1 9 o THE POPULAR SCIENCE MONTHLY. 



dilatation, like its contraction, is not so instantaneous as it was when 

 first observed under the miscroscope. In a short time longer the sen- 

 sibility of the heart is greatly diminished, for the blood is seen to be 

 accumulating in both the auricle and ventricle, but especially in the 

 former, from both of which cavities it is now only partly expelled by 

 the contraction of the heart. 



At length, just before death, the blood is seen to flow from tue 

 auricle into the ventricle, thence into the branchial artery and along 

 it, the heart being passive during this time, and only now and then 

 at long intervals manifesting a very slight and partial contraction. 

 During the whole of this time the blood is coagulating more and more 

 in the auricle and ventricle, but especially in the former ; and, when 

 at length the heart has ceased to beat, the auricle and the vena cava 

 opening into it are fully distended, while the ventricle is only partly 

 distended with black-red blood. In the last moments of life, after the 

 heart has ceased to beat, the branchial artery is seen to be pushing 

 forward its slender current of blood, and to become at length quite 

 empty and transparent. 



Here, then, as the oxygen dissolved in the water in which the fish 

 is confined becomes exhausted, and as the carbonic acid increases, the 

 sensibility and contractility of the heart are diminished, and at length 

 entirely destroyed. The negation of oxygen, and the increase of car- 

 bonic acid, have culminated in the death of the fish. 



Precisely the same effect is produced upon the human heart by an 

 accumulation of carbonic acid in the air respired. 



In the ordinary condition of the atmosphere, in which carbonic 

 acid does not exceed one part in a thousand parts of that medium, its 

 effects upon the heart are inappreciable. When, however, the car- 

 bonic acid has accumulated to the extent of 1 per cent, of the ah 

 respired, it begins to produce a slight feeling of faintness, and some 

 degree of uneasiness across the brow. At 2 per cent, the heart's ac- 

 tion is quickened, the sense of faintness is greater, there is slight gid- 

 diness, with heaviness and constriction of the head, together with 

 nausea. At 3 per cent, all these symptoms are increased. There are 

 vertigo, fluttering of the heart, nausea and sickness, followed by an 

 overwhelming sense of muscular prostration. At this moment the 

 contractions of the heart become very feeble, the skin relaxes, and is 

 bedewed with a cool, clammy perspiration. These symptoms deepen 

 with the increasing quantity of carbonic acid in the air respired until 

 the utmost limit of toleration is reached, beyend which life can no 

 longer be maintained. At this stage lethargy supervenes ; and, at the 

 moment of its occurrence, the heart begins to beat less frequently 

 and much less powerfully than before. This condition is the parallel 

 of that observed in the young trout, when the blood begins to accu- 

 mulate and to coagulate in the auricle and ventricle, and when the 

 heart's sensibility and contractility are reduced in the greatest degree. 



