RESPIRATION. 379 



in death. Before this occurs the individual exhibits a succession 

 of phenomena, to the totality of which the term asphyxia is given. 



Asphyxia may be caused: (i) By a sudden mtenerence with the 

 entrance of oxygen into and the exit of carbon dioxid from the blood, 

 as in drowning, occlusion of the trachea from any cause, double 

 pneumothorax, etc. (2) By confinement in a small space the air 

 of which speedily undergoes a loss of oxygen and an accumulation of 

 carbon dioxid. In the first instance death may occur in a few minutes ; 

 in the second instance it may be postponed several hours or more, 

 the time varying with the size of the space. 



The succession of phenomena presented by an individual in the 

 asphyxiated condition is as follows : Increased rate and depth of the 

 respiratory movements, passing rapidly from hyperpnea to dyspnea, 

 with an active contraction of all the muscles concerned in respira- 

 tion, ordinary and extraordinary; a blue cyanosed condition of the 

 face from the rapid accumulation of carbon dioxid and disappearance 

 of the oxygen of the blood; a diminution in the depth of inspiration 

 and an increase in the force and extent of expiration, followed by 

 general convulsions; collapse, characterized by unconsciousness, loss 

 of the reflexes, relaxation of the muscles, a weak action of the heart, 

 a disappearance of the pulse and death. As shown by observation 

 of the circulatory apparatus in artificially induced asphyxia, there is 

 primarily an increase in the activity of the heart, soon followed by 

 retardation; a rise of blood-pressure in the early stages and a fall to 

 zero after collapse has set in. The retardation and final cessation of 

 the heart, as well as the rise of the blood-pressure, are to be attributed 

 to stimulation of the cardio-inhibitory and vasomotor centers from 

 the accumulation of the carbon dioxid. With the exhaustion of the 

 nerve- centers, there is a general relaxation of the muscles and a fall 

 of the pressure. 



THE NERVE MECHANISM OF RESPIRATION. 



The nerve mechanism by which the respiratory muscles are ex- 

 cited to action is extremely complex and involves the action of both 

 afferent and efferent nerves and their related nerve-centers in the cen- 

 tral nerve system. For the free introduction of air into the lungs it is 

 essential that the nasal and laryngeal passages and the cavity of the 

 thorax be simultaneously enlarged. The muscles by which these 

 results are accomplished have already been mentioned and described. 

 \fTheir simultaneous and coordinate contraction implies the coordinate 

 activity of motor nerves and their centers ; thus, the nasal and laryn- 

 geal muscles (the dilator naris and the posterior crico-arytenoid) 

 involve the activity of the facial and inferior laryngeal nerves re- 

 spectively, the centers of origin of which lie in the gray matter beneath 



