RESPIRATION. 605 



Sobbing is produced by a series of similar convulsive inspirations, 

 in which the glottis is closed earlier than in hiccoughing, and no air, con- 

 sequently, enters the chest. 



The foregoing are the principal modifications of the inspiratory 

 phase of the respiratory rhythm. 



The following owe their characteristics mostly to some modification 

 of expiration : — 



Coughing is produced by a long, deep inspiration, after which the 

 glottis becomes firmly closed. A forced expiration is then made through 

 the violent contraction of the abdominal muscles and serves to force 

 open the glottis, the vibration of the vocal cords producing the charac- 

 teristic sound. The starting point of the act of coughing maj' be found 

 in the contact of any foreign body with the mucous membrane of the 

 air-passages, the impression being conducted through the superior laryn- 

 geal nerve. Other parts of the body may, however, also serve as the 

 starting point to the act of coughing. Thus, stimulation of the auricular 

 branch of the pneumogastric, as is often the case in numerous ear dis- 

 eases, may produce coughing, as may the stimulation of other nerves. 



Sneezing is produced by a mechanism closely similar to that of 

 coughing, with the exception that the expelled column of air passes 

 through the nose instead of the mouth. 



The point of origin of the afferent impulses is usually found in the 

 mucous membrane of the nose and is conducted through the fifth pair of 

 nerves. In certain individuals the optic nerve may also be the path over 

 which the afferent impulses travel to originate the act of sneezing. 



Laughing consists of a long inspiration followed by a series of short 

 expirations, during which the glottis is open and the vocal cords thrown 

 into vibration by the movement of the column of air. 



Crying is produced by the same mechanism as laughing, the only 

 difference being in the facial expression. As a consequence, laughing 

 frequently verges into crying, and the reverse, and the two are frequently 

 indistinguishable. 



6. Influence op the Respiration on the Circulation. — In the exami- 

 nation of a tracing obtained in a blood-pressure experiment, in addition 

 to the undulations caused by the variations in the blood pressure clue to 

 each individual heart contraction, a larger series of waves, each com- 

 posed of a number of cardiac pulsations, may be noticed. These undula- 

 tions have already been referred to as being due to the influence of the 

 respiratory movements on arterial blood pressure. Their mode of pro- 

 duction is now to be considered. Roughly speaking, it may be stated 

 that during every inspiration the blood pressure falls, and during every 

 expiration it rises. This statement is, however, not absolutely correct, 

 but requires some slight modification. 



