REPORT ON ANIMAL PHYSIOLOGY. 141 



removed from the body, responds to stimulation as long as the 

 composition of its tissue is such as to render it capable of doing 

 so. The stimulus may be the blood remaining in its vessels, if 

 its cavities be empty ; or may be the nervous influence which, 

 there is reason to suppose, remains in its nerves for a time after 

 they have been cut oft" from communication with the brain and 

 cord. Under these two conditions it is even possible that a low 

 degree of assimilation may yet go on, which however can never 

 completely restore the state which the fibre possessed at the 

 time when any individual contraction was performed. The ex- 

 citability at length ceases. But even without this supposition, 

 stimuli which do not at the same time afford food for the nutri- 

 tive process, can, if food be not otherwise supplied, merely 

 exhaust. The final cause of this perdurance of a certain degree 

 of irritability in the heart, even when nutritive supply and ner- 

 vous energy are suspended or imperfect, is obvious. 



Dr. Carson has pointed out the effect which the empty state 

 of the auricles produces upon the circulation in the veins, im- 

 parting to the heart the power of a sucking instrument ; and 

 also the effect of the resilient or elastic nature of the pulmonary 

 tissue in subjecting the heart to a less atmospheric pressure 

 than the rest of the body*. 



Poisseuille, by means of his barometrical instrument, has 

 confirmed Sir J. Barry's conclusions respecting the effect of in- 

 spiration on the venous circulation, as far as the large vessels 

 near the heart are concerned. When the instrument was in- 

 troduced into the vein of a Dog, towards the heart, the mercury 

 rose 60 millimetres above the zero point during expiration, and 

 fell 70 m. below it during inspiration ; the degree of rise and fall 

 varying with the struggles of the animal, but occurring syn- 

 chronously with the respiratory movements. He did not find 

 that inspiration at all influenced the veins of the exti-emities j 

 but he confirmed Majendie's observation, that expiration assists 

 not only the motion of the blood in the arteries, but that its 

 effect extends through the capillaries to the veins ; the blood 

 rose in them all during expiration, (the instrument being at- 

 tached to the peripheral portion in the case of the veins,) and 

 during the systole of the heart. 



From all these experiments we conclude, that the heart sup- 

 plies the power which effectually moves the blood in the higher 

 animals, not only through the arteries (to which Bichat con- 

 fined its effect), but also through the capillaries and the whole 

 venous system ; that it is assisted by the elastic power of the 

 arteries after they have been distended by the heart's action upon 

 • Inquiry into the Causes of Respiration, Sfc. 



