CHAP. XXIX.] RATIO OF RESPIRATIONS TO PULSE. 403 



although we have a limited power of checking or accelerating them, 

 of varying their rhythm or force, and can become aware of them 

 by an effort of attention. After holding the breath for fifteen or 

 twenty seconds during ordinary respiration, or forty seconds after a 

 deep inspiration, there arises an insupportable sensation over the 

 whole chest, concentrated under the sternum, and no effort can 

 maintain the interruption of the respiratory acts. This urgent sen- 

 sation of want of breath, when carried to its full extent by any 

 mechanical impediment to the aeration of the blood, is one of the 

 most painful and oppressive kind, and is referable to the pulmo- 

 nary plexuses distributed on the bronchia, and perhaps on the walls 

 of the lobular passages and cells. The impression made on these 

 peripheral nerves by the absence of oxygen, and the undue presence 

 of carbonic acid in the air in contact with them, is propagated to 

 the spinal cord and medulla oblongata by the sympathetic and vagus, 

 and there excites those combined actions of the muscles of inspira- 

 tion which lead to the renewal of the air; and it may be fairly con- 

 cluded, that the ordinary motions of respiration depend on the same 

 circle of nervous actions, which, thus voluntarily arrested, become 

 apparent by their accumulated force. (See ante, p. 124). The mus- 

 cular actions of expiration, which seem to follow so evenly on those 

 of inspiration, are probably due, in part, at least, to the stimulus of 

 elongation of their fibres. When the ribs diverge, the fibres of 

 the internal intercostals, except in front, are extended. 



Frequency of Respirations, and Ratio to the Pulse. The number 

 of respirations per minute in healthy adults, is from eighteen to 

 twenty ; but, according to Dr. Hutchinson's observations on more 

 than 1700 persons (when sitting), the range in health may be from 

 six to forty, though most persons breathe from sixteen to twenty- 

 four times per minute. The proportion which the respirations 

 bear to the beats of the heart is liable to much variation, but is in 

 general not far from that of one to four. Dr. Guy has shown that 

 the respirations are rather more frequent in the evening than in the 

 morning, whereas the pulse is rather slower. He has also discovered 

 that the proportion between the respirations and pulsations is much 

 deranged by changes of posture, probably by these modifying the 

 expansion of the lungs at each inspiration, without inducing any 

 corresponding change in the transmission of the blood. 



Aerial Capacity of the Lungs , and Amount of Air breathed. 

 When the lungs have been emptied as much as possible of air by 

 a forced expiration, they still contain a residual quantity, which 

 may be estimated at about forty cubic inches. The mechanical 



