cent, solution of chloride of sodium. The heart having 

 been exposed, and its empty condition noted, the clip is 

 opened. Its cavities at once distend, and it acts as vigorously 

 and effectually as before the destruction of the nervous cen- 

 tres. The experiment may be varied thus: Two frogs are 

 suspended side by side, one of which has been pithed in the 

 manner above described. In both, the heart is exposed and 

 the ventricle cut across. In the pithed frog, a small quantity 

 of blood escapes, the quantity contained in the heart itself and 

 the commencement of the arterial system. In the other, blood 

 continues to flow for some minutes, in consequence of the con- 

 tinned contraction of the arterial system. To what extent the 

 veins may participate in it is uncertain. 



These simple experiments show, first, that in the frog the 

 arteries, unaided by the heart, continue the circulation for a 

 certain time after equilibrium of pressure has been established, 

 by virtue of their contractility ; and secondly, that in this ani- 

 mal the influence of arterial contractility in aid of the circula- 

 tion is so considerable that, when it is abolished, circulation is 

 no longer possible. 



It ma}' be well to point out that this fact affords no ground 

 for supposing that the arteries take any active part in main- 

 taining the circulation. All that is proved is, that in the re- 

 laxed state the vascular system of the frog is relatively so 

 capacious that it is more than large enough to contain the 

 whole mass of the blood, which consequently comes to rest in 

 it out of reach of the influence of the heart. During life, the 

 arterial tonus is usually constant ; so long as, and in so far as 

 this is the case, the function of the arteries is a passive one, 

 the motion they give to the blood-stream during diastole being 

 a mere restitution of that received by them from the heart 

 during systole. On the other hand, whenever they contract, 

 they originate motion of themselves ; but in this case the dura- 

 tion of the effect is limited by that of the contraction, and can 

 never be continuous. 



48. (2.) Direct Excitation of the Spinal Cord in the 

 Frog The requirements are as follows : a. A thin board of 

 soft wood about 8 inches long and 2 inches broad, one end of 

 which lias a V-shaped notch cut out of it, corresponding in form 

 and size to one of the interdigital membranes of the web of the 

 frog's foot. b. A pair of common strong sewing-needles ; 

 around the blunt end of each of these needles, the end of a 

 length of thin copper wire is closely coiled ; they are then cov- 

 ered nearly to their points with a protective and insulating 



