RESPIRATION. 553 



During the second stage the excitation of the expiratory portion of the respiratory 

 centre is more intense than that of the inspiratory portion, so that the respira- 

 tions become slow and deep, prolonged and convulsive, and the movements of 

 inspiration are feeble and in striking contrast to the violent spasmodic expira- 

 tory efforts. During the third stage the dyspnoea is followed by general 

 exhaustion ; the respirations are shallow and occur at longer and longer inter- 

 vals, the pupils become dilated, the motor reflexes disappear, consciousness is 

 lost, the inspiratory muscles contract spasmodically with each inspiratory act, 

 convulsive twitches are observed in the muscles of the extremities and else- 

 where, gasping and snapping respiratory movements may be present, the legs 

 are rigidly outstretched and the head and body are arched backward, feces and 

 urine are usually voided, respiratory movements cease, and finally the heart 

 stops beating. During these stages the circulation has undergone considerable 

 disturbances. During the first and second stages the blood has been robbed 

 of nearly all its O, the gums, lips, and skin become cyanosed, and, owing to the 

 venous condition of the blood, the cardio-inhibitory centre has been decidedly 

 excited, so that the heart's contractions are rendered less frequent ; the vaso- 

 constrictor centre for the same reason has also been excited, causing a con- 

 striction of the capillaries and an increase of blood-pressure. During the 

 third stage these centres are depressed and finally are paralyzed. 



If asphyxia be caused by ligating the trachea, the whole series of events 

 covers a period of four to five minutes, the first stage lasting for about one 

 minute, the second a little longer, and the third from two to three minutes. 

 If asphyxia be produced gradually, as by placing an animal within a relatively 

 large confined air-space, death may occur without the appearance of any motor 

 disturbances (p. 544). 



The heart usually continues beating feebly for several minutes after the 

 cessation of respiration, so that by means of artificial respiration it is possible 

 to restore the respiratory movements and other suspended functions. After 

 death the blood is very dark, almost black. The arteries are almost if not 

 entirely empty, while the veins and lungs are engorged. 



Death from drowning occurs generally from the failure of respiration, 

 occasionally from a cessation of the heart's contractions. It is more difficult 

 to revive an animal asphyxiated in this way than one which, out of water, has 

 simply been deprived of air for the same length of time. Dogs submerged 

 for one and a half minutes can rarely be revived, but recovery can usually be 

 accomplished after deprivation of air, out of water, for a period four to 

 five times longer. After a person has been submerged for five minutes it is 

 extremely difficult to effect resuscitation. 



H. ARTIFICIAL RESPIRATION. 



Effective methods for maintaining ventilation of the lungs are important 

 alike to the experimenter and to the clinician. In the laboratory the usual 

 method is to expose the trachea, insert a cannula (Fig. 139), and then period- 

 ically force air into the lungs by means of a pair of bellows or a pump. Some 



