THE RESPIRATORY MECHANISM. 425 



slowly, though, at first, more powerfully. This slowing is 

 due to the fact that the unusual want of oxygen leads to 

 stimulation of the cardio-inhibitory centre in the medulla 

 and this, through the pneumogastric nerves, slows the 

 heart's beat. Soon, however, the want of oxygen affects 

 the heart itself and it begins to beat more feebly, and also 

 more slowly, from exhaustion, until its final stoppage. Dur- 

 ing the second and third stages the heart and the venae cavae 

 become greatly overfilled with blood, because the violent 

 muscular contractions facilitate the flow of blood to the 

 heart, while its beats become too feeble to send it out again. 

 The overfilling is most marked on the right side of the heart 

 which receives the venous blood from the Body generally. 



During the first and second stages of asphyxia arterial 

 pressure rises in a marked degree. This is due to excita- 

 tion of the vaso-motor centre by the venous blood, and 

 the consequent constriction of the muscular coats of the 

 arteries and increase of the peripheral resistance. In the 

 third stage the blood-pressure falls very rapidly, because the 

 feebly acting heart then fails to keep the arteries tense, even 

 although their diminished calibre greatly slows the rate at 

 which they empty themselves into the capillaries. 



Another medullary centre unduly excited during asphyxia 

 is that from which proceed the nerve-fibres governing those 

 muscular fibres of the eye which enlarge the pupil. During 

 suffocation, therefore, the pupils become widely dilated. 

 At the same time all reflex irritability is lost, and touching 

 the eyeball causes no wink; the reflex centres all over the 

 Body being rendered, through want of oxygen, incapable of 

 activity. The same is true of the higher nerve-centres; un- 

 consciousness comes on during the convulsive stage, which, 

 horrible as it looks, is unattended with suffering. 



Modified Respiratory Movements. Sighing is a deep 

 long-drawn inspiration followed by a shorter but correspond- 

 ingly large expiration. Yawning is similar, but the air is 

 mainly taken in by the mouth instead of the nose, and the 

 lower jaw is drawn down in a characteristic manner. Hic- 

 cough depends upon a sudden contraction of the diaphragm, 

 while the aperture of the larynx closes; the entering air, 

 drawn through the narrowing opening, causes the peculiar 

 sound. Coughing consists of a full inspiration followed by a 

 violent and rapid expiration, during the first part of which 



