Circulation of the Blood. 189 



In a paroxysm of asthma the lungs become obstructed with 

 mucous secretions, and the rapidity of oxidation is therefore 

 interfered with. Under such circumstances the passage of the 

 blood is retarded, as is shown by the great dilatation of the 

 jugular veins. 



Whatever therefore deranges the process of oxidation de- 

 ranges the flow of the blood. In violent expirations, such as in 

 coughing, the observations of Haller show that the blood 

 moves tardily in the lungs, and in delicate persons its retar- 

 dation is so complete that it regurgitates in the great veins. 



In a violent and continuous explosion of laughter, the 

 jugular veins become excessively distended ; the right cavities 

 of the heart having no power to push the venous blood through 

 the pulmonary capillaries, and owing to the expulsion of air 

 from the air-cells, the blood itself fails to effect the passage 

 with its usual speed. In this instance it must again accumu- 

 late in the veins. 



The various cases here cited depend on retarded oxidation. 

 I might now consider the reverse of this, or where oxidation 

 goes on too rapidly, as when protoxide of nitrogen is breathed. 

 Owing to the great solubility of this gas in serum, and its 

 power of supporting combustion, we should expect to find it 

 exert that control over the circulation which is well known to 

 be one of its peculiarities. This paper is however extended 

 to so great a length, that here I must stop, though I have 

 made no allusion to the movements in the lymphatics or lac- 

 teals, or to the flow of sap in trees, or to the circulatory move- 

 ments of the lower animals. These can all be explained upon 

 the same principle; thus the descent of the sap follows as a 

 necessary consequence of the decomposition of carbonic acid 

 in the leaf. Nor have I said anything of the obvious control 

 which certain classes of nerves have over the systemic oxida- 

 tion. There are many facts which prove that the nervous 

 system regulates this operation, and can either facilitate it or 

 keep it in check. In this there is nothing extraordinary. A 

 piece of amalgamated zinc exhibits no tendency to oxidize in 

 acidulated water, but by the touch of si'ver or platina it is 

 made to submit itself to the action of that medium. The act 

 of blushing, and all local inflammations, show that changes 

 in the relations of the nervous system control the oxidizing 

 action of arterial blood ; but to these things I propose to re- 

 turn on a future occasion. What is here stated is sufficient 

 to illustrate the general principle to which I wish to draw at- 

 tention, that the chemical changes which are impressed on these 

 circulating fluids are the true causes of their flow. 



