604 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



. The duration of asphyxia varies considerably in different animals 

 and in the same animal under different circumstances. As a rule, young 

 animals will bear a longer deprivation of fresh air than adults. It has 

 thus been stated that while a full-grown dog will rarely recover from an 

 immersion in water over one and one-half minutes, a puppy has been 

 known to recover after an immersion of fifty minutes. This is evidently 

 to be explained by the fact that in younger animals the processes of 

 tissue change are less active than in adults, and, as a consequence, the 

 demand on the oxygen in the blood is less. 



During asphyxia the circulation is the seat of various departures 

 from normal. 



In the stage of dyspnoea and convulsions the blood pressure is 

 reduced, while in the final stage it rapidly falls far below normal, until 

 at the period of death the blood pressure does not exceed the atmos- 

 pheric pressure. 



The heart also is modified in its activity. At first it beats more 

 rapidly than normal and then is slowed, but each pulsation is greatly, 

 increased in force. As a rule, the heart continues to beat for some 

 time after the final cessation of respiration. During the first and 

 second stages of asphyxia, the great increase in blood pressure from the 

 contraction of the arterioles leads to great increase in the resistance 

 met with by the heart in emptying itself. We find, therefore, if the 

 chest of an animal is opened during asphyxia, that the right side of the 

 heart especially is gorged with blood. The lungs and the pulmonary ves- 

 sels are overfilled and distended, and, as a consequence, blood collects 

 in the venous system, so accounting for the cyanosed appearance seen 

 on all external surfaces, and so characteristic of asphyxia. 



Modified Respiratory Movements. — In addition to the normal respira- 

 tory rhythm, various modifications of the respiratory movement are often 

 noticed. Many are involuntary, and yet nearly all of them ma}' be repro- 

 duced by a direct effort of the will. 



Sighing is produced by a long, deep inspiration, air being inhaled 

 through the nose and the inspiration followed by a number of shorter 

 expirations. 



Yawning is accomplished by a deep inspiration through the mouth, 

 and is accompanied by a depression of the lower jaw and elevation of 

 the shoulders. 



Hiccough is produced by a sudden contraction of the diaphragm 

 and a sudden closure of the glottis, so arresting the entrance of air into 

 the lungs, and, by the striking of the column of air against the closed 

 vocal cords, producing the sound characteristic of this phenomenon, 

 Hiccough is, as a rule, due to the stimulation of the gastric branches of 

 the pneumogastric, especially in some disturbance of digestion. 



