VENTILATION 495 



as into water or mercury, which is drowning, or by the simple withdrawal 

 of oxygen from the air; or it may be induced more slowly by closing the 

 air-tubes with the products of secretion, as in diphtheria or croup and 

 laryngitis, or by paralysing the muscles of respiration, as by the action 

 of curara, or by opening both pleural sacs simultaneously, thus preventing 

 the lungs from following the expansion of the chest walls, or, lastly, by 

 loss of blood. It is usually divided into three stages, the first of which, 

 in rapidly-induced asphyxia, lasts about a minute, and is characterized by 

 violent inspiratory eftbrts. Then the abdominal muscles contract power- 

 fully, and strong expiratory eftbrts occur, which are succeeded by irregular 

 spasms of the limbs, chiefly aftecting the flexor muscles. The blood 

 pressure rises to a great height, owing to the stimulation of the smaller 

 vessels by the non-arterialized blood. In the second stage, which also 

 lasts about a minute, the convulsive movements cease, the expiratory 

 movements are hardly perceptible, the pupil dilates, and touching the 

 eyeball excites no reflex movement of the lids. The blood - pressure 

 gradually falls, and the muscles generally are relaxed. In the third stage, 

 which lasts two or three minutes, the inspiratory eff'orts become more 

 feeble, and only take place at long intervals spasmodically, the extensor 

 muscles are convulsed, the head is thrown back, and death closes the 

 scene. 



VENTILATION 



It may be taken for granted that air which has been once breathed 

 is unfit for further respiration. Such air, we have seen, contains about 

 4^ per cent of carbon dioxide, and experiment has shown that a far smaller 

 proportion is poisonous, and it is generally admitted that a disagreeable 

 or stufiy sensation is perceived by man when the air in a room contains 

 al)0ut 1 per cent of carbon dioxide in 1000 of pure air. It is probable, 

 however, that the feeling of closeness is not altogether due to the carbon- 

 dioxide, but is in part owing to the exhalation from the skin and to the 

 volatile products resulting from chemical changes in the body that are 

 discharged from the lungs. M. Boussingault has estimated that a horse 

 of average size eliminates 4800 litres, or between 8000 and 9000 pints, 

 of carbon dioxide per diem, from which it is clear that a very large 

 quantity of pure air must be supplied to prevent the proportion of the 

 CO., from rising above 1 per cent. The object of good ventilation is to 

 eft'ect this renewal of the air without creating a draught, or, in cold periods 

 of the year, c&using a sudden inrush of air at a low temperature. In the 

 case of the human subject it is held that to preserve the air in a fit state 

 of purity, with ordinary or natural ventilation, at least 2000 cubic feet 



