152 REPORT—1840. 
been ascertained, a bladder of similar size, containing atmospheric 
air, was substituted for the nitrogen, and the resulis compared. The 
difference between the effects of the respiration of the nitrogen gas and 
the atmospheric air was most narked; for while the mercury continued 
to fall in the instrument during the respiration of the nitrogen gas, it 
rose very rapidly soon-after the atmospheric air had entered the lungs. 
In this experiment, the mechanical movements of the chest, which 
failed to renew the free circulation through the lungs when nitrogen 
was inspired, rapidly effected that object when atmospheric air was 
permitted to enter the lungs even of the same animal, tried subse- 
quently to the failure of the nitrogen, and consequently at a more ad- 
vanced period of the process of asphyxia. This experiment was re- 
peated several times, and when care was taken to procure pure nitrogen, 
invariably with the same result. In performing these experiments, a 
very unexpected phenomenon presented itself. The mercury actually 
stood higher in the instrument, and the arteries were more distended 
and tense for about two minutes after the animal had ceased to struggle, 
or, in other words, had become insensible, and when the respiratory 
process was nearly brought to a stand, than when it was breathing freely 
through the tube introduced into the trachea. It was suspected that 
this arose from an impediment to the passage of the venous blood 
through the capillary arteries of the tissues generally, by which the 
force of the left side of the heart was principally concentrated in the 
arterial system; and on placing a hemodynamometer in the vein of the 
opposite limb, and comparing its indications with the other fixed in the 
artery, this supposition appeared verified. In these experiments it was 
repeatedly observed, that however rapidly the respiratory movements 
were performed, provided they were not more forcibly exerted than in 
natural respiration in the quiescent state, this had no obvious effect 
upon the force with which the blood was sent along the arteries. 
Though there can be no doubt that violent movement, either of the 
muscles of the trunk or limbs, exerts a very marked influence upon the 
force with which the blood is sent along the arteries (in one experi- 
ment the mercury stood nine inches higher in the tube during a violent 
struggle and a forcible expiration, than during the opposite circum- 
stances of deep inspiration and intermission of the struggle), yet it is 
obvious, from these experiments, that the impediment to the passage of 
the blood through the lungs does not depend upon the arrestment of 
mechanical movements of the chest, but upon cessation of the chemical 
changes between the blood and the atmospheric air in the lungs. 
The explanations which have been given of the cause of the arrest- 
ment of the sensorial functions, are two in number, viz. Ist, the circu- 
lation of venous blood in the arteries of the brain; 2nd, that it princi- 
pally depends upon the blood being sent to the brain in diminished 
quantity, in consequence of the impediment to its passage through the 
lungs. It has already been stated, that the arterial pressure is not di- 
minished at the time that the animal becomes insensible, and the same 
thing was repeatedly ascertained in experiments on the veins. The in- 
sensibility cannot, therefore, depend upon any diminution in the vascu- 
