402 THE HUMAN BODY. 



tense, even although, their diminished calibre greatly slows 

 the rate at which they empty themselves into the capilla- 

 ries. 



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, in- 

 capable of activity. The same is true of the higher nerve- 

 centres; unconsciousness comes on during the convulsive 

 stage, which, horrible as it looks, is unattended with suffer- 

 ing. 



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 

 the laryngeal opening is kept closed; being afterwards sud- 

 denly opened, the air issues forth with a rush, tending to 

 carry out with it anything lodged in the windpipe or larynx. 

 Sneezing is much like coughing, except that, while in a 

 cough the isthmus of the fauces is held open and the air 

 mainly passes out through the mouth, in sneezing the- 

 fauces are closed and the blast is driven through the 

 nostrils. It is commonly excited by irritation of the nasal 

 mucous membrane, but in many persons a sudden bright 

 light falling into the eye will produce a sneeze. Laughing 

 consists of a series of short expirations following a single 

 inspiration; the larynx is open all the time, and the vocal 

 cords (Chap. XXXVI.) are set in vibration. Crying is, phy- 

 siologically, much like laughing and, as we all know, one 

 often passes into the other. The accompanying contrac- 



