RESPIRATION. 603 



either through some mechanical obstruction of the air-passages .or 

 through the reduction of the amount of oxygen in the air, as by breathino- 

 in a confined space, normal respiration gradually is replaced by dyspnoea, 

 in which simply all the accessory muscles of respiration are brought into 

 play, and respiration then, instead of being an almost imperceptible 

 involuntary movement, now becomes convulsive. If this interference 

 with the normal oxygenation of the blood continues, the animal then 

 passes into a state of asphyxia, which proves rapidly fatal. 



When the reduction of oxygen in the blood supplied to the respira- 

 tory centre becomes decreased, the overexcitation of the respiratory 

 centre becomes evident in the increased violence of the respiratory 

 movements; expiration becomes gradually convulsive, until often every 

 muscle which may react on the thorax is brought into play. Very 

 soon almost all of the muscles of the body are implicated and the animal 

 is then in a state of active convulsions. It would appear that these con- 

 vulsions are due simply to the extension of the activity of the respira- 

 tory centre to other closelj- associated centres in the medulla. For we 

 notice that first the ordinary respiratory muscles contract with greater 

 vigor, w r e then have the accessory muscles brought into play, and event- 

 ually the entire muscular system of the body ; and although a con- 

 vulsive centre in the medulla is sometimes spoken of, it is evident 

 that this centre must be closely connected with the respiratory centre. 

 After a variable period the violent muscular contractions suddenly 

 disappear, from the exhaustion of the nervous sj^stem, the pupils are 

 then found to be dilated to their utmost and are unaffected by light, 

 and the cornea insensible to the touch. All the muscles of the body 

 are now in a condition of relaxation ; and although the respiratory 

 movements have not entirely ceased, they are, however, far between, 

 are gasping, accompanied by twitching of other muscles, especially of 

 the face, and the intervals between them become gradually increased ; the 

 rhythm of the respiratory movements becoming irregular, each inspira- 

 tion shallower and shallower, again implicating the other muscles of the 

 body, with the head thrown, back and the mouth open, the face drawn 

 and the nostrils dilated, the last breath is taken in. 



It is thus evident that the phenomena resulting from the deprivation 

 of oxygen may be divided into three different stages : — 



First, a stage of dyspnoea, characterized by an extraordinary mus- 

 cular effort in the muscles of respiration, expiration being especially con- 

 vulsive. Second, an extension of the convulsive muscular contractions 

 to all the other muscles of the body, and therefore characterized by 

 general convulsions, while the final stage is one of exhaustion, in which 

 the inspirations become slower and slower, more and more shallow, 

 and are finally arrested. 



