ASPHYXIA. 307 



that they are active during normal respiration, for which the integrity 

 of the medullary centre and vagi is all that is necessary. 



Asphyxia. This condition is brought about by all circumstances 

 which hinder the normal respiratory changes of the blood passing to the 

 medullary centres. It may therefore be caused by any agency which 

 prevents the respiratory movements as well as by obstructing the 

 trachea, or by opening both pleural cavities, so that the respiratory 

 movements no longer dilate the lungs. Asphyxia may be brought about 

 by cutting off the blood supply to the medulla oblongata, either by 

 ligature of all the vessels going to the brain, as in the Kussmaul-Tenner 

 experiments, 1 or by producing universal anaemia by a large haemorrhage. 

 So far as the respiratory movements are concerned, the phenomena of 

 asphyxia may be divided into three stages. 



In the first stage, the respiratory movements are increased in 

 rhythm and amplitude. This stage is sometimes known as hyperpnwa. 

 If any tonic contraction of the diaphragm had been previously present, 

 this disappears at the onset of dyspnoea, the relaxation of the diaphragm 

 at the end of each inspiration being complete. 



In the second stage (dyspnoea) the expiration becomes active, and 

 gradually the expiratory movements become increased out of all propor- 

 tion to the inspiratory, so that finally respiration is made up of a series of 

 prolonged expiratory convulsions, separated by short inspirations. In 

 these forcible expirations nearly every muscle of the body may be involved. 



At the end of the second minute after ligature of the trachea (if 

 this mode of producing asphyxia has been adopted), the expiratory con- 

 vulsions cease almost suddenly, and give way to slow deep inspirations. 

 With each inspiratory spasm, the animal stretches itself out, and opens 

 its mouth widely, as if gasping for breath. The whole stage is one of ex- 

 haustion ; the pupils are widely dilated, and the animal is perfectly insen- 

 sitive, being unaffected by the strongest sensory stimulation. The pauses 

 between each inspiration become longer and longer and each inspiration 

 shallower, till, at the end of four or five minutes, " with extended limbs 

 and straightened trunk, with the head thrown back, the mouth widely 

 open, the face drawn and the nostrils dilated, the last breath is taken." 2 



The effects of deprivation of oxygen upon the circulation are 

 different in the different stages of asphyxia, and naturally vary accord- 

 ing to the mode by which asphyxia is produced. If by occlusion of 

 the trachea, the variations in intrathoracic pressure which are brought 

 about by the dyspnoeic movements of the chest are enormous, and 

 produce corresponding effects upon the arterial and venous pressures. 

 In addition to this, and whatever be the method by which asphyxia is 

 caused, the venous blood circulating in the medulla oblongata stimulates 

 both the vasomotor and the cardio -inhibitory centres, so that the 

 arterioles are contracted and the heart-beat slowed. The effect of the 

 vaso-constriction is, however, greater than that of cardio-inhibition, and 

 the blood pressure markedly rises. There are also usually produced 

 very well-marked Traube-Hering curves, indicating a rhythmic action 

 of the vasomotor centre. These changes go on during the first and 

 second stages of asphyxia, but after this the heart muscle, over-strained 

 by the raised peripheral resistance and inadequately supplied with 



1 "On Epileptiform Convulsions from Haemorrhage," by Kussmaul and Tenner, New 

 Syd. Soc. Translation, London, 1859. 



2 M. Foster, "Text-book of Physiology," 6th edition, p. 640. 



