ON POISONS. 27 



experiments of MM. Allen and Pepys, Lavoisier and Seguin, Davy and Ber- 

 zelius, concerning the exact quantity of this gas evolved during respiration, still 

 occupy the attention of the scientific world ; but its effects upon the system 

 seem to me to be yet greatly overlooked and disregarded. The injurious 

 consequences which it produces to those who, by accident or design, may be 

 exposed to its effects, have led to less useful results, practically, than might 

 have been anticipated from the nature of the symptoms, and the interesting 

 phenomena which result from its action. Year after year numbers flock to 

 witness the experiment of submitting a wretched animal to the deadly atmo- 

 sphere of the " Grotto del Cane," without drawing those deductions, or de- 

 riving that advantage from its sufferings, which alone can palliate or justify 

 their infliction. 



Carbonic acid has many sources out of the body, and it is abundantly fur- 

 nished by respiration. The lungs, however, are by no means its only outlet 

 from the animal body. It is given off' by the skin, it is secreted as well by 

 the serous as mucous membranes*, points of much interest, not only as afford- 

 ing an example of vicarious action, but as explanatory of various bodily dis- 

 orders. It cannot here be necessary to controvert old errors respecting the 

 source of the carbonic acid yielded by respiration, nor to dwell upon the 

 more probable views of modern times. The opinions and experiments of 

 Mr. Edwards, which prove that this acid is extricated from the lungs, although 

 no oxygen is respiredf , the observations of Professor Magnus, and his con- 

 clusions that all blood contains carbonic acid, the belief of Miiller that the 

 quantity held in solution in itij^ is sufficiently large to account for the whole 

 exhaled by the lungs, are facts well known to all whom it is my pleasure to 

 address. Thus we find that a most important series of changes takes place 

 during the circulation of the blood leading to the formation of carbonic acid, 

 which is set free from various surfaces, chiefly, however, from the parietes of 

 the air-cells, which allow it to pass through them in order to be exhaled§. 

 Next we find that many circumstances greatly influence the amount of this 

 gas yielded by respiration ; these may be ranked in two classes, one of which 

 may be considered as natural or regular, the other as accidental or abnor- 

 mal. With regard to the first of these, to the regular performance of the 

 functions of the system, we find that more carbonic acid is eliminated during 

 the day than by night ; that it increases at day-break, and diminishes at sun- 

 set II ; that it is produced in larger quantity by exercise and during digestion ; 

 and, what is extremely interesting, we see a tendency to equilibrium in the 

 whole amount ; for if given off in excess at one time, at another it wiU, as a 

 consequence, be lessened. With regard to the second class of circumstances 

 which influence the secretion, we find that it is diminished by depressing 

 passions, debilitating causes, low diet, and injuries to the par vagumf . 



If we now look to the actual effects of carbonic acid, when placed in con- 

 tact with the living body, many interesting consequences result which serve 

 to indicate its use in the human economy, and its agency in disease. 



It imparts an acid taste, produces a sense of burning in the uvula**, and acts 

 instantaneously as a powerful irritant to the muscles of the larynx, occasion- 

 ing by their spasmodic action the complete and firm closure of the glottisff. 

 Applied externally to the skin, or taken into the stomach, it occasions giddi- 

 ness, pain and weight in the head, obscurity of sight, and ringing in the earstJ. 



* Mayo, Phys., pp. 120, 131. Mayo, Path., 336. Mullcr, Phys., 656. Robert Lee, Cy- 

 clopscd. Pract. Med. vol. iv. p. 383. 



t Mavo, Phys., p. 63. t MUller, p. 328. § Miiller, p. 330. 



II Miiller, ibid. % Brodie, Phil. Trans., c. ii. p. 390. 



** Davy on Nit. Ox., p. 472. Christison on Poisons, 3rd ed., 745. ft Ibid. 



Jt Collaid de Martigni, Archives, 211. Christisou, 2ad ed., pp. 703-706. 



