126 PHYSIOLOGY FOR BEGINNERS CHAP. 



respiration is called dyspnoea. The demand for oxygen of 

 the brain, especially of the spinal bulb, leads to a fuller flow of 

 blood through the brain. This is brought about by the venous 

 blood acting on the vasomotor centre, which is situated, as we 

 have seen, in the spinal bulb, causing it to send out more 

 powerful constrictor impulses to the small arteries of other 

 parts of the body, especially of the abdominal organs, cut- 

 ting off much of the blood from those parts, and causing an 

 increase of pressure in the aorta, so that more blood is driven 

 to the brain. As the blood becomes more and more venous, 

 the dyspnoua increases in violence till almost all the muscles in 

 the body are thrown into contraction ; that is, convulsions 

 occur in the struggle to get fresh air into the lungs. If the 

 obstruction is not removed these soon suddenly cease, 

 exhaustion occurs, the beats of the heart become weaker, the 

 blood pressure falls, and the circulation of the blood becomes 

 feeble. At this stage all or nearly all the oxygen in the 

 blood has been used up, and the blood will be so dark 

 as to justify the expression that the man is " black in the 

 face." The respiratory centre deprived of oxygen then 

 rapidly fails, and after a few feeble respiratory efforts, ceases 

 altogether to send out impulses, and so the breathing stops. 

 When the respiration has ceased, the heart, after a few flicker- 

 ing beats, stops also. Such a death is called a death by 

 asphyxia. 



The Loss from the Body by the Lungs. An adult 

 man breathes in and out 1 7 x 30 = 5 1 o cubic inches of air in one 

 minute, so that he will charge about 500 cubic inches of air to 

 the extent of 4 per cent of CO., in one minute. Or, in other 

 words, he breathes out 20 cubic inches of CO 2 in one minute, 

 or 1 200 cubic inches in one hour. By actual experiment it is 

 found that the amount of CO. 3 a man, when at rest, expires in 

 one hour, may be taken as about 1000 cubic inches. When 

 doing hard work he would expire 2000 to 3000 cubic inches. 

 Of the 24,000 cubic inches of CO., which a man, at rest, 

 gives off in twenty-four hours, the oxygen is supplied by the 

 air inspired, but the carbon is derived from the tissues of 

 the body. This amount of CO., contains about eight ounces of 

 carbon. The water, expired as aqueous vapour, amounts to 

 about half a pint in twenty-four hours. 



